Broadband X‐Ray Spectra of the Black Hole Candidate GRO J1655−40

Abstract

We present broad-band (2 keV to 2 MeV) x-ray spectra of GRO J1655-40, a luminous X-ray transient and occasional source of relativistic radio jets, obtained with the Rossi X-Ray Timing Explorer (RXTE) and the Oriented Scintillation Spectrometer Experiment (OSSE). In one observation, the luminosity is found to be 18% of the Eddington limit, which is one of the highest luminosities ever observed from GRO J1655-40. For this observation, we find that an adequate fit is obtained when a broad iron line and a reflection component are added to a model consisting of a power-law plus a soft excess component. The 95% confidence lower limit on the rms line width is 0.86 keV. The power-law component has a photon index of 2.72 +0.03 −0.08 and extends to at least 800 keV without a cutoff.After this observation, a significant drop in the (5-12 keV)/(1.5-5 keV) hardness ratio occured on a timescale less than 2 hours. From an RXTE observation of GRO J1655-40 made after the hardness transition, we find that the power-law index is harder (α = 2.415 ± 0.011), the flux of the power-law component is lower, and the total luminosity is 10% of the Eddington limit. The change in the power-law component is consistent with the correlation between the spectral index and power-law flux previously reported for GRO J1655-40.

Discovery of X‐Ray Burst Oscillations from a Neutron Star X‐Ray Transient in the Globular Cluster NGC 6440

“…The fluxes reported from the RXTE PCA with PCUs 0, 1, 2, and 3 operating (which was the case during bursts 1 and 7) are systematically a factor of 1.2 higher than from other instruments. Assuming that this factor represents an error in the absolute calibration of the PCA (Tomsick et al 1999), we reduce the flux by this factor and find an average peak luminosity for these two bursts of ð3:6 AE 0:4Þ Â 10 38 ergs s À1 , which is consistent with the Eddington luminosity for a 1:4 M and 10 km radius neutron star with hydrogen-poor matter (Kuulkers et al 2003). The other bursts have lower peak fluxes.…”

X-Ray Properties of Black-Hole Binaries

“…During this period, Gamma-ray observations (∼40-500 keV) of seven BHBs brought clarity to the distinction between the soft and hard types of X-ray PL components (Grove et al 1998;Tomsick et al 1999). Sources in the low/hard state (Γ ∼ 1.7) were found to suffer an exponential cutoff near 100 keV, whereas sources with soft X-ray spectra (Γ ∼ 2.5) maintained a steep, strong and unbroken PL component out to the sensitivity limit of the gamma-ray detectors (∼ 1 MeV).…”

Photospheric radius expansion X-ray bursts as standard candles

“…Tomsick et al 1999;Wilms et al 1999;Kuiper et al 1999;Barret et al 2000). Although the calibration of the RXTE/PCA instrument has been improved, the absolute calibration has still not been optimized (see e.g.…”