The optical counterpart of the bright X-ray transient Swift J1745-26

TL;DR

This study monitored the optical counterpart of Swift J1745-26, revealing its outburst characteristics, spectral features, and supporting its classification as a new black hole candidate within 7 kpc.

Contribution

First detailed optical monitoring and spectroscopic analysis of Swift J1745-26, providing insights into its outburst timing, spectral properties, and black hole candidacy.

Findings

Optical peak occurred 3 days after X-ray peak.

Outburst amplitude > 4.3 magnitudes suggests a short orbital period.

Spectroscopy revealed broad, double-peaked H_alpha emission.

Abstract

We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19 minutes after the discovery of the source. We observe the system peaking at i' ~17.6 on day 6 (MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the optical peak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 magnitudes, which favours an orbital period < 21 h and a companion star with a spectral type later than ~ A0. Spectroscopic observations taken with the GTC-10.4 m telescope reveal a broad (FWHM ~ 1100 km/s), double-peaked H_alpha emission line from which we constrain the radial velocity semi-amplitude of the donor to be K_2 > 250 km/s. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ~7 kpc.