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Further observations of GRO J1750-27 (AX J1749.1-2639) with INTEGRAL

ATel #1400; S. Brandt (DNSC, Denmark), S. Shaw (Southampton, UK/ ISDC, Switzerland), A. Hill (Southampton, UK), P. Kretschmar (ESA/ESAC, Spain), R. Wijnands (UvA, The Netherlands), T. Oosterbroek (ESA/ESTEC, The Netherlands), E. Kuulkers (ESA/ESAC, Spain), V. Beckmann (ISDC, Switzerland), S. Soldi (ISDC, Switzerland), J. Chenevez (DNSC, Denmark), T. J.-L. Courvoisier (ISDC, Switzerland), A. Domingo (LAEFF/INTA, Spain), K. Ebisawa (ISAS, Japan), P. Jonker (SRON, The Netherlands), C. Markwardt (GSFC, USA), A. Paizis (INAF-IASF, Italy), D. Risquez (LAEFF/INTA, Spain), C. Sanchez-Fernandez (ESA/ESAC, Spain)
on 28 Feb 2008; 15:22 UT
Credential Certification: Søren Brandt (sb@spacecenter.dk)

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

Referred to by ATel #: 1401, 7096

The transient accreting X-ray pulsar GRO J1750-27 (AX J1749.1-2639), which became active end of January 2008 (ATel #1376), has been repeatedly observed by the INTEGRAL Galactic Bulge monitoring program since mid February (ATel #1385) on 11, 20 and 23 Feb. 2008.

During the three observations, totalling about 27 ks, the source has remained bright at a roughly constant level with the following flux averaged over the 3 observations: 186+/-9 mCrab (3-10 keV), 319+/-15 mCrab (10-25 keV), 198+/-2 mCrab (18-40 keV), and 20+/-3 mCrab (40-100 keV).

We observe a continuing spin-up with barycentric periods measured with the JEM-X and ISGRI instruments from 4.4547+/-0.0001 s (MJD=54507.7; see also ATel #1385), to 4.4527+/-0.0001 s (MJD=54516.7), and to 4.4518+/-0.0001 s (MJD=54519.7). Applying the orbital parameters of Scott et al. (ApJ 488, 831, 1997) from the 1995 outburst, we estimate the intrinsic spin-up to be 60+/-20 pico-Hz/s, where about 11-15 pHz/s of the error is due to the uncertainties in the 29.817 day orbital period. We note that the spin-up is likely to be higher than the 38 pHz/s observed during the 1995 outburst.

The pulse profile clearly shows two broad peaks, while Scott et al. reported a single broad peak in the BATSE data. This may be explained by our broader spectral coverage at lower energies and by better statistics. The peaks differ in their spectral shape: while they are of similar strength below 5 keV, one peak begins to dominate with increasing energy.

The pulsed fraction is estimated to about 30% in the ISGRI 18-40 keV band and about 15% in the JEM-X 3-10 keV band.

The INTEGRAL Galactic Bulge monitoring program resumes observing the source with 3 day intervals starting on Feb. 29, 2008.