Swift detects an X-ray burst and renewed activity from KS 1741-293

ATel #3632; M. Linares (MIT), D. Chakrabarty (MIT), J. R. Cummings (GSFC/UMBC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (NASA/GSFC)
on 6 Sep 2011; 18:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Manuel Linares (linares@mit.edu)

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An X-ray burst from the direction of the neutron star (NS) transient and thermonuclear burster KS 1741-293 triggered the Swift-BAT on 2011-09-01 at 12:07:22 UTC. The burst was detected only in the 15-25 keV band, lasted for about 10s, had a peak flux of (9.0+/-2.5)e-9 ergs/cm^2/s and an integrated fluence of (5.5+/-1.2)e-8 erg/cm^2 (both in the 15-150 keV band). The average BAT spectrum can be fitted with a 2.2+/-0.6 keV blackbody model. These characteristics are all typical of thermonuclear X-ray bursts from accreting neutron stars, which have triggered in several occasions the BAT onboard Swift (e.g., Linares et al. 2009, MNRAS, 392L 11; Degenaar et al. 2011, MNRAS, 414L 104).

Pointed imaging Swift-XRT observations began at 2011-09-01 15:03:48 UTC for an approximate total good time of 1 ksec. We clearly detect one source with a total net rate of (2.9+/-0.6)e-2 c/s, at a position (4.3 arcsec error circle) consistent with the proposed Chandra counterpart of KS 1741-293: CXOGC J174451.6-292042 (ATel #1531; Muno et al. 2009, ApJS, 181 110). The XRT position that we find is not consistent with none of two previously proposed arcsec positions for KS 1741-293 (0.6 arcmin from the counterpart in Marti et al. 2007, A&A, 462 1065; and 1.4 arcmin from that in Bird et al. 2010, ApJS, 186 1). Using the observed XRT count rate and assuming the spectral shape measured during the 2008 outburst (ATel #1541; absorbing column density of 2e23 cm^-2 and a power law with photon index 2.3) we estimate a persistent 0.5-10 keV luminosity of ~2.2e35*[d/8kpc]**2 erg/s.

We do not detect any source in the Swift-BAT X-ray burst 1-sigma error circle (1 arcmin radius), corresponding to a 90% upper limit on the 0.5-10 keV luminosity of L<3.6e34 erg/s (at 8 kpc and assuming the spectral parameters above). However, the BAT 99% c.l. error circle (3.2 arcmin radius) includes both the KS 1741-293 XRT (3 arcmin away) and Chandra (3.1 arcmin away from CXOGC J174451.6-292042; Muno et al. 2009) positions.

Our results support the association of KS 1741-293 with CXOGC J174451.6-292042 (ATel #1531) and show that the source is again in an active phase after its last detection in March 2010 (ATel #2465), with a luminosity of about 0.1% of the Eddington limit. Thermonuclear bursts at such low inferred mass accretion rates are rare, and may probe the sedimentation of heavy elements in the envelope of accreting NSs (e.g., Peng et al. 2007, ApJ, 654 1022).