INTEGRAL sees XTE J1739-285 change to hard state

ATel #615; S. E. Shaw (Southampton, UK / ISDC, Geneva); E. Kuulkers (ESA/ESAC, Spain); T. Oosterbroek (ESA/ESTEC, Netherlands); T. J.-L. Courvoisier (ISDC, Geneva); A. Paizis (ISDC, Geneva / IASF, Milano); K. Ebisawa (NASA/GSFC, USA); P. Kretschmar (ESA/ESAC, Spain); C. Markwardt (NASA/GSFC, USA); R. Wijnands (UvA, Netherlands); N. Mowlavi (ISDC, Geneva); A. Orr (ESA/ESTEC, Netherlands)
on 30 Sep 2005; 14:26 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)

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Continuing INTEGRAL Galactic Bulge Monitoring observations have detected the X-ray transient XTE J1739-285 above 20 keV.

XTE J1739-285 was first detected by INTEGRAL during JEM-X observations on 2005-08-25 with estimated fluxes of up to 120 and 70 mCrab in the 3-10 and 10-20 keV bands respectively (ATel #592). During those observations the source was not seen with the ISGRI instrument, suggesting an upper limit to the flux above 20 keV of < 5 mCrab.

In two recent INTEGRAL observations, each consisting of 7*30 minute pointings starting at 2005-09-26T19:32:00 and 2005-09-28T22:42:06 (UT), the source was seen in ISGRI with a 20-60 keV flux ~ 20 mCrab. In both observations the source was located within 2-3 arc minutes of the position reported by Swift/XRT in ATel #602. Contrary to the observations reported in ATel #592, in these observations the source was much weaker in JEM-X with an estimated 3-10 keV flux ~ 15 mCrab.

The ISGRI spectra from both observations of the source are modelled well by a hard power law extending above 100 keV with photon index ~ 1.6-1.7. A considerably steeper, possibly thermal, component is evident at energies < 30 keV.

The apparent change in the spectrum of the source over ~ 1 month indicates a recent change of state from high-soft to low-hard. The spectral shape and the observed changes indicate that XTE J1739-285 could be a black hole system. Follow-up observations at other wavelengths are encouraged.

Light curves and ISGRI images from the Galactic Bulge Monitoring observations can be found at http://isdc.unige.ch/Science/BULGE