Long term monitoring of the BHC 1E 1740.7-2942
M. Del Santo, A. Bazzano, N. Bezayiff, D. M. Smith, P. Ubertini, G. De Cesare, M. Federici
aa r X i v : . [ a s t r o - ph ] O c t LONG TERM MONITORING OF THE BHC 1E 1740.7–2942
M. Del Santo , A. Bazzano , N. Bezayiff , D. M. Smith , P. Ubertini , G. De Cesare , and M. Federici INAF/IASF-Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy Department of Physics, University of California, Santa Cruz, CA95064, USA
ABSTRACT
The microquasar 1E 1740.7–2942 is one of the mostappealing source of the Galactic Centre region. Thehigh energy feature detected once by SIGMA has beensearched in the last years by
INTEGRAL , but never con-firmed. Classified as a persistent source, on 2004 itshowed a quiescent-like state. In fact for few month 1E1740.7–2942 was below the detector sensitivity level.We present the long term temporal behaviour of 1E1740.7–2942 observed by
INTEGRAL and
RXTE in2004 and 2005, as well as preliminary results on possi-ble spectral transitions.Key words: X-ray: X-ray binaries; star: black-hole bi-nary; X-ray: observations; individual: 1E 1740.7–2942.
1. INTRODUCTION
1E 1740.7–2942 was classified as a Black Hole Candi-date and reported as the hardest persistent source in theGalactic Centre region [19]. Because of the two-sidedradio jets associated to the source, it was classified as amicroquasar [14].The nature of the normal star companion still remains amystery. This fact is likely due to the dense surround-ing medium, with high concentrations of dust and a largehydrogen column density (N H ∼ × cm − [5]),which depletes photons from the infrared up to soft X-rayenergies. RXTE long term observations provided an orbital and asuper-orbital modulations of 12.5 dy and 600 days, re-spectively [16]. The upper-limits on the magnitude ob-tained in the infrared band [11], as well as the modulationperiod measured by
RXTE suggest that the stellar com-panion in 1E 1740.7–2942 might be a low-mass star.The electron-positron annihilation radiation detected bySIGMA in 1992 [2] has been searched longly by nu-merous experiments. Reference [18] used simultaneousCGRO/BATSE data to search for the 1992 1E 1740.7–2942 transient, and a σ upper limit of 1.8 × − pho-tons s − cm − , which contradicted the SIGMA flux, was Figure 1. 1E 1740.7–2942 RXTE/PCA ligh curve basedon short pointings in the energy range 6–12 keV. given. OSSE was also used to observe that feature: noevidence of 511 keV line was found [9]. The history ofthese and other gamma-ray line transients is reviewed by[8]. Any 511 keV transient event from a compact objectin the Galactic Bulge has been searching by
INTEGRAL ;up to now only upper limits have been provided [3].Thanks to simultaneous
INTEGRAL and
RXTE observa-tions performed in 2003, [4] reported on the first broad-band spectral study of 1E 1740.7–2942. These authorsshowed a spectral transition from the canonical low/hardstate to a peculiar intermediate/soft state, which occurredwhen the source flux was decreasing. After few days thesource quenched in the soft gamma-ray and it was at thedetector sensitivity level when observed with X-ray in-struments, as also firstly reported by [7] and [12]. How-ever, a similar behaviour was found some years beforewith SIGMA observations [10].Recently a model of the source emission, from radio togamma-rays, with a cold-matter dominated jet has beenpresented [1]. These authors find out that jet emissioncannot explain the high fluxes observed at hard X-rayswithout violating at the same time the constraints fromthe radio data, favoring the corona origin of the hard X-rays.
2. OBSERVATIONS AND DATA ANALYSIS
Because of solar panels constraints, the Galactic Cen-tre is visible with
INTEGRAL for few months twice per igure 2. IBIS/ISGRI count rate vs MJD binned by pointing (1800-2200 s) is shown. Observations were performed inSpring 2005 (left) and Autumn 2005 (right). year. All
INTEGRAL public and Core Programme datacollected during 2004 and 2005 have been selected. Inthis work, we have used only pointings with 1E 1740.7–2942 in the IBIS fully coded field of view ( ◦ × ◦ ) [20],for a total of roughly 4 Ms.Since RXTE performs 1E 1740.7–2942 observationstwice per week, PCA data are sampled more frequentlythan the timescale of variability for most of the year.The IBIS/ISGRI light curves with pointing duration (scw)as a time bin (in the range 1800–2200 s) have beenextracted with the 5.1 version of the Off-Line Scien-tific Analysis (OSA). We chose the following six energyranges: 20–30 keV, 30–40 keV, 40–50 keV, 50-70 keV,70–100 keV and 100–200 keV.In order to compute the hardness ratio (HR) of 70–100keV and 30–40 keV bands, counts rate rebinned by orbitduration have been used.
3. 2004 AND 2005 TEMPORAL BEHAVIOUR
In Fig. 1, the
RXTE /PCA light curve in the energy range6–12 keV is shown. The superimposed periods corre-sponding to the
INTEGRAL observations are indicated.In 2004
INTEGRAL pointed towards 1E 1740.7–2942 before it quenched: at the end of February thesource was detected at 10 mCrab in 20-40 keV; after fewdays it was no longer visible [7] and [4]. In the Autumnit was still not detectable by IBIS.The IBIS/ISGRI light curves collected in Spring 2005and Autumn 2005, respectively, are shown (Fig. 2). At the beginning, 1E 1740.7–2942 was at roughly 50 mCrab(in 20–30 keV) and was detected up to 200 keV. The fluxvariability was not higher than a factor of 2 at lower en-ergy (30–40 keV) and a factor of 4 at higher energy (70–100 keV) (see rebinned light curves in Fig. 3).Spectral transitions during
INTEGRAL observations havebeen searched by using HR of 70–100 keV to 30–40 keV;as can be seen in Fig. 4 the resulted HR is almost flat.
4. DISCUSSION
Black holes in low mass systems (LMXB) are known tospend most of the time in the canonical Low/Hard state[5]. During this state, a radio activity due to jets emissioncan be observed; it is strongly suppressed in the high/softstate [6].1E 1740.7–2942 appears to show related radio and X-ray variability [15], although it is radio underluminousin the context of the radio-X-ray luminosity correlationfound for black hole candidate X-ray binaries and asso-ciated with the accretion/ejection activity [6]. This mightbe due to a particularly radiatively efficient corona.There is a wide debate about the possible origin of theX-rays in microquasars since both the corona and the jetscenarios seem to be roughly consistent in some caseswith observations [13]. In 1E 1740.7–2942 the hard X-ray emission has been interpreted with a corona emission[4]; later, [1] showed that the hard X-rays cannot be ex-plained if coming from a jet since it would imply an ener-getic efficiency in the jets significantly larger than for thecorona emission. igure 3. 1E 1740.7–2942 temporal behaviour averaged over INTEGRAL revolutions in the energy range 30–40 keV(left) and 70–100 keV (right). Clearly, the different exposure time for each revolution is resulted in the count rate errorbars.Figure 4. IBIS/ISGRI hardness ratio with orbit bin size.
As observed with
RXTE , for 1E 1740.7–2942 and GRS1758–258 the softest spectra observed are related to drop-ping photon fluxes [17]. In 2003, the first broad-bandstudy of 1E 1740.7–2942 showed a spectral transitionto the intermediate/soft state before the source quenching[4]. Our 2005 simultaneous
INTEGRAL and
RXTE ob-servations were performed in high flux levels; as expected1E 1740.7–2942 was in its Low/Hard state. Presum-ably, a spectral transition would have occurred after fewmonths (see Fig. 1). Temporal and spectral analysis of2006 are in progress.The so-called “dynamical” soft state [17] occurring dur-ing flux decreasing is explained in the context of two si-multaneous and independent accretion flows (i. e., thinand thick). It would occur when the inner geometricallythin disc has yet to respond to a drop in accretion ratethat has already depleted the external halo. The delaybetween the two accretion flows depleting is expected inthe LMXB because of the longer viscous timescale of theoptically thick disc compared with the halo [17].
ACKNOWLEDGMENTS
This work has been supported by the Italian SpaceAgency through the grant I/R/046/04. MDS thank Mrs.Catia Spalletta for the activity support at IASF-Rome.