A hard X-ray transient in the direction of Terzan 5 detected by INTEGRAL

ATel #2919; P. Bordas (ISDC/IAAT), E. Kuulkers (ESA/ESAC, Spain), J. Alfonso-Garzón (LAEX-CAB/INTA-CSIC, Spain), V. Beckmann (APC, France), T. Bird (Southampton, UK), S. Brandt (DTU Space, Denmark) J. Chenevez (DTU Space, Denmark), T. Courvoisier (ISDC, Switzerland), M. Del Santo (INAF/IASF-Roma, Italy), A. Domingo (LAEX-CAB/INTA-CSIC, Spain), K. Ebisawa (ISAS, Japan), C. Ferrigno (ISDC, Switzerland), P. Jonker (SRON, The Netherlands), P. Kretschmar (ESA/ESAC, Spain), C. Markwardt (GSFC, USA), T. Oosterbroek (ESA/ESTEC, The Netherlands), A. Paizis (INAF-IASF, Italy), K. Pottschmidt (UMBC/NASA GSFC, USA), C. Sánchez-Fernández (ESA/ESAC, Spain), R. Wijnands (UvA, The Netherlands)
on 10 Oct 2010; 23:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)

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During the Galactic bulge monitoring program, performed between 2010 October 10 08:45 and 2010 October 10 12:27, the IBIS/ISGRI instrument onboard INTEGRAL discovered an outburst from a hard X-ray source. The best determined position is at RA=267.01 DEC=-24.78 (J2000, associated error 1.3 arcmin, 90% c.l.), towards the globular cluster Terzan 5. We tentatively identify the source with the neutron star X-ray transient EXO 1745-248 = XB 1745-25, which is located within the INTEGRAL error box (RA=17:48:05.196± 0.015 DEC=-24:46:47.40 ± 0.20, Heinke et al, 2003, ApJ, 590, 809)

The source is detected in the IBIS/ISGRI mosaic at a significance of 20 sigma in the 20-40 keV energy band, and 11 sigma in the 40-100 keV energy band (effective exposure time 8.4 ks). The spectrum is best fit with a power-law of photon index 2.7+/-0.3 (90% c.l) and the estimated flux is 6E-10 erg/cm2/s in the 20-100 keV energy band (i.e., about 10 mCrab). The IBIS/ISGRI lightcurve of the source is compatible with a constant hard X-ray luminosity. The source is not detected by the JEM-X monitor, and we derive a 3 sigma upper limit of 22 mCrab in the 3-20 keV energy band (~5e-10 erg/s/cm2).

The association of the INTEGRAL source with EXO 1745-24 is strengthened by the similarities between the spectra. During the outburst in 2000, Kuulkers et al. (2003, A&A 399, 663) measured a spectrum for the persistent emission of EXO 1745-248 using Beppo-SAX, finding a high energy power-law tail with photon index 2.63+/-0.07 and a normalization of 6.1 +/-0.9 photons/cm^2/s at 1 keV. The photon index of the spectral component extending to the highest energies is thus similar to the spectrum measured by IBIS/ISGRI. EXO 1745-248 is also known to produce X-ray outbursts (Markwardt & Swank 2000, IAUC, 7454; Wijnands et al. 2005, ApJ, 618, 883). However, given the relatively large uncertainty on the INTEGRAL position, the association with other sources in the direction of Terzan 5 can not be ruled out.

Swift and RXTE ToO obseravtions have been requested. Further observations of the Galactic bulge with INTEGRAL are planned for 2010 October 11, and 16.

Galactic Bulge monitoring program web-page