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: SÃ¸ren Brandt (email@example.com)
Subjects: X-ray, Gamma Ray, Binary, Neutron Star, Transient, Pulsar
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.