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

¹ INAF – Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy
e-mail: demartino@oacn.inaf.it
² INAF – Osservatorio Astronomico di Brera, via E Bianchi 46, 23807 Merate (LC), Italy
e-mail: tomaso.belloni@brera.inaf.it; sara.motta@brera.inaf.it
³ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
e-mail: mfalanga@issibern.ch
⁴ Institut de Ciéncie de l’Espai (IEEC-CSIC) Campus UAB, Fac. de Ciéncies, Torre C5 08193 Barcelona, Spain
e-mail: papitto@ice.csic.es
⁵ INAF – Osservatorio Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra (CA), Italy
e-mail: apellizz@oa-cagliari.inaf.it
⁶ INAF – Istituto di Astrofisica Spaziale e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: yuri.evangelista@iasf-roma.inaf.it; giovanni.piano@iasf-roma.inaf.it
⁷ INAF – Istituto di di Astrofisica Spaziale, via Gobetti 101, 40129 Bologna, Italy
e-mail: nicola.masetti@iasfbo.inaf.it
⁸ CEA Saclay, DSM/Irfu/Service d’Astrophysique, 91191 Gif-sur-Yvette, France
e-mail: bonnetbidaud]@cea.fr
⁹ Laboratoire APC, Université Denis Diderot, 10 rue Alice Domon et Léonie Duquet, 75005 Paris, France and LUTH, Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, 92195 Meudon, France
e-mail: martine.mouchet@obspm.fr
¹⁰ CRESST and X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA and Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
e-mail: koji.mukai@nasa.gov

Received: 17 September 2012
Accepted: 2 December 2012

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.