A new Ultraluminous X-ray source in the galaxy NGC 5907

TL;DR

A newly discovered transient ULX in NGC 5907 reached a luminosity of 6.4x10^{39} erg/s, showing rapid onset and spectral characteristics consistent with a black hole or neutron star accretor, with no pulsations detected.

Contribution

This paper reports the discovery and detailed analysis of a new transient ULX in NGC 5907, including its luminosity evolution and spectral properties.

Findings

Transient ULX reached peak luminosity of 6.4x10^{39} erg/s.

The source's spectrum is consistent with a multicolour blackbody disc.

No pulsations detected, but neutron star origin cannot be ruled out.

Abstract

We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4x10^{39} erg/s. The source was undetected in previous 2012 Chandra observations with a 3 sigma upper limit on the luminosity of 1.5x10^{38} erg/s, implying a flux increase of a factor of >35. We analyzed three recent 60ks/50ks Chandra and 50ks XMM-Newton observations, as well as all the available Swift observations performed between August 2017/March 2018. Until the first half of October 2017, Swift observations do not show any emission from the source. The transient entered the ULX regime in less than two weeks and its outburst was still on-going at the end of February 2018. The 0.3-10 keV spectrum is consistent with a single multicolour blackbody disc (kT~1.5 keV). The source might be a ~30 solar mass black hole accreting at the Eddington limit. However, although we did not find evidence of pulsations, we cannot rule-out the possibility that this ULX hosts an accreting neutron star.