Renewed activity of the Galactic center transients Swift J174535.5-290135.6 and GRS 1741.9-2853 as observed with Swift/XRT

ATel #1006; R. Wijnands, M. Klein Wolt (UvA, The Netherlands), E. Kuulkers ESA/ESAC, Spain), S. Shaw (Southampton, UK), J. Chenevez, S. Brandt (DNSC, Denmark), T. Courvoisier (ISDC, Switzerland), A. Domingo (LAEFF/INTA, Spain), P. Kretschmar (ESA/ESAC, Spain), C. Markwardt (GSFC, USA), N. Mowlavi (ISDC, Switzerland), A. Paizis (INAF-IASF, Italy), D. Risquez (LAEFF/INTA, Spain), C. Sanchez-Fernandez (ESA/ESAC, Spain)
on 17 Feb 2007; 15:04 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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Following the detection of a new Galactic center transient in our new series of INTEGRAL Galactic bulge monitoring observations (ATEL #1005), we obtained a short pointing with the XRT aboard Swift to improve significantly on the source position of this transient. This observation was performed on 16-17 Feb 2007 from 21:39 to 00:37 UT for a total on-source time of nearly 4 ksec.

The XRT data shows a clear bright transient in the INTEGRAL JEM-X error circle (ATEL #1005) at a position (J2000) of R.A. = 17h 45m 35.4s and Dec. = -29d 01m 38s with an error radius of 3.6 arcseconds. This position is consistent with that of Swift J174535.5-290135.6 (ATEL #753) but not consistent with that of CXOGC J174535.5-290124 strongly indicating that it is Swift J174535.5-290135.6 which is active again. The XRT source spectrum could be fitted by a strongly absorbed power-law with a photon index of ~2.1 and a column density Nh of ~1.7E23 atoms/cm^2. The averaged absorbed and unabsorbed 2-10 keV flux was 6E-11 and 1.4E-10 erg/s/cm^2, respectively.

In addition to Swift J174535.5-290135.6, we detected another bright transient at R.A. = 17h 45m 02.0s and Dec.= -28d 54m 54s (J2000; error radius of 3.6 arcsecond). This position is ~6 arcseconds away from the neutron-star X-ray transient GRS 1741.9-2853 (Muno et al. 2003, ApJ, 598, 474). Although GRS 1741.9-2853 is located outside the XRT error circle, the source is located close to the edge of the FOV which likely increases the systematic error on the source position. Furthermore, we detected a type-I X-ray burst from the source consistent with the bursting behavior observed from GRS 1741.9-2853. Therefore, we think it is likely that we have detected another outburst of GRS 1741.9-2853. The source spectrum could be fitted with an absorbed power-law model with Nh = 9.5+/-2.5 E22 atom/cm^2 and an index of 1.4+/-0.5. The averaged absorbed and unabsorbed 2-10 keV flux was 9.1e-11 erg/s/cm^2 and 1.5e-10 erg/s/cm^2, respectively. We note that this transient is located in the error circle of the X-ray burst detected with Swift/BAT on 22 January 2007 (BAT trigger 257213; GCN #6020) and likely this transient was the origin of this burst (moreover, no other source was detected in the BAT error circle). This would suggest that the source has already been active for over 4 weeks.