A search for optical counterparts in quiescent black hole X-ray transients

Abstract

Dynamical mass measurements of compact stars in X-ray transients demand the detection of optical/near infrared counterparts in quiescence. Out of the 73 black-hole candidates in X-ray transients, optical and near-infrared quiescent counterparts have only been identified for 34 objects. We present ULTRACAM photometric observations of nine candidate black hole X-ray transients with no reported counterparts in quiescence, complemented with data from the public surveys DECaPS and Pan-STARRS. In addition, we analyze photometry of three sources (SWIFT J1539.2-6227, XTE J1817-330 and XTE J1818-245) obtained during their discovery outburst. The data provide the first optical identifications and precise astrometry of four targets (MAXI J1348-630, SWIFT J1539.2-6227, XTE J1726-476 and XTE J1817-330) plus $3σ$ lower limits to the quiescent optical magnitudes for an additional five (MAXI J0637-430, 4U 1755-338, MAXI J1803-298, XTE J1818-245 and MAXI J1828-249). Of these five, 4U 1755-338 was found to be active during our ULTRACAM observations and we use our images to derive refined astrometric coordinates. We use the photometric magnitudes and colors to place preliminary constraints on the orbital periods and spectral types of the companion stars. Finding charts of all the targets are also provided to facilitate future follow-up studies. Finally, we present updated astrometry for XTE J1650-500 using archival FORS2 images.

Figures (4)

• Figure 1: Limiting magnitudes as a function of the seeing cut-off for the X-ray transient SWIFT J1539.2-6227. The red and green dots represent the values obtained for the $r$ and $g$-band, respectively. The red and green numbers indicate the number of combined frames. The deepest magnitude is obtained by selecting images with a seeing up to 1.25 arcsec.
• Figure 2: Deepest $r$-band images for the sample of the nine BH candidates. The location of the objects are indicated by red ellipses, taking into account the positional errors reported in the literature (see Table \ref{['tab:coords']}). For the sake of visibility, we mark the position of J1803 and J1828 with red tick marks instead of ellipses, as the positional errors are too small to be resolved. The field of view is 20 x 20 arcsec each.
• Figure 3: PSF residual $r$-band images. Object locations are indicated by red ellipses, except for J1828, which is marked with red tick marks due to its small positional error. The field of view is 10 x 10 arcsec.
• Figure 4: Best identifications from DECaPS2 images, with each field of view measuring 20 × 20 arcsec. The positions of the objects are marked with red ellipses, accounting for the positional uncertainties reported in the literature (see Table \ref{['tab:coords']}), except for 4U 1755-338, where we plot our refined coordinates based on the outburst counterpart detected in the NTT images. For the sake of visibility, we mark the radio position of J1803 with red tick marks.