Studying the black widow pulsars PSR J0312–0921 and PSR J1627+3219 in the optical and X-rays

¹ Ioffe Institute, 26 Politekhnicheskaya, St., Petersburg, 194021, Russia
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 877, Ensenada, Baja California, 22800, Mexico
³ American Association of Variable Star Observers, 185 Alewife Brook Parkway, Suite 410, Cambridge, MA 02138, USA

^⋆ Corresponding author: bobakov_alex@mail.ru

Received: 28 July 2025
Accepted: 27 August 2025

Abstract

Context. PSR J0312−0921 and PSR J1627+3219 are black widow pulsars with orbital periods of 2.34 and 3.98 hours. They were recently detected in the radio and γ-rays.

Aims. Our goals were to estimate the fundamental parameters of both binary systems and their components.

Methods. We performed first phase-resolved multi-band photometry of both objects with the 10.4 m Gran Telescopio Canarias and fitted the obtained light curves with a model assuming direct heating of the companion by the pulsar. Archival X-ray data obtained with the Swift and XMM-Newton observatories were also analysed.

Results. For the first time, we firmly identified both systems in the optical. Their optical light curves show a rather symmetric single peak per orbital period and a peak-to-peak amplitude of ≳2 mag. We also identified the X-ray counterpart to J1627+3219, and for J0312−0921 we set an upper limit on the X-ray flux.

Conclusions. We estimated the masses of the pulsars, companion temperatures and masses, Roche lobe filling factors, orbital inclinations, and the distances to both systems. PSR J0312−0921 has a very light companion (≈0.02 M_⊙) that possibly has one of the lowest night-side temperatures of the known black widow systems (≈1600 K). We find that the distances to J0312−0921 and J1627+3219 are about 2.5 and 4.6 kpc, respectively. This likely explains their faintness in X-rays. The X-ray spectrum of PSR J1627+3219 can be described by a power-law model, and its parameters are compatible with those obtained for other black widows.