Searches for Pulsar-like Candidates from Unidentified Objects in the Third Catalog of Hard Fermi-LAT (3FHL) sources with Machine Learning Techniques

Abstract

We report the results of searching pulsar-like candidates from the unidentified objects in the $3^{\mathrm{rd}}$ Catalog of Hard Fermi-LAT sources (3FHL). Using a machine-learning based classification scheme with a nominal accuracy of $\sim 98\%$, we have selected 27 pulsar-like objects from 200 unidentified 3FHL sources for an identification campaign. Using archival data, X-ray sources are found within the $\gamma -$ray error ellipses of 10 3FHL pulsar-like candidates. Within the error circles of the much better constrained X-ray positions, we have also searched for the optical/infrared counterparts and examined their spectral energy distributions. Among our short-listed candidates, the most secure identification is the association of 3FHL J1823.3-1339 and its X-ray counterpart with the globular cluster Mercer 5. The $\gamma -$rays from the source can be contributed by a population of millisecond pulsars residing in the cluster. This makes Mercer 5 as one of the slowly growing hard $\gamma -$ray population of globular clusters with emission $>10$ GeV. Very recently, another candidate picked by our classification scheme, 3FHL J1405.1-6118, has been identified as a new $\gamma -$ray binary with an orbital period of $13.7$ days. Our X-ray analysis with a short Chandra observation has found a possible periodic signal candidate of $\sim 1.4$ hrs and a putative extended X-ray tail of $\sim 20$ arcsec long. Spectral energy distribution of its optical/infrared counterpart conforms with a blackbody of $T_{\mathrm{bb}}\sim 40000$ K and $R_{\mathrm{bb}}\sim 12R_{\odot }$ at a distance of 7.7 kpc. This is consistent with its identification as an early O star as found by infrared spectroscopy.

MNRAS in press, 19 pages, 13 figures, 8 tables

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