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21st of January 2021
Determining the distance to a new X-Ray Binary
by Chauhan et al.
The distance to an astronomical object is a key physical parameter in understanding its physical quantities. For nearby objects, measurements of parallax (small changes in the position of the object due to the Earth's motion about the sun) are possible, but for more distant objects constraining the distance is more difficult. One technique is to use the 21-cm line of neutral hydrogen. Galactic HI clouds along the line of sight to an object are rotating about the centre of our Galaxy, and their Doppler-shifted absorption features allow a kinematic distance to the source to be determined.

ASKAP observed the new X-Ray Binary MAXI J1348–630 on 2019 February 13, with ASKAP's large field of view and high angular resolution (∼25 arcseconds) allowed the simultaneous detection the HI absorption towards both MAXI J1348–630 and a set of nearby extragalactic sources, enabling upper and lower limits to be placed on the distance to the source.

The red and blue curves above show the HI absorption against MAXI J1348–630 and the stack of the eight extragalactic sources, respectively. The black dashed vertical line represents the rest frequency of the HI line. The dotted (−31 km/s) and dot-dashed vertical (−50 km/s) lines highlight the maximum negative radial velocities (with respect to the Local Standard of Rest) observed from the spectra of MAXI J1348–630, and the merged extragalactic sources, respectively. The maximum negative velocity of −31±4 km s−1 towards MAXI J1348–630 determines the most probable distance as 2.2 (+0.5, −0.6) kpc, More details are given in the paper, published in Monthly Notices of the Royal Astronomical Society.

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