I-band photometry of the quiescent black hole X-ray nova GROJ0422+32=V518 Per: modelling of the orbital light curve and estimation of the black hole mass

TL;DR

This study provides new I-band photometry of the black hole X-ray nova GROJ0422+32, revises its orbital period, models its light curve to estimate the black hole's mass, and discusses implications for the mass gap of compact objects.

Contribution

It offers the first detailed modeling of the orbital light curve to estimate the black hole and companion star masses in GROJ0422+32, refining previous orbital period measurements.

Findings

Revised orbital period of 5h 04m 35.50s.

Estimated black hole mass of 6.5±2.9 solar masses.

Black hole mass overlaps with the known mass gap.

Abstract

We present new photometric observations of the X-ray nova GROJ0422+32 (V518 Per) carried out in the I$_c$ band over 14 nights in 2020 -- 2023. We had to revise the orbital period of the system, the new value $P_{\rm orb}=5^{\rm h}04^{\rm m}35.50^{\rm s} \pm 0.04^{\rm s}$ is about one minute shorter than the period by Webb et al. (2000) but close to the result reported earlier by Filippenko et al. (1995). The obtained folded light curve has a regular shape with a clear ellipticity effect and signatures of a slight heating effect. The simulations of this light curve in terms of the model of an interacting binary system allowed us to estimate the orbital inclination $i = 33^\circ - 49^\circ$ of the system and derive masses of the black hole $M_x=(6.5\pm2.9)M_\odot$ and companion star $M_v=(0.47\pm0.21)M_\odot$. This range of the black hole masses overlaps with the known gap of (2--5)\,M$_\odot$ in the distribution of compact objects masses but mostly it lies above the upper boundary of this gap. To obtain more precise estimates one needs to know the degree of heating of the donor star, so synchronous X-ray/optical observations of this system are desirable.