A. de Ugarte Postigo
M. Armas Padilla
- The optical counterpart of the bright X-ray transient Swift J1745-26
- Monthly Notices of the Royal Astronomical Society
- Volume | Issue number
- 432 | 2
- Pages (from-to)
- Document type
- Faculty of Science (FNWI)
- Anton Pannekoek Institute for Astronomy (API)
We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745−26, starting only 19 min after the discovery of the source. We observe the system peaking at i′ ∼ 17.6 on day six (MJD 561 92) to then decay at a rate of ∼0.04 mag d−1. We show that the optical peak occurs at least 3 d later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 mag, which favours an orbital period ≲21 h and a companion star with a spectral type later than ∼A0. Spectroscopic observations taken with the Gran Telescopio de Canarias 10.4 m telescope reveal a broad (full width at half-maximum ∼1100 km s−1), double-peaked Hα emission line from which we constrain the radial velocity semi-amplitude of the donor to be K2 > 250 km s−1. 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.
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