The intriguing nature of the high-energy gamma ray source XSS J12270-4859

Abstract

Context. The nature of the hard X-ray source XSS J12270-4859 is still unclear. It was claimed to be a possible magnetic cataclysmic variable of the Intermediate Polar type from its optical spectrum and a possible 860 s X-ray periodicity in RXTE data. However, recent observations do not support the latter variability, leaving this X-ray source still unclassified. Aims. To investigate its nature we present a broad-band X-ray and gamma ray study of this source based on a recent XMM-Newton observation and archival INTEGRAL and RXTE data. Using the Fermi/LAT 1-year point source catalogue, we tentatively associate XSS J12270-4859 with 1FGL J1227.9-4852, a source of high-energy gamma rays with emission up to 10 GeV. We further complement the study with UV photometry from XMM-Newton and ground-based optical and near-IR photometry. Methods. We have analysed both timing and spectral properties in the gamma rays, X-rays, UV and optical/near-IR bands of XSS J12270-4859. Results. The X-ray emission is highly variable, showing flares and intensity dips. The flares consist of flare-dip pairs. Flares are detected in both X-rays and the UV range, while the subsequent dips are present only in the X-ray band. Further aperiodic dipping behaviour is observed during X-ray quiescence, but not in the UV. The broad-band 0.2-100 keV X-ray/soft gamma ray spectrum is featureless and well described by a power law model with Γ = 1.7. The high-energy spectrum from 100 MeV to 10 GeV is represented by a power law index of 2.45. The luminosity ratio between 0.1-100 GeV and 0.2-100 keV is ∼0.8, indicating that 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. The X-ray spectrum however hardens during the post-flare dips, where a partial covering absorber is also required to fit the spectrum. Optical photometry acquired at different epochs reveals a period of 4.32 hr that could be ascribed to the binary orbital period. Near-IR, possibly ellipsoidal, variations are detected. Large amplitude variability on shorter (tens mins) timescales is found to be non-periodic. Conclusions. The observed variability at all wavelengths together with the spectral characteristics strongly favour a low-mass atypical low-luminosity X-ray binary and are against a magnetic cataclysmic variable nature. The association with a Fermi/LAT high-energy gamma ray source further strengths this interpretation.

Unveiling the nature of INTEGRAL objects through optical spectroscopy

“…This confirms the importance of this identification work on catalogued but unidentified high-energy sources, because peculiar objects can be found within the considered samples (see, for instance, Paper IX; Masetti et al 2007Masetti et al , 2008bBassani et al 2012;de Martino et al 2010de Martino et al , 2013. With the present data, we also correct two results given in Paper IX, namely the actual redshift (z = 2.02) of the Type 1 QSO IGR J16388+3557 and the correct counterpart of source IGR J06293−1359.…”

Coordinated X-Ray, Ultraviolet, Optical, and Radio Observations of the PSR J1023+0038 System in a Low-Mass X-Ray Binary State

“…de Martino et al (2010) attributed the dips occurring immediately after flares to a rapid episode of accretion onto the neutron star (corresponding to the flare) and the corresponding emptying of a reservoir of accreting material (the dip) and subsequent filling up of the inner regions of the accretion disk. In contrast, in the long Xray exposures of PSR J1023+0038 presented here, we do not find preceding flaring episodes for all the dips but there is evidence for dips occuring before the majority of intense flares.…”

Different twins in the millisecond pulsar recycling scenario: Optical polarimetry of PSR J1023+0038 and XSS J12270-4859

“…Subsequent independent multi-wavelength studies (Pretorius 2009;Saitou et al 2009;de Martino et al 2010de Martino et al , 2013Hill et al 2011; casted doubts on this classification, suggesting instead an identification as a LMXB showing unusual dipping and flaring behaviour on timescales of few hundreds of seconds. XSS J12270−4859 was subsequently recognised to be spatially coincident with a moderately bright gamma-ray source that was detected by Fermi-LAT emitting up to 10 GeV (de Martino et al 2010;Hill et al 2011), and is now known as 3FGL J1227.9−4854.…”