Near-Infrared and X-Ray Observations of XSS J12270-4859

TL;DR

This study combines near-infrared and X-ray observations to analyze the enigmatic source XSS J12270-4859, revealing simultaneous flares and spectral characteristics that suggest it is a low-luminosity microquasar with a synchrotron jet.

Contribution

First simultaneous NIR and X-ray flare detection in XSS J12270-4859, providing new insights into its nature and possible microquasar characteristics.

Findings

Detected simultaneous NIR and X-ray flares for the first time.

No significant NIR polarization was observed.

Spectral energy distribution suggests a low-luminosity microquasar with a jet.

Abstract

XSS J12270-4859 (J12270) is an enigmatic source of unknown nature. Previous studies revealed that the source has unusual X-ray temporal characteristics, including repetitive short-term flares followed by spectral hardening, non-periodic dips, and dichotomy in activity; i.e. intervals filled with flares and those without. Together with a power-law X-ray spectrum, it is suggested to be a low-mass X-ray binary (LMXB). In order to better understand the object, we present the results of our near-infrared (NIR) photometry and linear polarimetry observations as well as X-ray spectroscopy observations, which overlap with each other partially in time, taken respectively with the InfraRed Survey Facility (IRSF) and the Rossi X-ray Timing Explorer (RXTE). We detected several simultaneous NIR and X-ray flares for the first time. No significant NIR polarization was obtained. We assembled data taken with IRSF, RXTE, Suzaku, Swift, and other missions in the literature and compared the flare profile and the spectral energy distribution (SED) with some representative high-energy sources. Based on some similarities of the repetitive NIR and X-ray flaring characteristics and the broad SED, we argue that J12270 is reminiscent of microquasars with a synchrotron jet, which is at a very low luminosity state of ~1e-4 Eddington luminosity for a stellar mass black hole or neutron star at a reference distance of 1 kpc.