M. Falanga

Swings between rotation and accretion power in a binary millisecond pulsar

It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star’s rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales.

Are There Magnetars in High-Mass X-Ray Binaries? The Case of Supergiant Fast X-Ray Transients

In this paper we survey the theory of wind accretion in high-mass X-ray binaries hosting a magnetic neutron star and a supergiant companion. We concentrate on the different types of interaction between the inflowing wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star surface is largely inhibited; these include inhibition through the centrifugal and magnetic barriers. Expanding on earlier work, we calculate the expected luminosity for each regime and derive the conditions under which transition from one regime to another can take place. We show that very large luminosity swings (~104 or more on timescales as short as hours) can result from transitions across different regimes. The activity displayed by supergiant fast X-ray transients, a recently discovered class of high-mass X-ray binaries in our galaxy, has often been interpreted in terms of direct accretion onto a neutron star immersed in an extremely clumpy stellar wind. We show here that the transitions across the magnetic and/or centrifugal barriers can explain the variability properties of these sources as a result of relatively modest variations in the stellar wind velocity and/or density. According to this interpretation we expect that supergiant fast X-ray transients which display very large luminosity swings and host a slowly spinning neutron star are characterized by magnetar-like fields, irrespective of whether the magnetic or the centrifugal barrier applies. Supergiant fast X-ray transients might thus provide a new opportunity to detect and study magnetars in binary systems.

Irradiation of an accretion disc by a jet: general properties and implications for spin measurements of black holes

X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the RELLINE model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given

INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth

Aims. We study the spectrum of the cosmic X-ray background (CXB) in energy range ∼5−100 keV. Methods. Early in 2006 the INTEGRAL observatory performed a series of four 30 ks observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background (CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. Results. The spectrum of the cosmic X-ray background in the energy band 5−100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is ∼10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20−50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin.

THE EVOLUTION OF THE γ- AND X-RAY LUMINOSITIES OF PULSAR WIND NEBULAE

Pulsar wind nebulae are a prominent class of very high energy (E > 0.1 TeV) Galactic sources. Their -ray spectra are interpreted as due to inverse Compton scattering of ultrarelativistic electrons on the ambient photons, whereas the X-ray spectra are due to synchrotron emission. We investigate the relation between the - and-X-ray emission and the pulsars’ spin-down luminosity and characteristic age. We find that the distance-independent - to X-ray flux ratio of the nebulae is inversely proportional to the spin-down luminosity, (∝ u E −1.9 ), while it appears proportional to the characteristic age, (∝ � 2.2 c ), of the parent pulsar. We interpret these results as due to the evolution of the electron energy distribution and the nebular dynamics, supporting the idea of so-called relic pulsar wind nebulae. These empirical relations provide a new tool to classify unidentified diffuse -ray sources and to estimate the spin-down luminosity and characteristic age of rotation powered pulsars with no detected pulsation from the X- and −ray properties of the associated pulsar wind nebulae. We apply these relations to predict the spin-down luminosity and characteristic age of four (so far unpulsing) candidate pulsars associated to wind nebulae. Subject headings: pulsars : general — radiation mechanisms: non-thermal — supernova remnants — X-rays : stars — gamma rays: observations

INTEGRAL and RXTE observations of accreting millisecond pulsar IGR J00291+5934 in outburst

Simultaneous observations of the accretion-powered millisecond pulsar IGR J00291+5934 by the International Gamma-Ray Astrophysics Laboratory and Rossi X-ray Timins Explorer during the 2004 December outburst are analysed. The average spectrum is well described by thermal Comptonization with an electron temperature of 50 keV and Thomson optical depth τ T ∼ 1 in a slab geometry. The spectra] shape is almost constant during the outburst. For the first time we detect a spin-up, ν = 8.4 x 10 -13 Hz s -1 , of an accreting millisecond pulsar. The ISGRI data reveal the pulsation of X-rays at a period of 1.67 milliseconds up to -150 keV. The pulsed fraction is shown to increase from 6 per cent al 6 keV to 12-20 per cent at 100 keV. This is naturally explained by the action of the Doppler effect on the exponentially cutoff Comptonization spectrum from the hot spot. The nearly sinusoidal pulses show soft lags with a complex energy dependence, increasing up to 7 keV, then decreasing to 15 keV, and seemingly saturating at higher energies.

X-ray Follow-ups of XSS J12270-4859: A Low-mass X-ray Binary with Gamma-ray Fermi-LAT Association

XSSJ1227.0-4859 is a peculiar, hard X-ray source recently positionally associated to the Fermi-LAT source 1FGLJ1227.9-4852/2FGLJ1227.7-4853. Multi-wavelength observations have added information on this source, indicating a low-luminosity low-mass X-ray binary (LMXB), but its nature is still unclear. To progress in our understanding, we present new X-ray data from a monitoring campaign performed in 2011 with the XMM-Newton, RXTE, and Swift satellites and combine them with new gamma-ray data from the Fermi and AGILE satellites. We complement the study with simultaneous near-UV photometry from XMM-Newton and with previous UV/optical and near-IR data. The X-ray history of XSSJ1227.0-4859 over 7yr shows a persistent and rather stable low-luminosity (~6x10^33 d_{1\,kpc}^2 erg/s) source, with flares and dips being peculiar and permanent characteristics. The associated Fermi-LAT source 2FGLJ1227.7-4853 is also stable over an overlapping period of 4.7\,yr. Searches for X-ray fast pulsations down to msec give upper limits to pulse fractional amplitudes of 15-25% that do not rule out a fast spinning pulsar. The combined UV/optical/near-IR spectrum reveals a hot component at ~13\,kK and a cool one at ~4.6\,kK. The latter would suggest a late-type K2-K5 companion star, a distance range of1.4--3.6kpc and an orbital period of 7--9 h. A near-UV variability (>6\,h) also suggests a longer orbital period than previously estimated. The analysis shows that the X-ray and UV/optical/near-IR emissions are more compatible with an accretion-powered compact object than with a rotational powered pulsar. The X-ray to UV bolometric luminosity ratio could be consistent with a binary hosting a neutron star, but the uncertainties in the radio data may also allow an LMXB black hole with a compact jet. In this case it would be the first associated with a high-energy gamma-ray source.

What ignites on the neutron star of 4U 0614+091?

The low-mass X-ray binary 4U 0614+091 is a source of sporadic thermonuclear (type I) X-ray burs ts. We find bursts with a wide variety of characteristics in serendipitous wide-field X-r ay observations by the WATCH on EURECA, the ASM on RXTE, the WFCs on BeppoSAX, the FREGATE on HETE-2, the IBIS/ISGRI on INTEGRAL, and the BAT on Swift, as well as pointed observations with the PCA and HEXTE on RXTE. Most of the bursts are bright, i.e., they reach a peak flux of a bout 15 Crab, but a few are weak and only reach a peak flux below a Crab. One of the bursts shows a ver y strong photospheric radius-expansion phase. This allows us to evaluate the distance to the source, which we estimate to be 3.2 kpc. The burst durations vary generally from about 10 sec t o 5 min. However, after one of the intermediate-duration bursts, a f aint tail is seen to at least about 2.4 hours after the start of the burst. One very long burst was observed, which lasted for several hours . This superburst candidate was followed by a normal type-I burst only 19 days later. This is, to our knowledge, the shortest burst- quench time among the superbursters. The observation of a superburst in this system is diffi cult to reconcile if the system is accreting at about 1% of the Eddington limit. We describe the burst properties in relation to the persistent emission. No strong correlation s are apparent, except that the intermediate-duration burs ts occurred when 4U 0614+091’s persistent emission was lowest and calm, and when burs ts were infrequent (on average roughly one every month to 3 months). The average burst rate increased significantly af ter this period. The maximum average burst recurrence rate i s about once every week to 2 weeks. The burst behaviour may be partly understood if there is at least an appreciable amount of helium pre sent in the accreted material from the donor star. If the system is an ultra-compact X-ray binary with a CO white-dwarf donor, as has been suggested, this is unexpected. If the bursts are powered by h elium, we find that the energy production per accumulated mas s is about 2.5 times less than expected for pure helium matter.

From outburst to quiescence: The decay of the transient AXP XTE J1810-197

Aims. XTE J1810-197 is the first transient anomalous X-ray pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude. Methods. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four year baseline (September 2003– September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse-phase spectroscopic studies were also carried out for observations with a high enough signal-to-noise. Results. We find that (i) a three-blackbody model reproduces the spectral properties of XTE J1810-197 over the entire outburst statistically better than the two blackbodies model previously used in the literature, (ii) the coldest blackbody is consistent with the thermal emission from the whole surface and has temperature and radius similar to those inferred from ROSAT observations before the outburst onset, (iii) there is a spectral feature around 1.1 keV during six consecutive observations (since March 2005). If this stems from proton resonant cyclotron scattering, it would imply a magnetic field of ∼2 × 10 14 G. This closely agrees with the value from the spin period measurements.

INTEGRAL spectroscopy of the accreting millisecond pulsar XTE J1807-294 in outburst

the date of receipt and acceptance should be inserted later Abstract. The fourth transient accreting millisecond pulsar XTE J1807-294 was observed during its February/March 2003 outburst by INTEGRAL, partly simultaneously with the XMM-Newton and RXTE satel- lites. We present here the first study of the 0.5-200 keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature � 40 keV, considerably larger than the value (� 10 keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200 keV with a luminosity in the energy band (0.1-200) keV of 1.3 × 10 37 erg s −1 (assuming a distance of 8 kpc). 22 days later the luminosity dropped to 3.6 × 10 36 erg s −1 . A re-analysis of XMM-Newton data yields the orbital Doppler variations of the pulse period and refines the previous ephemeris. For this source having shortest orbital period of any known binary radio or X-ray millisecond pulsar, we constrain the companion mass Mc < 0.022M⊙, assuming minimum mass transfer driven by gravitational radiation. Only evolved dwarfs with a C/O composition are consistent with the Roche lobe and gravitational radiation constraints, while He dwarfs require an unlikely low inclination.