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Swift/XRT confirms the INTEGRAL detection of a faint outburst from SAX J1750.8-2900

ATel #3181; L. Natalucci (INAF/IASF-Roma), M. Fiocchi (INAF/IASF-Roma), A. Bazzano (INAF/IASF-Roma), E. Kuulkers (ESA/ESAC Madrid), C. Sanchez (ESA/ESAC Madrid)
on 22 Feb 2011; 14:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Lorenzo Natalucci (Lorenzo.Natalucci@iasf-roma.inaf.it)

Subjects: X-ray, Binary, Neutron Star, Transient

We report on a Swift TOO observation of the neutron star transient SAX J1750.8-2900, detected by INTEGRAL/IBIS on 2011 Feb 10 and Feb 12, during monitoring observations of a sky region centered on RX J1713.7-3946. The reported intensity of ~15 mCrab (ATel#3170) is consistent with a faint outburst. Soon after, the source had declined in a time scale of about one day by at least a factor two, as it was not detected during the Galactic Bulge monitoring observations performed on Feb 13 (ATel#3172).
SAX J1750.8-2900 is found to be bright at a level of 0.94 +/- 0.01 counts/s in the XRT images taken on Feb 19, in four snapshots between 12:10 and 15:22UT for a total exposure time of 3,850s. The events, recorded by XRT in photon counting mode, were extracted adopting the standard grade filtering (0--12) and following the science threads listed on the Swift/XRT website (available at http://heasarc.nasa.gov/docs/swift/analysis/xrt_swguide_v1_2.pdf ). For spectral analysis a circular region of radius 1 arcmin, centered on the source position has been used. The background was taken from source-free regions close to the target.
The 0.3-10 keV spectrum is compatible with an absorbed power law giving N_h=(3.2+/-0.3)x10^22 cm-2 and photon index 2.0+/-0.2. The quoted errors are 90% confidence and the reduced chi-square is 1.02 for 33 degrees of freedom. The absorbed 0.5-10 keV flux is 8.3x10^(-11) erg cm-2 s-1. Assuming a 5kpc distance, the unabsorbed flux of 1.9x10^(-10) erg cm-2 s-1 corresponds to a luminosity of 5.7x10^35 erg/s in the 0.5-10 keV range. We note that currently, upper limits to the source distance have been determined by BeppoSAX and RXTE (Natalucci et al. 1999, ApJ 523, L45; Kaaret et al. 2002, ApJ 575, 1018) that constrain the source to within ~7 kpc.
Our Swift/XRT observation rules out the possibility that the nearby X-ray transient IGR J17507-2856, discovered by IBIS in 2004 during a faint emission episode (ATel #342), could be responsible of the observed hard X-ray activity. The 2-10 keV intensity of SAX J1750.8-2900 on 2011, Feb 19 is 4 mCrab and is then compatible with the latest INTEGRAL results, confirming the scenario of a faint outburst with declining source activity.
We thank the Swift Team for the prompt scheduling of this observation and for providing the quick-look data.