The intriguing nature of the high energy gamma ray source XSSJ12270-4859

TL;DR

This study investigates the high-energy gamma-ray source XSSJ12270-4859 using broad-band X-ray, gamma-ray, UV, optical, and near-IR data, revealing its likely nature as a low-mass X-ray binary with significant gamma-ray emission.

Contribution

The paper provides a comprehensive multi-wavelength analysis that clarifies the nature of XSSJ12270-4859 as an atypical low-mass X-ray binary, challenging previous Cataclysmic Variable classification.

Findings

X-ray emission shows flares and dips with specific spectral characteristics.

Gamma-ray emission up to 10 GeV is significant and correlates with X-ray data.

Optical and near-IR variability suggests binary orbital period and non-periodic short-term changes.

Abstract

The nature of the hard X-ray source XSSJ12270-4859 is still unclear though it was claimed to be a magnetic Cataclysmic Variable. We here present a broad-band X-ray and gamma ray study based on a recent XMM-Newton observation and archival INTEGRAL and RXTE data. From the Fermi/LAT 1-year point source catalogue, we tentatively associate XSSJ12270-4859 with 1FGLJ1227.9-4852, a source of high energy gamma rays with emission up to 10GeV. We complement the study with UV photometry from XMM-Newton and ground-based optical and near-IR photometry. The X-ray emission is highly variable showing flares and intensity dips. The X-ray flares consist of flare-dip pairs. Flares are also detected in the UV range but not the dips. Aperiodic dipping behaviour is also observed during X-ray quiescence but not in the UV. The 0.2-100keV spectrum is featureless and described by a power law model with Gamma=1.7. The 100MeV-10GeV spectrum is instead represented by a power law index of 2.45. The luminosity ratio between 0.1-100GeV and 0.2--100keV is ~0.8, hence the GeV emission is a significant component of the total energy output. Furthermore, the X-ray spectrum does not greatly change during flares, quiescence and the dips seen in quiescence but it hardens during the post-flare dips. Optical photometry reveals a period of 4.32hr likely related to the binary orbit. Near-IR, possibly ellipsoidal, variations are detected. Large amplitude variability on shorter (tens mins) timescales are found to be non-periodic. The observed variability at all wavelengths and the spectral characteristics strongly favour a low-mass atypical low-luminosity X-ray binary and are against a Cataclysmic Variable nature. The association with a Fermi/LAT high energy gamma ray source further strengths this interpretation.