An INTEGRAL hard X-ray survey of the Large Magellanic Cloud

Abstract

Two observation campaigns in 2003 and 2004 with the INTEGRAL satellite have provided the first sensitive survey of the Large Magellanic Cloud with an imaging instrument in the hard X-ray range (15 keV-10 MeV). The high energy flux and long-term variability of the black hole candidate LMC X-1 was measured for the first time without contamination by the nearby (∼25 ) young pulsar PSR B0540-69. We studied the accreting pulsar LMC X-4 by constraining the size of the hard X-ray emitting region (≤3 × 10 10 cm) from analysis of its eclipses and by measuring its spin period (13.497 ± 0.005 s) in the 20-40 keV band. As it was in a soft state during the first observation and possibly in an extremely low state in the second one, LMC X-3 was not detected. Thanks to the large field of view of the IBIS instrument, we could also study other sources falling serendipitously in the observed sky region around the LMC: the Galactic low mass X-ray binary EXO 0748-676, the accreting pulsar SMC X-1 in the Small Magellanic Cloud, and the Active Galactic Nucleus IRAS 04575-7537. In addition we discovered five new hard X-ray sources, two of which most likely belong to the LMC.

States and Transitions in Black Hole Binaries

“…LMC X-1 appears to be locked in the HSS, as does LMC X-3, which however has shown brief transitions to a harder state, even so extreme as not to be detected with RossiXTE. It is difficult to say more about them [100,73,36]. Moreover, it is impossible to ignore the prototypical microquasar GRS 1915+105 (see [24]) which, despite its peculiarity, needs to be compared with other systems.…”

The X-ray stellar population of the LMC

“…larger than the size of a NS and more typical of that of a hot corona. LMC X-4 is also varying at these high energies by an order of magnitude (Götz et al 2006;Tsygankov & Lutovinov 2005). Finally, LMC X-4 experiences X-ray flares and at these times, mHz quasi-periodic oscillations have been detected (Moon & Eikenberry 2001): strong, burstlike-features with a timescale of 700-1500 s and weak oscillation with periods of 50-500 s. The former could be explained by beating frequencies between the pulsar frequency and the orbital frequencies of big clumps on the verge of being accreted, while the latter is more compatible with Keplerian periods of clumps outside the corotation radius (Moon & Eikenberry 2001).…”

“…The X-ray flux is modulated with a 30-d timescale (Lang et al 1981, see also Naik &Paul 2003 andGötz et al 2006), which likely reflects the precession period of the tilted accretion disk as in Her X-1. While the orbital period appears very stable, the disk precessing period may vary non-uniformly (Tsygankov & Lutovinov 2005).…”

“…This yields rather good results, especially when the photon statistics is poor, but is clearly oversimplistic (presence of an intense gravitational field in the inner regions, interrelations between the comptonized emission and the seed blackbody spectrum,...) and can be improved in many ways (Yao et al 2005, and references therein). Götz et al 2006). Only a disk blackbody was required to fit its spectrum and no discrete features were found in the high-resolution X-ray spectrum, though one would have expected the Ne K edge to be detected (Cui et al 2002).…”

“…With a detection limit of 3×10 33 erg s −1 , they detected 460 sources in the 2-8 keV band, in vast majority AGNs, and focused on 9 good XRBs candidates and 19 possible XRBs (see below for more details). Finally, a sensitive survey of the LMC in the hard X-ray domain (15 keV-10 MeV) has been performed with Integral (Götz et al 2006). Only a few sources have been detected: the X-ray binaries LMC X-1, LMC X-4, and PSR B0540-69, as well as two hard sources which might correspond to LMC binaries.…”