Swift XRT observations of SWIFT J1626.6-5156
ATel #688; S. Campana (INAF-OAB), T. Belloni (INAF-OAB), J. Homan (MIT), J. Kennea (PSU), C. B. Markwardt (U. Maryland & NASA/GSFC), N. Gehrels (NASA/GSFC)
on 13 Jan 2006; 21:17 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)
Subjects: X-ray, Binary, Transient, Pulsar
We observed the transient pulsar SWIFT J1626.6-5156 (Palmer et al., ATEL #678, Markwardt & Swank, ATEL #679) with Swift on Jan 12, 2006. The source has been observed about 900 s in Window Timing (WT) mode and 900 s in Photon Counting (PC) mode, allowing for a precise determination of the source position. SWIFT J1626.6-5156 is located at RA(J2000): 16:26:36.24, DEC(J2000): -51:56:33.5 with a 90% error radius of 3.5 arcsec. Within the error box there is just one object (R=16.2, object A) within the USNO-B catalog (and other two within 4 arcsec: object B with R=15.5 and object C with R=15.8). In the 2MASS catalog there is just one object within 4 arcsec consistent with object C (J=13.4, H=12.9 and K=12.9).
The WT count rate is 9.0+/-0.1 counts s^{-1}. The PC count rate is much lower 2.6+/-0.1 counts s^{-1} due to pile-up. The WT and PC spectrum are well fit with an absorbed power law model. The column density is (9.4+/-1.0)x10^{21} cm^{-2} (errors at 90% level). Given the lower energy band coverage of XRT (0.3-10 keV) with respect to RXTE this determination of the column density should be more reliable. This value is in line with the Galactic column density in the direction of SWIFT J1626.6-5156
(1.0x10^{22} cm^{-2}). The power law photon index is Gamma=0.67+/-0.06. No iron line is present in
the data. This is only mildly consistent with previous RXTE results based on 19-31 December 2005 observations (Belloni et al., ATEL 687). The 0.5-10 keV unabsorbed flux is (1.2+/-0.1) x 10^{-9} erg cm^{-2} s^{-1}.
Strong pulsations are still observed at the 15.37+/-0.02 s. The pulsed (semi-)amplitude is very large of about 40%. No flares were detected.