RXTE/PCA and Swift/XRT observations of GRO J1655-40 during decay

ATel #644; Jeroen Homan (MIT), Albert Kong (MIT), John Tomsick (UCSD), Jon Miller (U. Mich), Sergio Campana (INAF/OAB), Rudy Wijnands (U. Amsterdam), Tomaso Belloni (INAF/OAB), Walter Lewin (MIT)
on 27 Oct 2005; 22:45 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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Following its transition to the hard state (ATels #607,#612), we have continued our daily RXTE/PCA observations of the black hole X-ray transient GRO J1655-40 (see http://tahti.mit.edu/opensource/1655 ). Between September 23, when the source reached the hard state, and October 10, the RXTE/PCA count rate decreased exponentially, with an e-folding time of ~7 days. After October 10 the decrease started to slow down and data from the last few days suggest that the count rate may have reached a constant level. We suspect that, given the small distance of GRO J1655-40 to the Galactic plane, this departure from an exponential decay is likely the result of contamination by the diffuse emission from the Galactic ridge. Such contamination could possibly also explain the softening of the spectrum (as observed with RXTE/PCA) to power-law indices much higher than usually seen in the hard state of black hole X-ray binaries.

With the aim to investigate this possible contamination, we observed GRO J1655-40 with Swift/XRT in 'Photon Counting' mode on October 22 (once) and 24 (twice), for 4.2 ks, 5 ks, and 7.6 ks, respectively. We extracted spectra for all three observations and fitted them simultaneously. The spectra are well fitted by a single power law (reduced Chi^2 = 0.9, for 69 dof). We find an average Nh of 1.3(2)e21 atoms/cm^2 and an average power-law index of 1.60+/-0.16. This index is similar to what was measured with RXTE/PCA at the start of the hard state. The unabsorbed 0.5-10 keV fluxes for the three Swift/XRT observations were:

Oct 22 00:27 UT : 8.4 +/- 1.1 x 10^-12 erg/cm^2/s
Oct 24 04:59 UT : 7.2 +/- 0.8 x 10^-12 erg/cm^2/s
Oct 24 21:01 UT : 5.8 +/- 0.6 x 10^-12 erg/cm^2/s

RXTE/PCA spectra from the same period (Oct. 22-24) cannot be fitted well with a single power law spectrum. With the Nh fixed to the value obtained from the Swift/XRT spectra we obtain a best fit (reduced Chi^2 = 2.0) with a power-law index of 2.4, which resulted in complex residuals below 5 keV. Moreover, the unabsorbed 0.5-10 keV flux from this fit is 6.2(6) x 10^-11 erg/cm^2/s, almost an order of magnitude higher than measured with Swift/XRT. We therefore conclude that the RXTE/PCA spectrum is dominated by emission other than that of GRO J16550-40, with the Galactic ridge being the obvious candidate. This effectively ends our RXTE/PCA monitoring campaign of the outburst of GRO J1655-40. Correcting the RXTE/PCA light curves for the additional background component, we find that the exponential decay (as observed between Sep 23 and Oct 10) continues well beyond Oct 10. The current Swift/XRT flux is still well above the quiescent flux of 2.6 x 10^-14 erg/cm^2/s as seen with Chandra (Kong et al. 2002, ApJ, 570, 277).

Finally, we note that the e-folding time of ~7 days in the hard state is approximately twice as short as the e-folding time we found for the soft state part of the decay (14.9 days). Changes in the decay rate are expected when a system changes from an accretion disk dominated regime to an ADAF or jet dominated regime, since these regimes have a different dependence of the X-ray luminosity on the mass accretion rate. The factor of ~2.1 change in the e-folding time is close to what is predicted by some models. Although it is not clear whether the observed change in the e-folding time can be used to distinguish between ADAF and jet models for the hard state, these measurements can potentially be very valuable for constraining the relative radiative efficiencies of the hard and soft states.