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29th of October 2019
An ASKAP HI absorption distance to MAXI J1535–571
by Chauhan et al.
On 2 September 2017, an uncatalogued X-ray binary system was discovered by both the Monitor of All-sky X-ray Image (MAXI) and the Neil Gehrels Swift Observatory, and designated MAXI J1535–571. Multi-wavelength studies of the system strongly suggest that the accreting compact object is a stellar-mass black hole. However, the physical parameters, including the black hole mass and distance, and the peak luminosity of the outburst were still uncertain. Chauhan et al. used the ASKAP Early Science array of 12 antennas to monitor the MAXI J1535–571 over seven epochs from 21 September to 2 October 2017. The radio flare accompanying the X-ray outburst was bright enough that the team could study the HI absorption spectrum from gas clouds along the line-of-sight and thereby constrain the distance to the source.

The image above shows the HI absorption spectrum observed from MAXI J1535–571 on 21 September 2017, when the source was brightest (~580 mJy). Spectra of MAXI J1535–571 and the nearby (in the sky plane) extragalactic source MGPS J153358–564218 are displayed in red and blue, respectively. The solid black horizontal line shows the zero flux level and the red and blue dotted lines represent the 3σ rms noise levels for the two sources (the latter is higher due to being located further out in the primary beam). The dashed vertical line shows the rest velocity. The maximum negative velocity of significant absorption in the MAXI J1535–571 spectrum (dash-dotted vertical line at −69 km/s) places a lower limit on the source distance of ~4.1 kilo-parsecs. The HI absorption spectrum of the extragalactic source MGPS J153358–564218 shows significant absorption out to a maximum negative velocity of −89 km/s, which is due to more distant gas clouds in our galaxy. Absorption at the tangent point velocity is not seen towards MAXI J1535–571, implying that MAXI J1535–571 should be located closer than the tangent point distance of ~6.7 kilo-parsecs, with the value of ~4.1 kilo-parsecs therefore being preferred. The study highlights the capabilities of new wide-field radio telescopes to probe Galactic transient outbursts, by allowing us to observe both a target source and a background comparison source in a single telescope pointing. More details are given in the paper, published in Monthly Notices of the Royal Astronomical Society.

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