Multi-Wavelength Observations Of A New Redback Millisecond Pulsar 4FGL J1910.7-5320

Abstract

We present the study of multi-wavelength observations of an unidentified Fermi Large Area Telescope (LAT) source, 4FGL J1910.7-5320, a new candidate redback millisecond pulsar binary. In the 4FGL 95% error region of 4FGL J1910.7-5320, we find a possible binary with a 8.36-hr orbital period from the Catalina Real-Time Transient Survey (CRTS), confirmed by optical spectroscopy using the SOAR telescope. This optical source was recently independently discovered as a redback pulsar by the TRAPUM project, confirming our prediction. We fit the optical spectral energy distributions of 4FGL J1910.7-5320 with a blackbody model, inferring a maximum distance of 4.1 kpc by assuming that the companion fills its Roche-lobe with a radius of R = 0.7R_sun. Using a 12.6 ks Chandra X-ray observation, we identified an X-ray counterpart for 4FGL J1910.7-5320, with a spectrum that can be described by an absorbed power-law with a photon index of 1.0+/-0.4. The spectrally hard X-ray emission shows tentative evidence for orbital variability. Using more than 12 years of Fermi-LAT data, we refined the position of the {\gamma}-ray source, and the optical candidate still lies within the 68% positional error circle. In addition to 4FGL J1910.7-5320, we find a variable optical source with a periodic signal of 4.28-hr inside the 4FGL catalog 95% error region of another unidentified Fermi source, 4FGL J2029.5-4237. However, the {\gamma}-ray source does not have a significant X-ray counterpart in a 11.7 ks Chandra observation, with a 3-{\sigma} flux upper limit of 2.4*10^-14 erg cm^-2 s^-1 (0.3-7 keV). Moreover, the optical source is outside our updated Fermi-LAT 95% error circle. These observational facts all suggest that this new redback millisecond pulsar powers the {\gamma}-ray source 4FGL J1910.7-5320 while 4FGL J2029.5-4237 is unlikely the {\gamma}-ray counterpart to the 4.28-hr variable.