The X-Ray Point-Source Population of NGC 1365: The Puzzle of Two Highly-Variable Ultraluminous X-ray Sources

TL;DR

This study investigates 26 X-ray point sources in NGC 1365, focusing on two highly-variable ultraluminous X-ray sources, suggesting they may involve intermediate or super-Eddington accretion onto black holes larger than typical stellar remnants.

Contribution

The paper provides detailed analysis of two extreme ULXs in NGC 1365, proposing they involve intermediate mass black holes and super-Eddington accretion, expanding understanding of ULX diversity.

Findings

NGC 1365 X1 is highly variable, with luminosity up to 3-5 x 10^40 erg/s.

NGC 1365 X2 is a newly discovered transient ULX with variability >90.

Estimated black hole masses are 40-60 Msolar for X1 and 80-500 Msolar for X2.

Abstract

We present 26 point-sources discovered with Chandra within 200" (~20kpc) of the center of the barred supergiant galaxy NGC 1365. The majority of these sources are high-mass X-ray binaries, containing a neutron star or a black hole accreting from a luminous companion at a sub-Eddington rate. Using repeat Chandra and XMM-Newton as well as optical observations, we discuss in detail the natures of two highly-variable ultraluminous X-ray sources (ULXs): NGC 1365 X1, one of the most luminous ULXs known since the ROSAT era, which is X-ray variable by a factor of 30, and NGC 1365 X2, a newly discovered transient ULX, variable by a factor of >90. Their maximum X-ray luminosities (3-5 x 10^40 erg/s, measured with Chandra) and multiwavelength properties suggest the presence of more exotic objects and accretion modes: accretion onto intermediate mass black holes (IMBHs) and beamed/super-Eddington accretion onto solar-mass compact remnants. We argue that these two sources have black-hole masses higher than those of the typical primaries found in X-ray binaries in our Galaxy (which have masses of <20 Msolar), with a likely black-hole mass of 40-60 Msolar in the case of NGC 1365 X1 with a beamed/super-Eddington accretion mode, and a possible IMBH in the case of NGC 1365 X2 with M=80-500Msolar.