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XTE and Swift observations of XTE J1652-453: in transition to the hard state ?

ATel #2219; M. Coriat (Univ. Paris Diderot & CEA Saclay), J. Rodriguez (CEA Saclay), S. Corbel (Univ. Paris Diderot & CEA Saclay), J. A. Tomsick (UC Berkeley/SSL)
on 30 Sep 2009; 14:42 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: S. CORBEL (corbel@discovery.saclay.cea.fr)

Subjects: X-ray, Request for Observations, Binary, Black Hole, Transient

The X-ray transient XTE J1652-453 was discovered by RXTE on June 28 (Atel #2107, #2120, #2125, #2135) when it went into outburst. Around 2009 September 10, the BAT monitor aboard Swift showed an increase of the hard X-ray (15-50 keV) count rate from XTE J1652-453 by a factor 1.5 - 2 within a couple of days, that may be indicative of a transition back to the hard state.

We analyzed RXTE observations of XTE J1652-453 (from 2009 Sept 14 to Sept 20). The first combined PCA and HEXTE spectra on Sept. 14 are consistent with a disk black body (kT= 0.6 +/- 0.15) and a power law (photon index 1.8 +/-0.1) with a line feature around 6 keV (eq. width ~1 keV). The unabsorbed 2-10 keV flux is 7.4 e-10 erg/cm^2/s and about 72 % of the flux is in the power law component. The next 3 observations do not require (according to the F-test) the presence of a disk component and the power law photon index is softer (2.12 +/- 0.03 to 2.14 +/-0.04). The 2-10 keV flux decreases from 7.4 to 5.5 e-10 erg/cm^2/s. For all observations we fixed the NH to the value of 5e22 cm-2 as reported from a previous Swift observation (Atel #2120). We also studied the timing properties of the source. On Sept. 14, the 0.1-10 Hz power density spectrum is well represented by a zero-centered Lorentzian, with an amplitude of 8.5 (-2.5 +3)% . Only the RXTE observation of Sept 14 is useful in this respect since the source count rate decreases quickly over this period, and therefore so does the significance of any variability with respect to the white noise level.

On Sept. 23 we triggered a Swift ToO observation to better constrain the soft X-ray evolution. As significant pile up affects the detector, we extracted the spectrum from an annulus region (with an outer radius of 40 pixels, and excluding the inner 10 pixels). The spectrum is well described by a simple absorbed power law with an hydrogen column density of (4.9 +/- 0.8)10^22 cm-2 and a photon index of 1.97 +/- 0.3 consistent with our previous results. The 2-10 keV unabsorbed flux is 5.84e-10 ergs/cm^2/s. The spectral evolution is consistent with XTE J1652-453 going through a transition to the hard state, although our spectral results indicate a rather soft photon index compared to other black hole binaries.

Black hole X-ray transients typically become bright at radio and infrared wavelengths in the hard state, and multi-wavelength observations are strongly encouraged to have a better understanding of the accretion-jet connection in late stages of black hole outburst.

We thank the Swift PI, Neil Gehrels, the Swift science team and the Swift mission operations team for their support of these observations.