A Multi-wavelength search for Black Widows and Redbacks counterparts of candidate $γ$-ray millisecond pulsars

Abstract

The wealth of detections of millisecond pulsars (MSPs) in γ-rays by Fermi has spurred searches for these objects among the several unidentified γ-ray sources. Interesting targets are a subclass of binary MSPs, dubbed "Black Widows" (BWs) and "Redbacks" (RBs), which are in orbit with low-mass non-degenerate companions fully or partially ablated by irradiation from the MSP wind. These systems can be easily missed in radio pulsar surveys owing to the eclipse of the radio signal by the intra-binary plasma from the ablated companion star photosphere, making them better targets for multi-wavelength observations. We used optical and X-ray data from public databases to carry out a systematic investigation of all the unidentified γ-ray sources from the Fermi Large Area Telescope (LAT) Third Source Catalog (3FGL), which have been pre-selected as likely MSP candidates according to a machine-learning technique analysis. We tested our procedure by recovering known binary BW/RB identifications and searched for new ones, finding two possible candidates. At the same time, we investigated previously proposed BW/RB identifications and we ruled out one of them based upon the updated γ-ray source coordinates.

Radio pulsations from the $γ$-ray millisecond pulsar PSR J2039-5617

“…With only a minority of BWs/RBs detected as X-ray pulsars, the search for Xray pulsations from PSR J2039−5617 is now one of the next steps. The case of PSR J2039−5617 confirms the validity of the multiwavelength approach in the identification of BW/RB systems and spurs systematic searches (e.g., Braglia et al 2020). New BW/RB candidates singled out through the detection of optical/X-ray flux modulations will hopefully be confirmed in the next years once radial velocity measurements provide the values of the orbital parameters to ease blind radio/γ-ray periodicity searches.…”