Uncovering the Nature of the X-ray Transient 4U 1730-22: Discovery of X-ray Emission from a Neutron Star in Quiescence with Chandra

Abstract

The X-ray transient, 4U 1730-22, has not been detected in outburst since 1972, when a single outburst was detected by the Uhuru satellite. This neutron star or black hole X-ray binary is presumably in quiescence now, and here, we report on X-ray and optical observations of the 4U 1730-22 field designed to identify the system's quiescent counterpart. Using Chandra, we have found a very likely counterpart. The candidate counterpart is close to the center of the Uhuru error region and has a thermal spectrum. The 0.3-8 keV spectrum is well-described by a neutron star atmosphere model with an effective temperature of 131+/-21 eV. For a neutron star with a 10 km radius, the implied source distance is 10(+12)(-4) kpc, and the X-ray luminosity is 1.9E33 ergs/s assuming a distance of 10 kpc. Accretion from a companion star is likely required to maintain the temperature of this neutron star, which would imply that it is an X-ray binary and therefore, almost certainly the 4U 1730-22 counterpart. We do not detect an optical source at the position of the Chandra source down to R > 22.1, and this is consistent with the system being a Low-Mass X-ray Binary at a distance greater than a few kpc. If our identification is correct, 4U 1730-22 is one of the 5 most luminous of the 20 neutron star transients that have quiescent X-ray luminosity measurements.

Accepted by the Astrophysical Journal

References in corpus (2)

• X-ray spectral properties of AGN in the Chandra Deep Field South
• The neutron star soft X-ray transient 1H1905+000 in quiescence

Cited by in corpus (12)

• Further Constraints on Thermal Quiescent X-ray Emission from SAX J1808.4-3658
• Cooling of Accretion-Heated Neutron Stars
• Statistical theory of thermal evolution of neutron stars
• Neutron Star Radius Measurement with the Quiescent Low-Mass X-ray Binary U24 in NGC 6397
• Thermal evolution and quiescent emission of transiently accreting neutron stars
• X-ray Spectral Identification of Three Candidate Quiescent Low-Mass X-ray Binaries in the Globular Cluster NGC 6304
• Type I X-ray bursts' spectra and fuel composition from the atoll and transient source 4U 1730$-$22
• Discovery of a 584.65 Hz Burst Oscillation in the Low Mass X-Ray Binary 4U 1730--22
• Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER
• The thermonuclear X-ray bursts of 4U 1730-22
• Return of 4U~1730--22 after 49 years silence: the spectral properties of the 2021/2022 outbursts observed by NICER and Insight-HXMT and the soft-to-hard state transition caused by the propeller effect
• Return of 4U~1730--22 after 49 years silence: the peculiar burst properties of the 2021/2022 outbursts observed by Insight-HXMT