Unraveling the Nature of Unidentified High Galactic Latitude Fermi/LAT Gamma-Ray Sources With Suzaku

Abstract

Here we report on the results of deep X-ray follow-up observations of four unidentified γ-ray sources detected by the Fermi/LAT instrument at high Galactic latitudes using the X-ray Imaging Spectrometers on board the Suzaku satellite. All of the studied objects were detected with high significance during the first three months of Fermi/LAT operation and subsequently better localized in the first Fermi/LAT catalog (1FGL). For some of them, possible associations with pulsars and active galaxies have subsequently been discussed, and our observations provide an important contribution to this debate. In particular, a bright X-ray point source has been found within the 95% confidence error circle of 1FGL J1231.1-1410. The X-ray spectrum of the discovered Suzaku counterpart of 1FGL J1231.1-1410 is well fitted by a blackbody with an additional power-law component. This supports the recently claimed identification of this source with a millisecond pulsar PSR J1231-1411. For the remaining three Fermi objects, on the other hand, the X-ray observations performed are less conclusive. In the case of 1FGL J1311.7-3429, two bright X-ray point sources were found within the LAT 95% error circle. Even though the X-ray spectral and variability properties for these sources were robustly assessed, their physical nature and relationship with the γ-ray source remain uncertain. Similarly, we found several weak X-ray sources in the field of 1FGL J1333.2+5056, one coinciding with the high-redshift blazar CLASS J1333+5057. We argue that the available data are consistent with the physical association between these two objects, although the large positional uncertainty of the γ-ray source hinders a robust identification. Finally, we have detected an X-ray point source in the vicinity of 1FGL J2017.3+0603. This Fermi object was recently suggested to be associated with a newly discovered millisecond radio pulsar PSR J2017+0603, because of the spatial coincidence and the detection of the γ-ray pulsations in the light curve of 1FGL J2017.3+0603. Interestingly, we have detected the X-ray counterpart of the high-redshift blazar CLASS J2017+0603, located within the error circle of the γ-ray source, while we were only able to determine an X-ray flux upper limit at the pulsar position. All in all, our studies indicate that while a significant fraction of unidentified high Galactic latitude γ-ray sources is related to the pulsar and blazar phenomena, associations with other classes of astrophysical objects are still valid options.