Renewed activity from the X-ray transient SAXJ 1810.8-2609 with INTEGRAL

TL;DR

This paper reports on INTEGRAL observations of the neutron star low mass X-ray binary SAX J1810.8-2609 during its first active phase since 1998, revealing low luminosity, spectral states, and burst characteristics that classify it as a faint soft X-ray transient.

Contribution

The study provides detailed spectral analysis and burst observations of SAX J1810.8-2609 during its 2007 outburst, offering new insights into its accretion state and burst composition.

Findings

Luminosity during outburst: 1.1-2.6x10^36 erg s^-1 in 20-100 keV

Spectral state: low hard state with thermal Comptonization

Burst recurrence time: ~1.2 days, with mixed hydrogen and helium fuel

Abstract

We report on the results of INTEGRAL observations of the neutron star low mass X-ray binary SAX J1810.8-2609 during its latest active phase in August 2007. The current outburst is the first one since 1998 and the derived luminosity is 1.1-2.6x10^36 erg s-1 in the 20-100 keV energy range. This low outburst luminosity and the long-term time-average accretion rate of ~5x10^-12Msolar/yr suggest that SAXJ 1810.8-2609 is a faint soft X-ray transient. During the flux increase, spectra are consistent with a thermal Comptonization model with a temperature plasma of ~23-30 keV and an optical depth of ~1.2-1.5, independent from luminosity of the system. This is a typical low hard spectral state for which the X-ray emission is attributed to the upscattering of soft seed photons by a hot, optically thin electron plasma. During the decay, spectra have a different shape, the high energy tail being compatible with a single power law. This confirm similar behavior observed by BeppoSAX during the previous outburst, with absence of visible cutoff in the hard X-ray spectrum. INTEGRAL/JEM-X instrument observed four X-ray bursts in Fall 2007. The first one has the highest peak flux (~3.5Crab in 3--25 keV) giving an upper limit to the distance of the source of about 5.7 kpc, for a LEdd~3.8x10^38 erg s^-1. The observed recurrence time of ~1.2 days and the ratio of the total energy emitted in the persistent flux to that emitted in the bursts (~73) allow us to conclude that the burst fuel was composed by mixed hydrogen and helium with X>0.4.