IGRJ17361-4441: a possible new accreting X-ray binary in NGC6388

TL;DR

This paper reports multi-wavelength observations of the transient X-ray source IGRJ17361-4441 in NGC6388, suggesting it is likely a new accreting neutron star binary rather than a black hole, with implications for the cluster's black hole mass.

Contribution

The study provides the first detailed X-ray and radio analysis of IGRJ17361-4441, challenging its previous classification as a black hole candidate and proposing it as a neutron star binary.

Findings

No significant radio emission detected, setting upper limits.

Spectral analysis favors an accreting neutron star over a black hole.

Upper limit on intermediate-mass black hole in NGC6388 is <600 solar masses.

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.