A state transition in the black-hole candidate SLX 1746-331

ATel #169; Jeroen Homan (INAF/OAB, Merate) and Rudy Wijnands (University of St Andrews)
on 30 Jul 2003; 10:12 UT
Credential Certification: Jeroen Homan (homan@merate.mi.astro.it)

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Since the detection in April 2003 of a new outburst of the black hole candidate SLX 1746-331 (ATEL #143, #144, and #145), the RXTE/PCA has been monitoring the source every 3-4 days. Currently, a total of 65 ksec of data has been obtained. The source has decreased slowly in flux (3-25 keV; unabsorbed) from 5.4E-9 ergs/s/cm^2 on April 28 to 2.0E-10 ergs/s/cm^2 on July 22. The latest observation, taken on July 26, shows an increase in the flux to 3.6E-10 ergs/s/cm^2. A figure showing the evolution of the count rate (3-25 keV) and spectral hardness (defined as the ratio of counts in the 6.1-9.8 keV and 3.3-6.1 keV bands) can be found at: http://www.merate.mi.astro.it/~homan/SLX1746-311/lc-hc.ps

A major spectral transition occurred between the observations on July 19 and July 21. Before the transition, the energy spectra were dominated by a soft component which could be fitted with a multi-color disk model with temperatures of 1.0-1.3 keV (contributing ~80-90% of the 3-25 keV flux), similar to the spectrum reported at the start of the outburst by Markwardt (2003, ATEL #143). No broad-band noise could be detected in the power density spectra with typical upper limits of 2-3 % r.m.s. (for the energy range 3.6-60 keV and the frequency range 0.01-100 Hz).

After the transition, the energy spectra were considerably harder and dominated by a power law component with index ~1.7 (contributing ~80% of the 3-25 keV flux); band-limited noise was detected in the power-density spectra with a strength of ~10 % r.m.s.. The spectral hardening and the increase in the strength of the variability strongly suggest that the source made, or is making, a transition from the black-hole soft state to the hard state. As transitions from the soft to the hard state are commonly associated with the onset of an outflow (e.g. Fender, R., 2003, astro-ph/0303339) the source may have become detectable as a radio source. More RXTE observations are scheduled to follow the X-ray behavior of the source.