Time lag in transient galactic and extragalactic accreting sources
aa r X i v : . [ a s t r o - ph . H E ] A ug Time lag in transient galactic and extragalacticaccreting sources
Franco Giovannelli ∗ INAF - Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00133Roma, ItalyE-mail: [email protected]
Gennady S. Bisnovatyi-Kogan
IKI, Moscow, Russian FederationE-mail: [email protected]
X-ray binaries are cauldrons of fundamental physical processes which appear along practicallythe whole electromagnetic spectrum. The sub-class of X-ray transient sources show multifre-quency behaviour which deserve particular attention in order to understand the causing physics.These binary systems consist of a compact star and an optical star, therefore there is a mutualinfluence between these two stars that drive the low energy (LE) (i.e. radio, IR, optical) andhigh energy (HE) (i.e. UV, X-ray, γ -ray) processes. The LE processes are produced mostly onthe optical star and the HE processes mostly on the compact star, typically a neutron star. Thusit appears evident that through the study of LE processes it is possible to understand also theHE processes and vice versa. In this paper we will discuss this problem starting from the ex-perimental evidence of a delay between LE and HE processes detected for the first time in theX-ray/Be system A0535+26/HDE245770 (e.g. Giovannelli & Sabau-Graziati, 2011; Giovannelli,Bisnovatyi-Kogan & Klepnev, 2013 (here after GBK13); Giovannelli et al., 2015b). This delay iscommon in cataclysmic variables (CVs) and other binary systems with either a neutron star or ablack hole.Since a delay between LE processes and HE processes has been experimentally observed in sev-eral active galactic nuclei (AGNs), we will discuss also the tidal disruption of stars by massiveBHs, following the original idea of Rees (1998): stars in galactic nuclei can be captured or tidallydisrupted by a central black hole. Some debris would be ejected at high speed, the remainderwould be swallowed by the hole, causing a bright flare lasting at most a few years.The outline of this paper is: • Antecedent fact • Introduction • X-ray Binary Systems • Old & News from the transient X-ray/Be system A0535+26/HDE245770 • The model for Galactic Accreting Sources • The model for AGNs • Discussion & Conclusions
Accretion Processes in Cosmic Sources - II - APCS20183-8 September 2018Saint Petersburg, Russian Federation c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ ime lag in transient accreting sources
Franco Giovannelli
1. Antecedent Fact
Figure 1 clearly explain all the mysteries of our Universe (Giovannelli, 2000). People who areable to read this sentence can understand that "
The truth is written in the book of the Nature.We must learn to read this book ". Figure 1:
Understanding our Universe (adopted from Giovannelli, 2000).
The experiments provide the basic alphabet, immersed in an apparently chaotic soup, butnecessary to understand the nature. From that soup we must extract words and phrases to composethe book of the nature. In other words, the data coming from the experiments constitute the basicalphabet that we use for constructing models that attempt to describe the nature. But we havea lot of models for interpreting the experimental data by the light of science. Depending on thehypotheses the results could run against the experiments. Then, in order to be acceptable, modelscan take into account and justify
ALL the available data .The same concept was expressed in much more incisive terms by Richard Phillips Feynman– Nobel laureate in Physics in 1965 – also known as
The Great Explainer : It doesn’t matterhow beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree withexperiment, it’s wrong .The talk originating this paper had not been scheduled in the original Final Program of theworkshop because the detailed description of the models that we will present is published in the ∗ Speaker. ime lag in transient accreting sources Franco Giovannelli proceedings of the Saint Petersburg workshop 2016 by G.S. Bisnovatyi-Kogan and F. Giovannelli,and other their publications. This talk came from the hole created in the program due to the sud-den absence of Nazar Ikhsanov and Mateusz Wisniewicz. Therefore, instead of the details of themodels, we will present the genesis of this work to the benefit above all of the young participants.This paper is a demonstration of how we tried to read the book of the nature.Undoubtedly the advent of new generation experiments ground– and space–based have givena strong impulse for verifying current theories, and for providing new experimental inputs for de-veloping a new physics for going, probably, over the standard model (SM). Recent results comingfrom Active Physics Experiments (APEs) – experiments in which we try to reproduce in a labora-tory the physical conditions of processes occurring in nature we want to understand – and PassivePhysics Experiments (PPEs) – experiments by which we observe the nature – have opened such anew path.An extensive review on the situation about the knowledge of the physics of our Universe hasbeen recently published by Giovannelli & Sabau-Graziati (2016). The reader interested is invitedto look at that paper.The correct procedure in passive physics experiments is: • to observe and collect experimental data; • to analyze these data without any a priori bias; • to attempt their interpretation on the base of current models; • if not possible, it is mandatory to search for other models that cannot be "ad hoc".This seems trivial, but, unfortunately, it is not so.
2. Introduction
Important observations started early in the past century with the discovery of inexplicableeffects - the supernovae. They had been observed in ancient times but it was only with the estab-lishing of the stellar and galactic distance scales that their true enormity was realized, namely therelease of ≥ erg within a matter of days.The beginning of the Space Astrophysics Era is commonly located around the end of thefifties of the last century with the first space experiments, in the energy range 0.2–0.5 MeV, onboard balloons. They were devoted to the detection of γ -rays generated in solar activity (Peterson& Winckler, 1958). But actually γ -ray astronomy was born in the last year of the XIXth centurywith the discoveries of penetrating gamma radiation (Villard, 1900), and the atmospheric ionization(Wilson, 1900). Wilson suggested that the extraterrestrial gamma radiation could be responsiblefor the atmospheric ionization. With balloon flights Hess (1912) demonstrated the extraterrestrialand extra-solar origin of the ionizing radiation, which was called Cosmic Rays .Until 1927 it was thought that cosmic rays consisted of γ -rays. Thanks to the discovery of thedependence of the cosmic ray flux on the geomagnetic latitude during a trip from Java to Genoamade by Clay (1927), it was recognized that the composition of cosmic rays included chargedparticles. Later Hayakawa (1952) determined the contribution of γ -rays to the composition of2 ime lag in transient accreting sources Franco Giovannelli cosmic rays as less than 1%. The experiments outside the atmosphere started in 1946, soon afterthe end of the second world war, when the Naval Research Laboratory (NRL) launched a V2 rocketwith a payload which observed the Sun’s UV spectrum.Since that time many space experiments were prepared and several fundamental results werereached. In our opinion the actual beginning of the Space Era for studying the Universe is the year1962. An X-ray experiment – prepared by Giacconi, Gursky, Paolini & Rossi – launched on boardan Aerobee rocket discovered a strong X-ray emission from an extra-solar object, namely Sco X-1(Giacconi et al., 1962). After this first historical experiment many others were launched on boardrockets and later balloons and satellites. These experiments lead to our knowledge of an X-ray skyhitherto unknown which started to give experimental proofs of the first theories of Baade & Zwicky(1934) about the possible existence of neutron stars.Indeed, Baade & Zwicky (1934) first suggested that the supernova was the result of the transi-tion from a normal star to a neutron star. The essential point (Zwicky, 1939) being that the energyreleases in such a process is comparable to the change in gravitational potential energy of a star,which collapses from its "normal" size of ∼ Km down to the size of a neutron star of ∼
10 Km.In the 1950’s, Burbidge et al. (1957) with their works on stellar nucleosynthesis suggestedrealistic models of stars prior to supernova explosion. The supernova process was seen as the resultof catastrophic change of state occurring in the core of a highly evolved star, e.g. the transformationof an iron core into a helium core.Contrary, Cameron (1958) suggested that this degenerate iron core would collapse to a neutroncore through inverse beta decay.The discovery (by chance) of the first X-ray source (Sco X-1) (Giacconi et al., 1962) acceler-ated the studies on neutron stars, until that Zel’dovich & Guseinov (1965) suggested the presenceof an unseen massive companion in a binary system.Space orbiting observatories with larger and more sophisticated experiments – from Uhurulaunched in 1970 (Giacconi et al., 1971) up to HEAO-1 launched in 1977 (Wood et al., 1984) –discovered the most luminous galactic and extragalactic X-ray sources, such as pulsars, X-ray bi-naries, supernova remnants (SNRs), bursters, and active galactic nuclei (AGNs). But the qualitativejump in the observational capabilities was obtained with the HEAO-2 satellite (Einstein) (Giacconiet al., 1979) in which the X-ray focussing optics of the instruments enhanced the sensitivity in thesoft X-ray range by a factor of about 1000 with respect to the old generation of detectors. Also theangular resolution was improved up to ∼ This was the first measurement that originated the Nobel Price of Riccardo Giacconi. ime lag in transient accreting sources Franco Giovannelli
3. X-ray binary systems
The trivial definition of X-ray binaries (XRBs) is that they are binary systems emitting X-rays.However it has been largely demonstrated that X-ray binary systems emit energy in IR, Optical, UV,X-ray, Gamma-ray and sometimes they show also valuable radio emission. They can be divided indifferent sub-classes • High Mass X-ray Binaries (HMXB) in which the optical companion is an early type giantor supergiant star and the collapsed object is a neutron star or a black hole. They are concen-trated around the galactic plane. The mass transfer is usually occurring via stellar wind; theyshow hard pulsed X-ray emission (from 0.069 to 1413 s) with KT ≥ to 10 erg s − , and the ratio of X-ray to optical luminosityis ∼ − –10. The HMXBs can be divided in two sub-classes – Hard Transient X-ray Sources (HXTS) in which the neutron star is eccentrically (e ∼ . orb >
10 days); they showstrong variable pulsed hard X-ray emission (L
Xmax /L Xmin > ≥
17 keV,and P spin ranging from 0.069 to 1413 s; L X = − erg s − . – Permanent X-ray Sources in which the neutron star or black hole is circularly orbiting(e ∼
0) around a giant or supergiant OB star (P orb <
10 days); they show an almoststeady permanent pulsed hard X-ray emission (L
Xmax /L Xmin ≪ spin rangingfrom 0.069 to 1413 s; L X ∼ erg s − . – Supergiant X-ray Binaries (SGXBs): obscured sources, which display huge amountof low energy absorption produced by the dense wind of the supergiant companion,surrounded by a weakly magnetized neutron star. – Supergiant Fast X-ray transients (SFXT), a subclass of SGXBs and a new subclass oftransients in which the formation of transient accretion discs could be partly responsiblefor the flaring activity in systems with narrow orbits. They show L
Xpeak ≈ erg s − ,and L Xquiecence ≈ erg s − . • Low Mass X-ray Binaries (LMXB) in which the optical companion is a low-mass-late-typestar and the collapsed object is a neutron star or a black hole (P orb from 41 min to 11.2 days).They are concentrated in the globular clusters, and in the halo around the galactic center.The mass transfer in these systems is usually occurring via Roche lobe overflow. Theiremission in soft X-ray range is usually not pulsed with KT ≤ to 10 erg s − and L X /L opt ∼ –10 ; many LMXBs show QuasiPeriodic Oscillations (QPOs) between 0.02 and 1000 seconds and few of them also pulsedX-ray emission, such as Her X1, 4U 1626-27 and GX 1+4.Many LMXB show transient behaviour in the form irregular X-ray bursts, when their lumi-nosity increase several tens or hundreds times. During these luminous stages steady period-ical signals, with milliseconds (ms) period, have been observed in several of them. In fewof them ms X-ray pulsars have been discovered in quiescent stages between bursts. The msX-ray pulsars in LMXB form a link between binary X-ray sources and recycled binary radio4 ime lag in transient accreting sources Franco Giovannelli pulsars, with ms periods and low magnetic fields (Bisnovatyi-Kogan and Komberg, 1974),which are formed on the place of these LMXB after ceasing of accretion, due to evolution ofthe companion star, transforming into low mass white dwarf, or a giant degenerate planet. • Cataclysmic Variables (CVs) in which the optical companion is a low-mass-late-type starand the compact object is a white dwarf. The detected CVs are spread roughly around thesolar system at distance of 200-300 pc. Orbital periods are ranging from tens of minutesto about ten hours. The mass transfer is occurring either via Roche lobe overflow or viaaccretion columns or in an intermediate way depending on the value of the magnetic field.Typical X-ray luminosity is ranging from 10 to 10 erg s − . Updated reviews about CVsare those by Giovannelli (2008) and Giovannelli & Sabau-Graziati (2015a); • RS Canum Venaticorum (RS CVn) type systems , in which no compact objects are presentand the two components are a F or G hotter star and a K star. Typical X-ray luminosity isranging from 10 to 10 erg s − . Usually in the current literature they are excluded fromthe class of X-ray binaries since historically they were discovered as X-ray emitters onlywith the second generation of X-ray experiments.Figure 2 shows a compendium of the characteristics of the X-ray binaries (adapted from Gio-vannelli, 2015).In binary systems there are essentially two ways for accreting matter from one star to theother: via accretion disk or via stellar wind (Giovannelli & Sabau-Graziati, 2001, adapted fromBlumenthal & Tucker, 1974) (left panel of Fig 3). But in some cases there is a third way whichis a mixture between the two, as for instance in eccentric binary systems close to the periastronpassage where a temporary accretion disk can be formed around the neutron star (e.g. Giovannelli& Ziółkowski (1990), like shown in the right panel of Fig. 3 (Giovannelli & Sabau-Graziati, 2001,after Nagase, 1989).XRBs are the best laboratory for the study of accreting processes thanks to their relative highluminosity in a large part of the electromagnetic spectrum. For this reason, multifrequency ob-servations are fundamental in understanding their morphology and the physics governing theirbehaviour.Because of the strong interactions between the optical companion and collapsed object, lowand high energy processes are strictly related.Often, it is easier to perform observations of low energy processes (e.g. in radio, near-infrared(NIR) and optical bands) since the experiments are typically ground-based, on the contrary toobservations of high energy processes, for which experiments are typically space-based. Among the X-ray binaries, the class of High Mass X-ray Binaries (HMXBs) constitutes an im-portant group for studying the interactions either via stellar wind either via accretion disk betweenthe optical and the compact companions.Figure 4 shows schematically the classification of HMXBs (adopted from Giovannelli, 2015).5 ime lag in transient accreting sources
Franco Giovannelli
Figure 2:
Classification of X-ray binaries (adapted from Giovannelli, 2015). ime lag in transient accreting sources Franco Giovannelli
Figure 3:
Left panel: accretion in X-ray binary systems disk-fed and wind-fed (adopted from Giovannelli& Sabau-Graziati, 2001, adapted from Blumenthal & Tucker, 1974). Right panel: mixed transfer (adoptedfrom Giovannelli & Sabau-Graziati, 2001, after Nagase, 1989).
The X-ray/Be binaries are the most abundant group of massive X-ray binaries in the galaxy,with a total inferred number of between 10 and 10 . The ones which do occasionally flare-up astransient X-ray/Be systems are only the "tip" of this vast "iceberg" of systems (van den Heuvel andRappaport, 1987). The mass loss processes are due to the rapid rotation of the Be star, the stellarwind and, sporadically, to the expulsion of casual quantity of matter essentially triggered by grav-itational effects close to the periastron passage of the neutron star. The long orbital period ( > > .
2) together with transient hard X-ray behavior are themain characteristics of these systems. Among the whole sample of galactic systems containing 114X-ray pulsars (Liu, van Paradijs & van den Heuvel, 2006), only few of them have been extensivelystudied. Among these, the system A 0535+26/HDE 245770 – HDE 245770 was nicknamed Flaviastar by Giovannelli & Sabau-Graziati, 1992) – is the best known thanks to concomitant favorablecauses, which rendered possible forty three years of coordinated multifrequency observations, mostof them discussed in the past by e.g. Giovannelli & Sabau-Graziati (1992), Burger et al. (1996),Piccioni et al. (1999), and later by Giovannelli & Sabau-Graziati (2011) and Giovannelli et al.(2015a,b). Accretion powered X-ray pulsars usually capture material from the optical companionvia stellar wind, since this primary star generally does not fill its Roche lobe. However, in somespecific conditions (e.g. the passage at the periastron of the neutron star) and in particular systems(e.g. A 0535+26/HDE 245770), it is possible the formation of a temporary accretion disk aroundthe neutron star behind the shock front of the stellar wind. This enhances the efficiency of theprocess of mass transfer from the primary star onto the secondary collapsed star, as discussed byGiovannelli & Ziolkowski (1990) and by Giovannelli et al. (2007) in the case of A 0535+26.Optical emission of HMXBs is dominated by that of the optical primary component, whichis not, in general, strongly influenced by the presence of the X-ray source. The behavior of theprimary stars can be understood in the classical (or almost) frame-work of the astrophysics ofthese objects, i.e. by the study of their spectra which will provide indications on mass, radius, and7 ime lag in transient accreting sources
Franco Giovannelli
Figure 4:
Classification of HMXBs (adopted from Giovannelli, 2015). ime lag in transient accreting sources Franco Giovannelli luminosity. Both groups of HMXBs (transient and permanent) differ because of the different originof the mass loss process: in the first, the mass loss process occurs via a strong stellar wind and/orbecause of an incipient Roche lobe over-flow; in the second group, the mass transfer is probablypartially due to the rapid rotation of the primary star and partially to stellar wind and sporadicallyto expulsions of a casual quantity of matter, essentially triggered by gravitational effects because ofperiastron passage where the effect of the secondary collapsed star is more marked. A relationshipbetween orbital period of HMXBs and the spin period of the X-ray pulsars is shown in Fig. 5(updated from Giovannelli & Sabau-Graziati, 2001 and from Corbet, 1984, 1986). It allows torecognize three kinds of systems, namely disk-fed, wind-fed [P pulse ∝ (P orb ) / ], and X-ray/Besystems [P pulse ∝ (P orb ) ]. Figure 5:
Spin period vs orbital period for X-ray pulsars. Disk–fed systems are clearly separated by systemshaving as optical counterparts either OB stars or Be stars (adopted from Giovannelli & Sabau-Graziati, 2001,after Corbet, 1984, 1986).
Most of the systems having a Be primary star are hard X-ray (KT >
10 KeV) transient sources(HXTS). They are concentrated on the galactic plane within a band of ∼ . ◦ . The orbits are quiteelliptic and the orbital periods large (i.e. A 0538-66: e = 0.7, P orb = 16.6 days (Skinner et al.,1982); A 0535+26: e = 0.47 (Finger, Wilson & Hagedon, 1994), P orb = 111.0 days (Priedhorsky &Terrell, 1983). The X-ray flux during outburst phases is of order 10-1000 times greater than duringquiescent phases. For this reason, on the contrary, the stars belonging to the class of permanentX-ray sources, which do not present such strong variations in X-ray emission, can be also named"standard" high mass X-ray binaries. In X-ray/Be systems, the primary Be star is relatively notevolved and is contained within its Roche lobe. The strong outbursts occur almost periodicallyin time scales of the order of weeks-months. Their duration is shorter than the quiescent phases.During X-ray outbursts, spin-up phenomena in several systems have been observed (i.e. A 0535+26and 4U 1118-61 (Rappaport & Joss, 1981). The observed spin-up rates during the outbursts are9 ime lag in transient accreting sources Franco Giovannelli consistent with torsional accretion due to an accretion disk (e.g. Ghosh, 1994). So, the formationof a temporary accretion disk around the collapsed object should be possible during outburst phases(e.g. Giovannelli & Ziolkowski, 1990).
4. Old & News from the transient X-ray/Be system A0535+26/HDE245770
The most studied HMXB system, for historical reasons and due to concomitant favourablecauses, is the X-ray/Be system A 0535+26/HDE 245770. By means of long series of coordinatedmultifrequency measurements, very often simultaneously obtained, it was possible to: • identify the optical counterpart HDE 245770 of the X-ray pulsar; • identify various X-ray outbursts triggered by different states of the optical companion andinfluenced by the orbital parameters of the system; • identify the presence of a temporary accretion disc around the neutron star at periastron.Multifrequency observations of A 0535+26 started soon after its discovery as an X-ray pulsarby the Ariel-5 satellite on April 14, 1975 (Coe et al., 1975). The X-ray source was in outburstwith intensity of ∼ ∼
104 s (Rosenberg et al., 1975). The hardX-ray spectrum during the decay from the April 1975 outburst became softer, so that the 19 Mayspectrum had E − . and the 1 June spectrum E − . (Ricketts et al., 1975). Between 13 and 19 April,1975, as the nova brightened, the spectra showed some evidence of steepening. The best fit of theexperimental data between roughly 27 and 28 April was compatible with an 8 keV black-bodycurve (Coe et al., 1975). The X-ray source decayed from the outburst with an e -folding time of 19days in the energy range of 3-6 keV (Kaluzienski et al., 1975).In the X-ray error box of the X-ray source A 0535+26, detected by Ariel V, were present 11stars up to 23 rd magnitude and one of them (HDE 245770) of magnitude around 9 showed the H α and H β in emission, H γ filled in with emisssion, and H δ , H ε ,..., H in absorption (Margon et al.,1977). A priori probability of finding a 9 mag star in such a field is 0.004, thus HDE 245770 wasconsidered as the probable optical counterpart of A0535+26.But in order to really associate this star with the X-ray pulsar, it was necessary to find a clearsignature proving that the two objects would belong to the same binary system. This happenedthanks to a sudden insight of one of us (FG), who predicted the fourth X-ray outburst of A 0535+26around mid December 1977. For this reason, Giovannelli’s group was observing in optical HDE245770 around the predicted period for the X-ray outburst of A 0535+26. Figure 6 shows the X-ray flux intensity of A 0535+26 as deduced by various measurements available at that time, withobvious meaning of the symbols used (Giovannelli, 2005). FG’s intuition was sparked by lookingat the rise of the X-ray flux (red line) and at the 24th May 1977 measurement (red asterisk): heassumed that the evident rise of the X-ray flux would have produced an outburst similar to the firstone, which occurred in 1975. Then with a simple extrapolation he predicted the fourth outburst,similar to the second: and this happened!Optical photoelectric photometry of HDE 245770 showed significant light enhancement of thestar relative to the comparison star BD +26 876 between Dec. 17 and Dec 21 (here after 771220-E)and successive fading up to Jan. 6 (Bartolini et al., 1978), whilst satellite SAS-3 was detecting an10 ime lag in transient accreting sources Franco Giovannelli
X-ray flare (Chartres & Li, 1977). The observed enhancement of optical emission followed by theflare-up of the X-ray source gave a direct argument strongly supporting the identification of HDE245770 – later nicknamed Flavia’ star by Giovannelli & Sabau-Graziati (1992) – with A 0535+26.
Figure 6:
X-ray flux versus time of A 0535+26. X-ray measurements are reported with red lines andasterisk, upper limits with green arrows, and predicted fluxes with light blue stars. Periods of real detectedX-ray outburst and optical measurements are also marked (adopted from Giovannelli, 2005).
Soon after, with spectra taken at the Loiano 152 cm telescope with a Boller & Chivens 26767grating spectrograph (831 grooves/mm II-order grating: 39 Å mm − ) onto Kodak 103 aO plates,it was possible to classify HDE 245770 as O9.7IIIe star. This classification was so good that itsurvives even to the recent dispute attempts made with modern technology. The mass and radius ofthe star are 15 M ⊙ and 14 R ⊙ , respectively; the distance to the system is 1 . ± . . ± .
05 mag, the rotational velocity of the O9.7IIIe star (v rot sin i = ±
45 km s − ), theterminal velocity of the stellar wind (v ∞ ≃
630 km s − ), the mass loss rate ( ˙M ∼ − M ⊙ yr − inquiescence (Giovannelli et al., 1982). During the October 1980 strong outburst, the mass loss ratewas ˙M ∼ . × − M ⊙ yr − (de Martino et al., 1989).Complete reviews of this system can be found in Giovannelli et al. (1985), Giovannelli &Sabau-Graziati (1992), and Burger et al. (1996).Briefly, the properties of this system, placed at a distance of 1 . ± . ∼ . ± . ⊙ (Joss& Rappaport 1984; Thorsett et al. 1993; van Kerkwijk, van Paradijs, J. & Zuiderwijk, 1995), and15 M ⊙ (Giangrande et al., 1980) for the secondary and primary star, respectively. The eccentricity11 ime lag in transient accreting sources Franco Giovannelli is e = 0.47 (Finger et al., 1994). Usually the primary star does not fill its Roche lobe (de Looreet al., 1984). However, the suggestion that there might be a temporary accretion disk around theX-ray pulsar when it approaches periastron (Giovannelli & Ziółkowski, 1990) was confirmed bythe X-ray measurements of Finger, Wilson & Harmon (1996) and was discussed by Giovannelli etal. (2007).The first suggestion of Bartolini et al. (1983) about the value of the orbital period (P orb = . ± .
002 days), allowed Giovannelli & Sabau-Graziati (2011) to discover a systematic delay( ∼ ≈ .
02 to ≈ . orb = . ± . opt − outb = JD (2,444,944) ± n(111.0 ±
5. Time delay between optical and X-ray outbursts in A 0535+26/HDE 245770
A description of the time–delay among many optical and X-ray events occurring around theperiastron passages in the system A 0535+26/HDE 245770 have been presented in the papers byGBK13 and Giovannelli et al. (2015a,b). However, just to remark the importance of the exper-imental evidence of such a time–delay we will present a few more examples that in our opiniondefinitively support the validity of the model developed in GBK13. Briefly, the model is the fol-lowing: in the vicinity of periastron the mass flux ˙M increases (depending on the activity of the Bestar) between ≈ − and ≈ − M ⊙ yr − . The outer part of the accretion disk – geometricallythin and optically thick without advection (Shakura & Sunyaev, 1973; Bisnovatyi-Kogan, 2011) –becomes hotter, therefore the optical luminosity (L opt ) increases. Due to large turbulent viscosity,the wave of the large mass flux is propagating toward the neutron star, thus the X-ray luminosity(L x ) increases due to the appearance of a hot accretion disk region and due the accretion flow chan-12 ime lag in transient accreting sources Franco Giovannelli neled by the magnetic field lines onto magnetic poles of the neutron star. The time–delay τ is thetime between the optical and X-ray flashes appearance. Figure 7 shows a sketch of this model. Figure 7:
Sketch of the viscous accretion disk model for explaining the time-delay between X-ray andoptical flashes (adopted from GBK13).
By using the ephemerides given by GBK13, namely:JD opt − outb = JD (2,444,944) ± n(111.0 ± ime lag in transient accreting sources Franco Giovannelli only to predict the arrival time of the X-ray outbursts following the optical flashes, but also theintensity I x of the X-ray flares, thanks to the relationship I x versus ∆ V mag , where ∆ V mag is therelative variations of the V magnitude of the Be star around the periastron passage with respectto the level before and after such a passage. This relationship is shown in Fig. 8 (adapted fromGiovannelli et al., 2015b). Figure 8:
Intensity of the X-ray flare of A 0535+26 versus the variation of V magnitude of HDE 245770around the periastron passage (adapted from Giovannelli et al., 2015b).
We have also found a relationship between the equivalent width (EW) of H α and I x . Thevalues of H α -EW have been taken only if measurements were performed around the periastronpassage ±
10 days. Unfortunately few measurements have been found in this time range. However,the trend of the relationship is rather good, as shown in Fig. 9, where data taken from Camero-Arranz et al. (2012), Yan, Li & Liu (2012), Giovannelli et al. (2015b) are reported (Fasano, 2015).It is interesting to note that in one occasion, at the 106th periastron passage (JD 2,456,710 = 21Feb 2014) after 811205-E, optical photometry and spectroscopy as well as X-ray measurementsfrom different experiments were obtained. A jump in the H α -EW and H β -EW in correspondencewith the rise of X-ray intensity was detected, being the jump of H β -EW delayed of ≈ α -EW. This important result deserves further investigations. However, thejumps of H α -EW and H β -EW could originate because of a contribution to the total emission inthose lines coming from the temporary accretion disk around the neutron star (Giovannelli et al.,2015a, and the references therein). And if so, the delay between H β -EW and H α -EW jumps shouldbe explicable within the framework of GBK13’s model.An impressive strong optical event have been detected on March 19, 2010 (JD 2,455,275).On the basis of such a strong optical activity – especially H γ in emission – Giovannelli, Gualandi& Sabau-Graziati (2010, ATel 2497) predicted the incoming X-ray outburst of A 0535+26, whichactually occurred (Caballero et al., 2010b, ATel 2541). The X-ray intensity reached was 1.18 Crabon April 3, 2010 in the range 15–50 keV of BAT/SWIFT (Caballero et al., 2010a,b,c,d; Caballeroet al., 2011). Figure 10 shows the March–April 2010 event. The X-ray flare started about 8 daysafter the 93th periastron passage after the 811205-E, just when optical spectroscopy was performed14 ime lag in transient accreting sources Franco Giovannelli
Figure 9:
Intensity of the X-ray flare of A 0535+26 versus the equivalent width of H α of HDE 245770around the periastron passage (data taken from Camero-Arranz et al., 2012; Yan, Li & Liu, 2012; Giovannelliet al., 2015b) (figure adopted from Fasano, 2015). by Giovannelli, Gualandi & Sabau-Graziati (2010), and reached the maximum about 12 days laterand decayed in about 20 days roughly as occurred in 1975 when A0535+26 was discovered by theAriel V satellite. Figure 10:
The predicted March–April 2010 X-ray outburst of A 0535+26 (Giovannelli, Gualandi & Sabau-Graziati, 2010) after the 93th passage at the periastron after 811205-E (Caballero et al., 2010a,b,c,d; Ca-ballero et al., 2011).
The astonishing fact that definitively demonstrate the goodness of GBK13’s ephemerides andthe mechanism triggering the X-ray outburst with a time–delay with respect to the optical flare15 ime lag in transient accreting sources
Franco Giovannelli around the periastron passage is reported in Fig. 11.
Figure 11:
The Periastron passage at the 22nd cycle before 811205-E (JD 2502) (red line) precedes of ∼ Indeed, in Fig. 11 the measurements of the first detection of A 0535+26 by Ariel V satelliteare reported. Unfortunately in 1975, around the time of the discovery of A 0535+26 no opticalmeasurements are available for obvious reasons. The vertical red line indicates the time of the peri-astron passage following GBK13’s ephemeris. This passage occurred at the 22nd cycle before the811205-E. The vertical blue line indicates the day April 7, 1975 (JD 2,442,510.4 ) just 115 cyclesbefore the strong optical spectroscopic activity detected on March 19, 2010 (JD 2,455,275.4), thatpreceded the strong X-ray outburst reported in Fig. 10.16 ime lag in transient accreting sources
Franco Giovannelli
The similarity between the first X-ray outburst and that of March–April 2010 is evident, andthe separation of the two events is exactly 115 cycles.
6. General model of time lag between optical and X-ray outbursts in binaryaccreting sources
In LMXBs (Low-Mass X-ray Binaries) the compact object can be either a neutron star or ablack hole and the optical companion is a low mass star. The exchange of matter occurs via Rochelobe overflow, like shown in the left panel of Fig. 3. In HMXBs (High-Mass X-ray Binaries) thecompact object can be either a neutron star or a black hole and the optical companion is a high massstar: giant or super-giant. The exchange of matter occurs mainly via stellar wind since usually theoptical star does not fill its Roche lobe (Fig. 3, left panel). However, sometimes, the exchange ofmatter can occur in a mixed way because of the formation of an accretion disk around the compactobject around the periatron passage (e.g. Giovannelli & Ziółkowski, 1990) (Fig. 3, right panel).McClintock, Narayan & Rybicki (2004) found a very interesting relationship between theminimum X-ray luminosity in the range 0.5–10 keV and the orbital periods of BH LMXBs and NSLMXBs. BH LMXBs are on average a factor of ∼
100 fainter than NS LMXBs with similar orbitalperiods (Fig. 12).
Figure 12:
Minimum X-ray luminosity (0.5-10 keV) versus orbital period for BH LMXBs and NS LMXBs.The diagonal hatched areas delineate the regions occupied by the two classes of sources and indicate thedependence of luminosity on orbital period (adapted from McClintock, Narayan & Rybicki, 2004). ime lag in transient accreting sources Franco Giovannelli
As well known, X-ray/Be systems are formed by a compact star and an optical star. Obviouslythere is a mutual influence between the two stars. Low-energy (LE) processes influence high-energy (HE) processes and vice versa. Never confuse the effect with the cause. There is a generallaw in the Universe:
Cause and Effect . The
Cause generates an
Effect and NOT vice versa!It is right to remind that the mechanism proposed by GBK13 for explaining the X-ray-opticaldelay in A 0535+26/HDE 245770 is based on an enhanced mass flux propagation through theviscous accretion disk. This mechanism, known as UV-optical delay (the delay of the EUV flashwith respect to the optical flash) was observed and modeled for cataclysmic variables (e.g. Smak,1984; Lasota, 2001). Time delays have been detected also in several other X-ray transient binaries.This is the reason that urged Bisnovatyi-Kogan & Giovannelli (2017, BKG17) to generalize theaforementioned model, developed for the particular case of A 0535+26/HDE 245770 (Flavia’ star).This general model provides the formula (6.1) of the time delay between the optical and X-rayflashes appearance in transient cosmic accreting sources: τ = . / ˙m / α / ( T ) / (6.1)where:m = M/M ⊙ ; ˙m = ˙M/(10 − M ⊙ /yr) ; T = T / K ; α = viscosity , andT = maximum temperature in optics.By using this formula it is possible to obtain an excellent agreement between the experimental andtheoretical delays found in: • X-ray/Be system A0535+26/HDE245770: τ exp ≃ τ th ≃ • Cataclysmic variable SS Cygni; τ exp = 0.9–1.4 days (Wheatley, Mauche & Mattei, 2003); τ th ≃ • Low-mass X-ray binary Aql X-1/V1333 Aql: τ exp ∼ τ th ≃ • Black hole X-ray transient GRO J1655-40: τ exp ∼ τ th ≃ α -viscosity parameter plays an important role, and usually it ishard to be determined. However, if the other parameters are known, because experimentally deter-mined, the formula (6.1) can be used for determining α , taking into account the experimental delaymeasured in a certain source.Over the last couple of decades we have witnessed the discovery of a multitude of highlyionized absorbers in high-resolution X-ray spectra from both BH and NS XRBs. The first detectionswere obtained thanks to ASCA on the BH binaries GROJ1655-40 and GRS 1915+105. Narrowabsorption lines in the spectra of these systems identified as Fe XXV and Fe XXVI indicatedthe first of many discoveries of photo-ionized plasmas in LMXBs (Chandra, XMM-Newton andSuzaku). Black hole hot accretion flows occur in the regime of relatively low accretion rates andare operating in the nuclei of most of the galaxies in the universe. One of the most important18 ime lag in transient accreting sources Franco Giovannelli progress in recent years in this field is about the wind or outflow. This progress is mainly attributedto the rapid development of numerical simulations of accretion flows, combined with observationson, e.g., Sgr A ⋆ , the SMBH in the Galactic center. The mass loss from a BH via wind is related tothe mass accretion rate onto the BH as (Yuan, 2016):˙M wind (r) = ˙M BH × (r/20r g ) s with s ≈ g = BH c (6.2)At this point it is useful to make a sort of summary about the number of XRBSs, including CVs.Liu, van Paradijs & van den Heuvel (2006, 2007) and Ziółkowski (2013) report 315 galactic XRBs:197 LMXBs (63%) and 118 HMXBs (37%), 72 of which are Be/X-ray systems; moreover there are62 BH candidates. Coleiro & Chaty (2013) report that in the Milky way there are ≥
200 HMXBs.Ritter & Kolb (2003) catalogue, in the 7.20 (Dec. 2013) version, reports 1166 CVs. Buckley (2015)reports about the discoveries of 530 new CVs from MASTER-Network and 855 CVs from CatalinaReal Time Survey (CRTS) (http://nesssi.cacr.caltech.edu/DataRelease/). Ferrario, de Martino &Gänsicke (2015) report the number of MCVs as ≈ ∼
60 of which IPs, and consideringthose systems for which the magnetic field intensity has not yet been determined, their number isof ≈ Table 1:
Comparison of numbers of different classes of X-ray Binary Systems in the Milky Way and in theMagellanic Clouds (Ziółkowski, 2013; Ferrario, de Martino & Gänsicke, 2015; Buckley, 2015).
Name of the Class Milky Way LMC SMCTotal mass of the galaxy(in M
SMC units) 100 10 1High Mass X-ray Binaries 118 26 83in this Be/X-ray 72 19 79Low Mass X-ray Binaries 197 2 -Black Hole Candidates 62 2 -Cataclysmic Variables ≈ ≈
250 - -IPs ∼
60 - -B not yet determined ≈
600 - -Grimm (2003) published: (i) a list of the 17 most luminous LMXBs contributing ≈
90% to theintegrated luminosity of LMXBs in the 2-10 keV band in the whole Galaxy, averaged over 1996-2000. The 12 most luminous sources (Cir X-1, GRS 1915+105, Sco X-1, Cyg X-2, GX 349+2,GX 17+2, GX 5-1, GX 340+0, GX 9+1, NGC 6624, Ser X-1, GX 13+1) contribute ≈
80% of theintegrated luminosity of the Galaxy; (ii) a list of the 10 most luminous HMXBs (Cyg X-3, Cen19 ime lag in transient accreting sources
Franco Giovannelli
X-3, Cyg X-1, X 1657-415, V 4641 Sgr, GX 301-2, XTE J1855-024, X 1538-522, GS 1843+009,X 1908+075- that contribute ≈
40% to the integrated luminosity of HMXBs in the 2-10 keV bandin the whole Galaxy, averaged over 1996-2000.Raguzova & Lipunov (1999) – using the "Scenario Machine" developed by Lipunov (1987)and Lipunov & Postnov (1988) – obtained an evolutionary track that can lead to the formation ofBe/BH systems. This result has been confirmed fifteen years later by Casares et al. (2014) whodiscovered MWC 656, the first Be/BH binary.Indeed, Raguzova & Lipunov (1999) calculations show that binary black holes with Be starsmust have 0.2 < e < 0.8. It is particularly difficult to detect such systems as most of their spectro-scopic variations occur in a relatively small portion of the orbit, and could easily be missed if thesystems are observed at widely separated epochs. This represents one more reason for asking con-tinuous multifrequency observations of different classes of cosmic sources in order to understandtheir true behaviour.The critical initial mass of the supernova star that collapses to a BH is accepted to be equal to55 < M cr <
75 M ⊙ , and the fraction of the presupernova mass (M ⋆ ) collapsing to the BH, k BH = M BH /M ⋆ = 0.5. The kick velocity v m = 0–200 km s − . The age of the system, according to theirevolutionary scenario is 4 × yr.The expected number of Be/BH binaries – with orbital period 10 d < P orb < 1000 d, andeccentricity 0.2 < e < 0.8 – is 1 Be/BH for 20-30 Be/NS.Belczynski and Ziółkowski (2009) used binary population synthesis models to show that theexpected ratio of Be/XRBs with neutron stars to black holes in the Galaxy is relatively high ( ∼ − ∼ ∼ stars. Roughly one out of everythousand stars that form is massive enough to become a black hole. Therefore, our galaxy mustharbor some 10 stellar-mass black holes. Most of these are invisible to us, and only nineteenhave been identified (Wiktorowicz, Belczynski & Maccarone, 2014) with masses up to ∼
15 M ⊙ .Theoretically the mass of a SBH depends on the initial mass of the progenitor, how much mass islost during the progenitor’s evolution and on the supernova explosion mechanism (Belczynski et al.,2010; Fryer et al., 2012). Mass is lost through stellar winds, and the amount of mass lost strongly20 ime lag in transient accreting sources Franco Giovannelli depends on the metallicity of the star. For a low metallicity star ( ∼ .
01 of the solar metallicity)it is possible to leave a black hole of ≤
100 M ⊙ (Belczynski et al., 2010). In the region of theUniverse visible from Earth, there are perhaps 10 galaxies. Each one has about 10 stellar-massblack holes. And somewhere out there, a new stellar-mass black hole is born in a supernova everysecond.However, some attempts of evaluation of the number of SBHs in the Galaxy have been done.For instance, taking into account the γ -ray emissivity of the Galaxy (1.3 × s − for E > γ =5/3 – found a possible upper limits to the number of black holes (M ∼
10 M ⊙ and ˙M ≈ − M ⊙ yr − ) of 10 − - 10 − of the total star population of the Galaxy.There is a class of intermediate-mass black holes (IMBHs), with masses >
100 M ⊙ up to ≈ M ⊙ . It contains a dozen systems, as listed in Johnstone (2004). However, black holes withmasses of several hundred to a few thousand solar masses remain elusive, as reported in a reviewby Casares & Jonker (2014) where a deeply discussion about the mass measurements of SBHs andIMBHs is contained.Supermassive black holes (SMBHs) are 10 –10 times more massive than our Sun and arefound in the centers of galaxies (see the exhaustive review by Kormendy & Ho, 2013). Mostgalaxies, and maybe all of them, harbor such a black hole. So in our region of the Universe, thereare some 10 SMBHs. The nearest one resides in the center of our Milky Way galaxy. The mostdistant one we know of resides in a quasar galaxy billions of lightyears away. SMBHs grow in sizeas they gorge on surrounding matter.A list of BH candidates has been reported by Robert Johnston (2004) and provides the inputfor constructing the map of sky locations of BH candidates. Source list includes results reported inKormendy & Gebhardt (2001), Orosz (2002), Tremaine et al. (2002), and Ziolkowski (2003).In the case of galactic compact sources, by using the softness and hardness ratios, coming forthe measurements of the many X-ray satellites, it is possible to construct a diagram in which BHsin high state are separated by those in low state, and by other kind of objects, such as X-ray pulsarsand other systems, as shown in Fig. 13 (Giovannelli, 2016, after Tanaka, 2001).
7. Model of time lag between optical and X-ray outbursts in AGNs
As already noted black hole accretion is a fundamental physical process in the universe. It isthe standard model for the central engine of active galactic nuclei (AGNs), and also plays a cen-tral role in the study of black hole X-ray binaries, Gamma-ray bursts, and tidal disruption events.According to the temperature of the accretion flow, the accretion models can be divided into twoclasses, namely cold and hot. The standard thin disk model belongs to the cold disk, since the tem-perature of the gas is far below the virial value (Shakura & Sunyaev, 1973) (see reviews by (Pringle,1981; Frank, King & Rayne, 2002; Bisnovatyi-Kogan, 2011; Abramowicz & Fragile, 2013; Blaes,2014). The disk is geometrically thin but optically thick and radiates multi-temperature black bodyspectrum. The radiative efficiency is high, ∼ ime lag in transient accreting sources Franco Giovannelli
Figure 13:
Softness ratio versus hardness ratio for galactic compact systems. Light yellow ellipse marks thezone where BHs in high state lie, light turquoise ellipse marks the zone of the BHs in low state and NSs, andlight fuchsia ellipse marks the zone of the X-ray pulsars (Giovannelli, 2016, after Yasuo Tanaka, 2001). binaries in the thermal state. The most recent review about the accretion onto black holes waspublished by Lasota (2016).The tidal disruption of stars by massive BHs has been discussed since many years by Rees(1988), and e.g. Magorrian & Tremaine (1999). Rees (1988) argued that stars in galactic nucleican be captured or tidally disrupted by a central black hole. Some debris would be ejected at highspeed, the remainder would be swallowed by the hole, causing a bright flare lasting at most a fewyears. Such phenomena are compatible with the presence of 10 –10 M ⊙ holes in the nuclei ofmany nearby galaxies. Stellar disruption may have interesting consequences in our own GalacticCenter if a ≈ M ⊙ hole lurks there.In a recent paper, BKG17 developed models of time lags between optical and X-ray flashes forclose-binary galactic sources with accretion disks and for an AGN with an SMBH that is embeddedin a quasi-spherical bulge. The flashes in an AGN are considered in the model when a disruptionof a star that is in the evolution phase of a giant enters the radius of strong tidal forces. The matterwith a low angular momentum that is released by the star falls into the SMBH in the form of aquasi-spherical flow with a velocity that is close to the free-fall velocity. An X-ray flash occurswhen the falling matter reaches the hot inner regions. The time lag observed in these sources isidentified with the time of the matter falling from the tidal radius onto the central region. Thevalues of the tidal radius that they calculated in this model were compared with the theoretical radiiof a tidal disruption that depends on the masses of the SMBH and of the star, and on the radius ofthe star. 22 ime lag in transient accreting sources Franco Giovannelli
Knowing the SMBH masses from observations, and making a reasonable suggestion for thestellar mass that is on the order of one solar mass, they obtained that the radii of the disrupted starare between a few tens and a few hundreds of R ⊙ (see Table 2). These radii are characteristic ofstars of moderate mass on the giant phase of evolution (e.g. Bisnovatyi-Kogan, 2011).The time delay between the optical and X-ray flashes is experimentally determined. The radiusat which the optical flash occurs is calculated for the motion with free-fall velocity V ff as:V ff = (2GM/r) / ; dr/dt = V ff ; τ ff = 2/3 [r / /(GM) / ]Taking τ ff = τ obs , BKG17 obtained a radius of the optical flash r opt as:r opt = 1 . × τ obs m / cm (6.3)where: τ obs expressed in days, and SMBH mass: m expressed in (M ⊙ ). Table 2:
Properties of stars tidally disrupted by SMBH in AGNs (adapted from Bisnovatyi-Kogan & Gio-vannelli, 2017). Here m s = M s M ⊙ , where M s is the mass of the disrupted star. Source name τ obs r opt = r t R s (days) (cm) (cm)Mrk 509 15 5 . × × m / s R ⊙ NGC 7469 4 8 . × × m / s R ⊙
3C 120 3.9-6.2? (10) 3 . × × m / s R ⊙ NGC 3516 100 1 . × × m / s R ⊙ NGC 4051 2.4 2 . × × m / s R ⊙ ASASSN-14li 5 1 . × × m / s R ⊙ It is necessary to mention that another possibility to interpret the short time delays in AGNsis based on the irradiation model (e.g. Ulrich, Maraschi, & Urry, 1997). This model could explainrecent extensive observations of NGC 5548 in X-rays (SWIFT), UV, and optical light (HST) in the"reverberation mapping" campaign (Edelson et al., 2015; Fausnaugh et al., 2016).However, with the formula (6.3) BKG17 justify the experimental time delay between opticaland X-ray flashes observed in AGNs.
8. Discussion and Conclusions
We have discussed the genesis of the work that allowed us to develop models of time lagsbetween optical and X-ray flashes for close-binary galactic sources with accretion disks and for anAGN with a SMBH that is embedded in a quasi-spherical bulge.The time lag in disk-accreting galactic close-binary sources is based on a sudden increase in theaccretion flow that starts at the disk periphery and is related to the optical maximum. The massiveaccretion layer propagates to the central compact source as a result of the turbulent viscosity. TheX-ray flash occurs when this massive layer reaches the inner hot regions of the accretion disk andfalls into the central compact object. The matter in the accretion disk moves inside with a speedthat is determined by the turbulent viscosity. We described this model quantitatively and derived an23 ime lag in transient accreting sources
Franco Giovannelli analytic formula that determines the value of the time lag. This formula gives results that agree wellwith observational values. The flashes in an AGN are considered in the model when a disruptionof a star that is in the evolution phase of a giant enters the radius of strong tidal forces. The matterwith a low angular momentum that is released by the star falls into the SMBH in the form of aquasi-spherical flow with a velocity that is close to the free-fall velocity. An X-ray flash occurswhen the falling matter reaches the hot inner regions. The time lag observed in these sources isidentified with the time of the matter falling from the tidal radius onto the central region. Thevalues of the tidal radius that we calculated in this model were compared with the theoretical radiiof a tidal disruption that depends on the masses of the SMBH and of the star, and on the radius ofthe star. Knowing the SMBH masses from observations, and making a reasonable suggestion forthe stellar mass that is on the order of one solar mass, we obtained that the radii of the disruptedstar are between a few tens and a few hundreds of R ⊙ . These radii are characteristic of stars ofmoderate mass on the giant phase of evolution (see, for instance, Bisnovatyi-Kogan, 2011).The matter with larger angular momentum that appeared in the disruption of the star is ex-pected to form an accretion disk through which the matter will move to the center as a result ofturbulent viscosity, similarly to flashes in close galactic binaries. This motion is much slower thanfree-fall velocity and may last for many years. After such a flash in AGNs, we therefore expect along-duration irregular variability in the whole electromagnetic spectrum.The variability properties observed in many AGNs, where optical and UV emission lags theX-ray light curve, may be explained by the model in which an X-ray flash in the center of AGN isfollowed by reradiation of the surrounding accretion disk. Acknowledgments
This research has made use of the NASA’s Astrophysics Data System;
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DISCUSSIONDMITRY BISIKALO:
What is the physical reason of mass-transfer changing in your "delaymodel"?
FRANCO GIOVANNELLI:
The physical reason for X-ray/Be is connected with eccentric orbitof NS or BH in the binary system, where the accretion rate is increasing in the vicinity of theperiastron of the orbit. For CV the increase of the accretion rate is, probably, connected with thedevelopment of instability in the outer parts of the accretion disk, leading to the turbulent state withhigh viscosity.
VICTOR DOROSHENKO:
You detected HeI line in 1999 in A0535+26, but there was no X-rayoutburst, so there seems to be persistent disc around the NS, which is consistent with our XMMresults in quiescence. What do you think about it?
FRANCO GIOVANNELLI:
Yes, it is possible. The INTEGRAL monitor also detected someactivity in quiescence.
VICTOR DOROSHENKO:
Did I understand correctly that there is no need for field amplificationfor magnetars?