Multiwavelength Observations of a New Redback Millisecond Pulsar 4FGL J1910.7−5320

Abstract

We present the study of multiwavelength 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, 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 = 0.7 R _☉ . 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 yr of Fermi-LAT data, we refined the position of the γ -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 γ -ray source does not have a significant X-ray counterpart in an 11.7 ks Chandra observation, with a 3 σ 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 γ -ray counterpart to the 4.28 hr variable.

Keywords

• Millisecond pulsars
• Gamma-ray sources
• Compact binary stars