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Broad band energy spectrum and a low frequency QPO from H1743-322 in the hard state revealed by INTEGRAL and Swift observations

ATel #3280; J. Rodriguez (CEA-SAp/Lab. AIM Saclay, France), M. Cadolle Bel (ESA/ESAC, Villanueva de la Canada, Spain), J. A. Tomsick (UC Berkeley/SSL, USA), D. Hannikainen (Aalto University Metsähovi Radio Observatory, Finland), K. Pottschmidt (CRESST & NASA-GSFC, Greenbelt), E. Kuulkers (ESA/ESAC Villanueva de la Canada, Spain), S. Corbel (U. Paris 7/Lab. AIM Saclay, France), M. Coriat (U. Southampton, UK), A. Goldwurm (CEA-SAp/Lab. APC Paris, France), D. M. Russell (Anton Pannekoek Institute, Amsterdam, Netherlands), J. Wilms (ECAP, Bamberg Germany)
on 13 Apr 2011; 22:11 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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

Referred to by ATel #: 3295, 3301, 3842

Following the report of renewed activity of the microquasar H1743-322 (aka IGR J17464-3213, ATels #3263, #3267) we have triggered a Swift ToO. The observation was performed on April 10, 2011 between 7.9h UT and 16.12h UT, therefore in quasi-simultaneity with the INTEGRAL bulge monitoring program (April 10, 2011 18.0h UT to 21.7h UT). The Swift/XRT was operated in Window Timing mode.

Contrary to the previous INTEGRAL observations (e.g. ATel #3267), a high energy cut-off is required in the fits to the 21-500 keV IBIS/ISGRI spectrum (red. chi^ 2=1.28 for 24 dof). We obtain the following parameters Gamma=1.74 (-0.07/+0.06), Ecut=53 (-12/+11) keV, Efold=224 (-60/+115) keV.

We then fitted the broad band Swift+INTEGRAL spectrum between, respectively, 0.7-8 keV and 21-500 keV. We remark the presence of large residuals in the 0.5-3 keV region that that may be due to systematic uncertainties in the XRT calibration (see e.g. http://heasarc.nasa.gov/docs/swift/proposals/swift_responses.html ), but we cannot rule out the possibility of a soft thermal component from the disk. To account for these we added a Gaussian at around 2.8 keV whose parameters were left free to vary.
The best fit model (red. chi^2=1.30 for 79 dof) to the broad band spectrum consists of an absorbed powerlaw with an high energy cut-off and a reflection component. Note that to limit the number of free parameters, and since these were poorly constrained otherwise, the cut off and folding energies were frozen to the value obtained from to fit the single IBIS spectrum. The parameters we obtain are Nh=1.86(-0.09/+0.16)e22 cm-2, Gamma=1.63 (-0.05/+0.07), and a reflection scaling factor of 0.6 (-0.5/+0.6).
The 2-10 keV flux (IBIS normalization) is 1.3 e-9 erg/s/cm2 and the 20-100 keV one 3.4e-9 erg/cm2/s

The 0.01-30 Hz source power density spectrum obtained from an 1.8 ms time resolution XRT light curve shows the presence of a QPO on top of a zero centered Lorentzian (red. chi^2 1.05 for 129 dof). The zero centered feature has a power of 18.3 (+-1.7)% RMS.
The QPO has a centroid frequency F=0.38 (-0.03 +0.02) Hz a coherence Q(=Freq/FWHM)=4.6 and a power A=12 (-3 +2)%. This level of variability and the presence of the QPO are compatible with the results of an RXTE observation performed two days later (ATel #3277).
Another possible feature is also visible in the PDS. The addition of a Lorentzian slightly improves the fit to red. chi^2=1.01 (for 126 dof) with an F-test value of 0.06, indicating a low significance of this feature. This potential peak has a frequency 0.70 (+-0.05) Hz marginally compatible with being harmonically related to the first QPO, and a power A=8 (+- 3)%.
We note that (type C) QPOs at similar frequency have previously been reported during the 2003 initial hard state of this source (Homan et al. 2005, ApJ, 623, 383).

These results are typical of a black hole in a hard state. We note that the Swift/BAT light curve (http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ17464-3213/) shows that H1743-322 has almost reached the maximum of the hard X-ray flux seen during the previous outbursts. The source may therefore soon undergo a spectral transition to a softer state.

Following the mention of a potential contaminating source in the RXTE fov, we inspected the latest INTEGRAL IBIS and JEMX images. No active source is found within ~2 degrees from H1743-322.

We acknowledge the use of public data from the Swift data archive, and warmly thank the swift Project Scientist, and planners for having accepted and scheduled the Swift observation reported here.