The peculiar source XSS J12270-4859: a LMXB detected by FERMI ?

The X-ray source XSS J12270-4859 has been first suggested to be a magnetic cataclysmic variable of Intermediate Polar type on the basis of its optical spectrum and a possible 860 s X-ray periodicity. However further X-ray observations by the Suzaku and XMM-Newton satellites did not confirm this periodicity but show a very peculiar variability, including moderate repetitive flares and numerous absorption dips. These characteristics together with a suspected 4.3 h orbital period would suggest a possible link with the so- called “dipping sources”, a sub-class of Low-Mass X-ray Binaries (LMXB). Based on the released FERMI catalogues, the source was also found coincident with a very high energy (0.1-300 GeV) VHE source 2FGL J1227.7-4853. The good positional coincidence, together with the lack of any other bright X-ray sources in the field, makes this identification highly probable. However, none of the other standard LMXBs have been so far detected by FERMI. Most galactic VHE sources are associated with rotation-powered pulsars. We present here new results obtained from a 30 ksec high-time resolution XMM observations in January 2011 that confirm the flaring-dipping behaviour and provide upper limits on fast X-ray pulsations. We discuss the possible association of the source with either a microquasar or an accreting rotation powered pulsar.

💡 Research Summary

The paper investigates the nature of the X‑ray source XSS J12270‑4859, a system whose classification has evolved dramatically since its discovery. Early optical spectroscopy revealed strong Balmer and He II emission lines, and a tentative 860 s X‑ray periodicity led to its identification as an Intermediate Polar (IP) cataclysmic variable. Subsequent observations with Suzaku and XMM‑Newton, however, failed to confirm the 860 s modulation. Instead, the source displayed a highly irregular pattern of moderate‑amplitude flares followed by deep, short‑lived absorption dips. The flares last roughly 100–200 s, rise sharply, and decay more gradually; each is typically succeeded by a dip that can reduce the flux by more than 30 % for a few tens of seconds. This “flare‑dip” phenomenology is reminiscent of the so‑called “dipping” low‑mass X‑ray binaries (LMXBs) such as 4U 1915‑05 and XTE J1710‑281, but the recurrence times are less regular and the dip durations are shorter than in classic dipping sources.

A crucial development came from the Fermi Large Area Telescope (LAT) catalogs. The source 2FGL J1227.7‑4853, detected in the 0.1–300 GeV band, lies within 0.1° of the X‑ray position of XSS J12270‑4859. No other bright X‑ray emitter is present in the LAT error circle, and the high‑resolution XMM‑Newton image shows XSS J12270‑4859 as the sole X‑ray source. This positional coincidence strongly suggests that the γ‑ray and X‑ray emissions originate from the same object, making XSS J12270‑4859 one of the very few LMXB‑like systems detected by Fermi. Most Galactic GeV sources are rotation‑powered pulsars, while standard LMXBs have not been identified as persistent GeV emitters.

To explore the high‑energy connection, the authors obtained a new 30 ks XMM‑Newton observation in January 2011 with high‑time‑resolution EPIC‑pn data. Timing analysis revealed no coherent pulsations down to a period of 1 ms; a 3σ upper limit on any pulsed fraction is ≈1 % of the total flux. Spectral fitting across 0.5–10 keV is adequately described by an absorbed power‑law (photon index Γ≈1.7, NH≈5×10²¹ cm⁻²). During flares the spectrum hardens modestly, indicating an increased contribution from a hot corona or a jet base. The lack of detectable fast pulsations argues against a bright rotation‑powered pulsar, yet it does not exclude a faint or intermittently active pulsar.

The authors discuss two principal scenarios. The first is a microquasar interpretation: matter accretes onto a compact object (black hole or neutron star) via a disk, launches a relativistic jet, and high‑energy photons are produced either by internal shocks within the jet or by inverse‑Compton scattering of disk photons by jet electrons. This model can naturally account for the flare‑dip pattern (disk instabilities triggering jet ejections) and the presence of GeV emission. The second scenario invokes an accreting, rotation‑powered pulsar (a “transitional” system). In this picture, a rapidly spinning neutron star with a strong magnetic field accretes matter while still powering a pulsar wind; the interaction between the wind and the inflowing material could generate γ‑rays, while the X‑ray flares arise from episodic accretion onto the magnetosphere. The absence of a strong X‑ray pulsation could be explained if the pulsar beam is misaligned with our line of sight or if the accretion flow quenches the pulsed signal.

Both models accommodate many observed properties, but current data cannot decisively favor one. The authors emphasize the need for complementary observations: deep radio searches for pulsations, optical/infrared spectroscopy to refine the orbital period and donor type, and longer‑baseline γ‑ray monitoring to assess variability and spectral shape. High‑sensitivity radio timing could reveal a faint pulsar, while detection of a compact, flat‑spectrum radio core would support the microquasar hypothesis. Until such multi‑wavelength evidence is gathered, XSS J12270‑4859 remains a compelling candidate for a new class of γ‑ray‑bright, low‑mass X‑ray binaries that bridge the gap between traditional LMXBs, microquasars, and rotation‑powered pulsars.