Discovery of the optical counterpart of MAXI J1535-571

ATel #10702; Simone Scaringi, ASTR211 students (University of Canterbury)...
on 3 Sep 2017; 20:20 UT
Credential Certification: Simone Scaringi (simone.scaringi@canterbury.ac.nz)

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On 2017 September 02, both Swift (GCN #21788, ATel #10700) and MAXI(ATel #10699) co-detected a new X-ray source designated MAXI J1535-571. The localisation of this transient near the Galactic plane and long image trigger detected by BAT, makes this source a new Galactic Transient. Here we report on new optical observations of the field around MAXI J1535-571 with the 0.61m B&C Telescope operated by the University of Canterbury at Mt. John Observatory, New Zealand. We performed imaging observations on September 3 between 08:20 and 10:30 UT. 12x600 second images have been obtained simultaneously in the SDSS g', r' and i' bands. Several targets are detected close to the 1.5 arcsec Swift-XRT error circle (ATel #10700). We calibrated the fields using VPHAS+ stars (Drew et al. 2014) in all 3 bands. One target is found inside the Swift-XRT error circle in the i'-band image at magnitude 21.79 +- 0.20. The i'-band finding chart can be found in the link below. No detection of the same target is achieved in the r' and g' bands. There are no objects within 5.8 arcsec of the MAXI J1535-571 position in the VPHAS+ catalogues, or any other optical/infrared catalogue. The clear detection in the i'-band images suggests this source is variable not only in X-rays, but also at optical/infra-red wavelengths. The fact that no detection has been achieved in the g' and r' bands is not surprising given the high extinction level towards the Galactic plane (ATel #10700). The nature of this source is uncertain. However, from the Swift, MAXI and i'-band detections, together with the location in the Galactic plane, it is probable that MAXI J1535-571 is a compact interacting binary, possibly a new low mass x-ray binary. Multiwavelength observations are encouraged to help constrain the nature of MAXI J1535-571.

MAXI J1535-571 finding chart