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=17:48:05.196Â± 0.015 DEC=-24:46:47.40 Â± 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.
|Publication date||10 Oct 2010|
|Publication status||Published - 10 Oct 2010|
|Series||The Astronomer's telegram|
- Gamma Ray
- Neutron Star