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)
Subjects: X-ray, Binary, Black Hole, Transient
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.