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

Understand

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.