X-ray follow-ups of XSS J12270-4859: a low-mass X-ray binary with gamma-ray Fermi-LAT association

Abstract

Context. XSS J1227.0-4859 is a peculiar, hard X-ray source recently positionally associated to the Fermi-LAT source 1FGL J1227.9-4852/2FGL J1227.7-4853. Multi-wavelength observations have added information on this source, indicating a low-luminosity low-mass X-ray binary (LMXB), but its nature is still unclear.
Aims: To progress in our understanding, we present new X-ray data from a monitoring campaign performed in 2011 with the XMM-Newton, RXTE, and Swift satellites and combine them with new gamma-ray data from the Fermi and AGILE satellites. We complement the study with simultaneous near-UV photometry from XMM-Newton and with previous UV/optical and near-IR data.
Methods: We analysed the temporal characteristics in the X-rays, near-UV, and gamma rays and studied the broad-band spectral energy distribution from radio to gamma rays.
Results: The X-ray history of XSS J1227 over 7 yr shows a persistent and rather stable low-luminosity (6 × 10³³ d_{1 kpc}² erg s^-1) source, with flares and dips being peculiar and permanent characteristics. The associated Fermi-LAT source 2FGL J1227.7-4853 is also stable over an overlapping period of 4.7 yr. Searches for X-ray fast pulsations down to msec give upper limits to pulse fractional amplitudes of 15-25% that do not rule out a fast spinning pulsar. The combined UV/optical/near-IR spectrum reveals a hot component at ~13 kK and a cool one at ~4.6 kK. The latter would suggest a late-type K2-K5 companion star, a distance range of 1.4-3.6 kpc, and an orbital period of 7-9 h. A near-UV variability (≳6 h) also suggests a longer orbital period than previously estimated.
Conclusions: The analysis shows that the X-ray and UV/optical/near-IR emissions are more compatible with an accretion-powered compact object than with a rotational powered pulsar. The X-ray to UV bolometric luminosity ratio could be consistent with a binary hosting a neutron star, but the uncertainties in the radio data may also allow an LMXB black hole with a compact jet. In this case, it would be the first associated with a high-energy gamma-ray source.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, with Swift, a NASA science mission with Italian participation, with Rossi-XTE, a NASA science mission, and with Fermi a NASA mission with contributions from France, Germany, Italy, Japan, Sweden, and USA, and with AGILE, an Italian Space Agency mission with participation of the Italian Institute of Astrophysics and the Italian Institute of Nuclear Physics.