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Swift observations of the continuing outburst of GRO J1750-27

ATel #1401; K. Pottschmidt (CRESST/GSFC/UMBC), W. Baumgartner (CRESST/GSFC/UMBC), J. Wilms (Remeis Observatory, FAU Erlangen-Nuremberg, Germany), C. Graemer (Remeis Observatory, FAU Erlangen-Nuremberg, Germany), I. Kreykenbohm (Remeis Observatory, FAU Erlangen-Nuremberg, Germany), S. D. Barthelmy (GSFC), J. Cummings (CRESST/GSFC/UMBC), H. A. Krimm (CRESST/GSFC/USRA), E. Kuulkers (ESA/ESAC, Spain)
on 28 Feb 2008; 16:24 UT
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Gamma Ray, Binary, Neutron Star, Transient, Pulsar

The current outburst of the accretion powered pulsar GRO J1750-27 (AX J1749.1-2639), first detected by Swift BAT on 2008 January 29 (ATEL#1376) and observed during the INTEGRAL Galactic Bulge monitoring (ATEL#1385, ATEL#1400), is continuing. The BAT light curve shows a stable level of emission at ~250 mCrab in the 15-50 keV band, see http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/AXJ1749.1-2639/. As mentioned in ATEL#1376, this is only the second outburst observed for this source, with the first one detected and monitored over ~60 days with CGRO BATSE in 1995 (Scott et al. 1997, ApJ 488, 831).
A 3.3 ks pointed Swift observation was performed on 2008 February 15. The 1-10 keV XRT spectrum can be best described with an absorbed cutoff power law, with an absorption column of (3.2+/-0.3)x10^22 atoms/cm^2, a photon index of -1.1+/-0.2, and a cutoff energy of 2.7+/-0.3 keV (reduced chi-squared of 1.08 for 633 degrees of freedom). The Galactic absorption of (1-2)x10^22 atoms/cm^2 in this direction (http://heasarc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl) alone is not sufficient to explain the XRT data. No iron K line is detected. The 2-10 keV flux amounts to 2.9x10^-9 ergs/cm^2/s.
The pulse period is clearly detected in this high time-resolution window timing mode observation and is determined to be 4.454+/-0.001 s, consistent with the INTEGRAL (ATEL#1385, ATEL#1400) and CGRO BATSE values (Scott et al.). Based on this period a background corrected XRT pulse profile with 32 phase bins has been constructed. While the previously published 20-70 keV BATSE profile shows a single asymmetric peak, the XRT pulse profile is clearly double peaked, with an asymmetric main peak and a more symmetric secondary peak, independently confirming the INTEGRAL results. The pulsed signal is also seen in BAT up to at least 50 keV.
Further pointed Swift observations, to be coordinated with the upcoming INTEGRAL Galactic Bulge monitoring exposures, have been requested.