Colours of black holes: infrared flares from the hot accretion disc in XTE J1550-564

Outbursts of the black hole (BH) X-ray binaries are dramatic events occurring in our Galaxy approximately once a year. They are detected by the X-ray telescopes and often monitored at longer wavelengths. We analyse the X-ray and optical/infrared (OIR) light curves of the BH binary XTE J1550&ndash;564 during the 2000 outburst. By using the observed extreme colours as well as the characteristic decay time-scales of the OIR and X-ray light curves, we put strong constraints on the extinction towards the source. We accurately separate the contributions to the OIR flux of the irradiated accretion disc and a non-thermal component. We show that the OIR non-thermal component appears during the X-ray state transitions both during the rising and the decaying part of the outburst at nearly the same X-ray hardness but at luminosities differing by a factor of 3. The line marking the quenching/recovery of the non-thermal component at the X-ray hardness&ndash;flux diagram seems to coincide with the &lsquo;jet line&rsquo; that marks the presence of the compact radio jet. The inferred spectral shape and the evolution of the non-thermal component during the outburst, however, are not consistent with the jet origin, but are naturally explained in terms of the hybrid hot flow scenario, where non-thermal electrons emit synchrotron radiation in the OIR band. This implies a close, possibly causal connection between the presence of the hot flow and the compact jet. We find that the non-thermal component is hardening during the hard state at the decaying stage of the outburst, which indicates that the acceleration efficiency is a steep function of radius at low accretion rate.&nbsp;</p

New fast X-ray transient IGR J18462-0223 discovered by the INTEGRAL observatory

Details of the discovery of a new X-ray source, IGR J18462-0223, on October 12, 2007, during a short (several hours), intense (~ 35 mCrab at the peak) outburst of hard radiation by the IBIS/ISGRI gamma-ray telescope onboard the INTEGRAL observatory are given. The detection of another earlier outburst from this source occurred on April 28, 2006, in the archival data of the telescope is reported. We present the results of the source's localization and our spectral/timing analysis of the observational data. The source may turn out to be yet another representative of the continuously growing population of fast X-ray transients, which are the focus of attention because of the identification of their optical counterparts with early-type supergiants

Unveiling Soft Gamma-Ray Repeaters with INTEGRAL

Thanks to INTEGRAL's long exposures of the Galactic Plane, the two brightest Soft Gamma-Ray Repeaters, SGR 1806-20 and SGR 1900+14, have been monitored and studied in detail for the first time at hard-X/soft gamma rays. This has produced a wealth of new scientific results, which we will review here. Since SGR 1806-20 was particularly active during the last two years, more than 300 short bursts have been observed with INTEGRAL. and their characteristics have been studied with unprecedented sensitivity in the 15-200 keV range. A hardness-intensity anticorrelation within the bursts has been discovered and the overall Number-Intensity distribution of the bursts has been determined. In addition, a particularly active state, during which ~100 bursts were emitted in ~10 minutes, has been observed on October 5 2004, indicating that the source activity was rapidly increasing. This eventually led to the Giant Flare of December 27th 2004, for which a possible soft gamma-ray (>80 keV) early afterglow has been detected. The deep observations allowed us to discover the persistent emission in hard X-rays (20-150 keV) from 1806-20 and 1900+14, the latter being in a quiescent state, and to directly compare the spectral characteristics of all Magnetars (two SGRs and three Anomalous X-ray Pulsars) detected with INTEGRAL.Comment: 8 pages, 7 figures, Presented at the conference "Isolated Neutron Stars: from the Surface to the Interior", London, UK, 24-28 April 200

The first multi-wavelength campaign of AXP 4U 0142+61 from radio to hard X-rays

For the first time a quasi-simultaneous multi-wavelength campaign has been performed on an Anomalous X-ray Pulsar from the radio to the hard X-ray band. 4U 0142+61 was an INTEGRAL target for 1 Ms in July 2005. During these observations it was also observed in the X-ray band with Swift and RXTE, in the optical and NIR with Gemini North and in the radio with the WSRT. In this paper we present the source-energy distribution. The spectral results obtained in the individual wave bands do not connect smoothly; apparently components of different origin contribute to the total spectrum. Remarkable is that the INTEGRAL hard X-ray spectrum (power-law index 0.79 +/- 0.10) is now measured up to an energy of ~230 keV with no indication of a spectral break. Extrapolation of the INTEGRAL power-law spectrum to lower energies passes orders of magnitude underneath the NIR and optical fluxes, as well as the low ~30 microJy (2 sigma) upper limit in the radio band.Comment: 6 pages, 1 figure. To be published in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface" (April 24-28, 2006, London, UK), eds. S. Zane, R. Turolla and D. Pag

The near-infrared counterpart of 4U 1636-53

The optical counterpart of the neutron star X-ray binary and well known X-ray burster, 4U 1636-53 (= 4U 1636-536 = V801 Ara) has been well studied for three decades. However to date, no infrared studies have been reported. Our aims are to identify and investigate the near-infrared (NIR) counterpart of 4U 1636-53. We present deep, K_s-band (2.2 micron) imaging of the region of 4U 1636-53 taken with the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope. Archival optical and UV data are used to infer the 0.2-2.2 micron spectral energy distribution (SED). One star is located at coordinates alpha =16:40:55.57, delta =-53:45:05.2 (J2000; 1 sigma positional uncertainty of ~ 0.3 arcsec) which is consistent with the known optical position of 4U 1636-53; its magnitude is K_s = 16.14 +- 0.12. This star is also detected in the 2MASS survey in J-band and has a magnitude of J = 16.65 +- 0.22. Under the assumption that the persistent emission is largely unvarying, the 0.4-2.2 micron de-reddened SED can be described by a power law; F_nu propto nu^(1.5 +- 0.3), with some possible curvature (F_nu propto nu^(<= 1.5)) at 0.2-0.4 microns. The SED can be approximated by a blackbody of temperature ~ 27 000 K. This is typical for an active low-mass X-ray binary, and the emission can be explained by the outer regions of a (likely irradiated) accretion disc. We therefore interpret this K_s-band star as the NIR counterpart.Comment: Accepted in A&A as a Research Note; 4 pages, 2 figure

A ten-year-long peak of the X-ray flux from the burster 4U 1724-307 in the globular cluster Terzan 2: Evolution of the donor star or the influence of a third star?

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Localization of the X-ray burster KS 1731-260 from Chandra data

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

RXTE observations of Galactic microquasar XTE J1748-288 during its 1998 outburst

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman