On the origin of two unidentified radio/X-ray sources discovered with XMM-Newton

FEDERICO GARCÍA; JORGE A. COMBI; MARÍA C. MEDINA; GUSTAVO E. ROMERO

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2015 vol. 584

0004-6361

Aims. We aim at clarifying the nature of the emission of two spatially related unidentified X-ray sources detected with XMM-Newtontelescope at intermediate-low Galactic latitudeMethods. We use the imaging and spectral capabilities of XMM-Newton to study the X-ray properties of these two sources. In addition,we complement our study with radio data obtained at different frequencies to analyze a possible physical association between thesources.Results. Observations reveal a point-like source aligned with elongated diffuse emission. The X-ray spectra of these sources is best-fitted by an absorbed power law with photon index Γ ∼ 1.7 for the point-like source and ∼2.0 for the extended source. Both sourcesshow nonthermal radio-continuum counterparts that might indicate a physical association. In addition, from the available data, wedid not detect variability on the point-like source in several timescales. Two possible scenarios are analyzed: one Galactic and oneextra-Galactic. First, based on HI line absorption, assuming a Galactic origin, we infer a distance upper bound of 2 kpc, which posesa constraint on the height over the Galactic plane of 10ergsand7.5×10ergs, for the point-like and extended sources, respectively. Second, an extra-Galactic∼∼nature is discussed, where the point-like source might be the core of a radio galaxy and the extended source its lobe. In this case,we compare derived fluxes, spectral indices, and spatial correlation with those typical from the radio galaxy population, showing thefeasibility of this alternative astrophysical scenario.Conclusions. From the available observational evidence, we suggest that the most promising scenario to explain the nature of thesesources is a system consisting of a one-sided radio galaxy, where the point-like source is an active galactic nucleus and the extendedsource corresponds to the emission from its lobe. Other possibilities include a pulsar and its associated pulsar wind nebula, wherethe radio/X-ray emission originates from the synchrotron cooling of relativistic particles in the pulsar?s magnetic field or a casualalignment between two unrelated sources, such as an active galactic nucleus and a Galactic X-ray blob. Deeper dedicated observationsin both radio and X-ray bands are needed to fully understand the nature of these sources.