Abstract
IGRJ17361-4441 is a newly discovered INTEGRAL hard X-ray transient, located in the globular cluster NGC6388. We report here the results of the X-ray and radio observations performed with Swift, INTEGRAL, RXTE, and the Australia Telescope Compact Array (ATCA) after the discovery of the source on 2011 August 11. In the X-ray domain, IGRJ17361-4441 showed virtually constant flux and spectral parameters up to 18 days from the onset of the outburst. The broad-band (0.5-100 keV) spectrum of the source could be reasonably well described by using an absorbed power-law component with a high energy cut-off (N_H\simeq0.8x10^(22) cm^(-2), {\Gamma}\simeq0.7-1.0, and E_cut\simeq25 keV) and displayed some evidence of a soft component below \sim2 keV. No coherent timing features were found in the RXTE data. The ATCA observation did not detect significant radio emission from IGRJ17361-4441, and provided the most stringent upper limit (rms 14.1 {\mu}Jy at 5.5 GHz) to date on the presence of any radio source close to the NGC6388 center of gravity. The improved position of IGRJ17361-4441 in outburst determined from a recent target of opportunity observation with Chandra, together with the X-ray flux and radio upper limits measured in the direction of the source, argue against its association with the putative intermediate-mass black hole residing in the globular cluster and with the general hypothesis that the INTEGRAL source is a black hole candidate. IGRJ17361-4441 might be more likely a new X-ray binary hosting an accreting neutron star. The ATCA radio non-detection also permits us to derive an upper limit to the mass of the suspected intermediate massive black hole in NGC6388 of <600 M\odot. This is a factor of 2.5 lower than the limit reported previously.
Highlights
IGR J17361-4441 was discovered by the IBIS/ISGRI telescope (Ubertini et al 2003) onboard INTEGRAL (Winkler et al 2003) on 2011 August 11 (Gibaud et al 2011)
We report here the results of the X-ray and radio observations performed with Swift, INTEGRAL, RXTE, and the Australia Telescope Compact Array (ATCA) after the discovery of the source on 2011 August 11
The improved position of IGR J17361-4441 in outburst determined from a recent target of opportunity observation with Chandra, together with the X-ray flux and radio upper limits measured in the direction of the source, argue against its association with the putative intermediate-mass black hole residing in the globular cluster and with the general hypothesis that the INTEGRAL source is a black hole candidate
Summary
IGR J17361-4441 was discovered by the IBIS/ISGRI telescope (Ubertini et al 2003) onboard INTEGRAL (Winkler et al 2003) on 2011 August 11 (Gibaud et al 2011). The source was located in the direction of the globular cluster (GC) NGC 6388 and detected at an X-ray flux of 9.7 × 10−11 erg cm−2 s−1 (20– 100 keV). Follow-up observations carried out with Swift and Chandra (Ferrigno et al 2011b; Pooley et al 2011) provided a first description of the source X-ray emission in the soft energy band (0.3–10 keV), a refined source position at RA = 17:36:17.418, Dec = –44:44:05.98 (J2000; the nominal Chandra positional accuracy is 0.6 arcsec, 90% c.l.) and confirmed its localization in NGC 6388. We report on the analysis of all the available X-ray data collected during the first 18 days of the outburst observed from IGR J17361-4441, and use a simultaneous observation in the radio domain with the ATCA telescope to investigate the nature of the source
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