RXTE shows spectral transition during decay in H 1743-322

ATel #2410; Nikolai Shaposhnikov (UMD/CRESST/GSFC), John A. Tomsick (SSL/UC Berkeley)
on 29 Jan 2010; 13:58 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Nikolai Shaposhnikov (nikolai.v.shaposhnikov@nasa.gov)

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The latest RXTE/PCA pointing observations on Jan 26, 27 and 28 (observation 1,2 and 3 hereafter) indicate that a galactic black hole candidate H 1743-322, which is currently is in an active (ATEL #2364,2378) but decaying phase, is undergoing a soft-to-hard spectral transition. While the 2-10 keV (absorbed nH=0.9*1022 cm-2) PCA flux has decreased from 1.9e-9 to 1.2e-9 erg/ cm2/s, the 10-30 keV flux has increased from 7.9e-10 to 8.9e-10 erg/cm2/s between observations 1 and 3. The power density spectrum for observation 1 showed little variability, which is characteristic of the high-soft state. However, during observations 2 and 3 clear quasi-periodic oscillations are seen at 4 and 3.1 Hz. In addition, during the third observation, the QPO is accompanied by a band-limited noise component common for the intermediate and low-hard states (i.e. C-type QPO). Notably, the temperature of the thermal component remained unchanged at about 0.95 keV, as measured by the sum of a disk black body and an empirical power law model (diskbb +powerlaw XSPEC model), while its normalization dropped significantly. The photon index of the non-thermal spectral component was 1.82+/-0.02, 1.95+/-0.04 and 1.88+/-0.02 for observations 1,2 and 3 respectively. The source is expected to enter the low hard state on a timescale of several days, with a subsequent fading to quiescence. RXTE will continue monitoring the source evolution. Radio coverage is encouraged as steady radio emission is often observed for low-hard spectral states of galactic black hole candidates.