Saturday, July 14, 2018

Maltese discovery of a new variable star in the constellation of Aquila.

We report the discovery of a previously unknown variable star in the constellation of Aquila. This new variable star was discovered on 31st July 2017 by Stephen Brincat and myself through a set of images acquired which I acquired for the purposes of asteroid photometry.

At the time I used a 0.20-m aperture Schmidt-Cassegrain telescope and a scientific-grade CCD for data acquisition.

Stephen has analysed the images for any new variable star among the background field stars. Analysis of the unfiltered CCD frames yielded a possible candidate at coordinates (J2000) 19:18:20.60 -09:25:56.29 where the resultant lightcurve for the possible variable star depicted a decline in brightness.  The suspected variable star was monitored for an additional six nights from Flarestar Observatory through a 0.25-m f/6.3 Schmidt-Cassegrain telescope. We performed differential CCD aperture photometry on the suspected variable star using four field stars as comparison stars. The selected comparison stars were all of near-solar colour in order to mitigate atmospheric extinction effects that could diminish the quality of our results.

The analysis of the observations revealed that this star (identified as UCAC4 403-115663) varies its light in a regular fashion at a period of around half a day. Consequently, each night we viewed the same part of the phase. In such circumstances, it was very difficult to determine the nature of the star through its light curve from a single location as each night overlaps the others. As a result, we searched for data on this star from several professional catalogues.

As the star’s maximum magnitude in V-band is of magnitude 15.85, we found out that a number of surveys did not capture it adequately. However we discovered that the ASAS-SN survey (All-Sky Automated Survey for Super Novae) had a good number of data points for this star. The ASAS-SN survey picked out our target once or twice every few days.  Luckily we had appropriate algorithms in our arsenal for such instances. We employed the Lomb-Scargle and the Phase Dispersion Minimization (PDM) algorithms that are capable to extract a reliable period despite that the survey had acquired their data at irregular intervals.   Our data combined with those from the ASASSN-SN survey yielded a period of 0.498115 days.

The lightcurve revealed that the regular variation in brightness is due to two stars revolving around each other, typical of a binary star system of the W UMa class binary star system where two stars orbit each other and are close enough to distort their spherical shapes into ellipsoids. Our study revealed that the amplitude of this system range from magnitude 15.85 to 16.35, due to continuous eclipses where each component occults the other.  We reported our discovery to the American Association of Variable Star Observers (AAVSO) where it has been officially recognised and catalogued in the AAVSO International Variable Star Database (VSX). This newly discovered star system is now designated as AUID (AAVSO Unique Identifier) 000-BMP-328.

We wish to take the opportunity to thank AAVSO external consultant Sebastian Otero for his help and guidance. 

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