This episode's coordinates point to the object A 0535+26 also known as HD 245770 located at a distance of roughly 6200 light years (1900 pc) [1, 2] in the constellation Taurus. The history of the "discovery" of this object is rather complicated. Originally, it was included in the Henry Draper Catalogue, which was compiled in the early twentieth century, as a really bright, relatively massive blue star, and it was thought to be just a single blue star for a few decades. In the 1970s, however, the Ariel V satellite detected variable X-ray emission from somewhere around this region of the sky, although because X-ray detectors were not that good in the 1970s, people weren't sure exactly where the X-ray emission was coming from. At this point, this X-ray source was given the designation A 0535+26, where the A stood for Ariel and the digits gave the location in the coordinate system that was in use at the time. Eventually, this variable X-ray emission was associated with variations in the light from HD 245770 seen in the visible part of the electromagnetic spectrum, which implied that the star was the source of the X-ray emission [3].;
So it turned out that the star was not a single large blue star but instead a special type of binary star system called a high mass X-ray binary. The progenitor for one of these star systems would have contained two very bright blue stars with masses multiple times the mass of the Sun that would have each initially fused hydrogen into helium in their cores. However, these stars only last a few million years in this stage, and the more massive one would have evolved into a red supergiant and then exploded as a supernova, leaving behind something like a neutron star or black hole, while the other star would still be in the hydrogen-fusing stage. In a typical high mass X-ray binary, the stars are close enough that the smaller object would gravitationally strip away gas from the bigger blue star, and as that gas falls onto the smaller object, it would get gravitationally compressed and really hot, thus leading to the production of X-ray emission.;
However, A 0535+26 is an abnormal high mass X-ray binary, so it doesn't do that. Instead, it belongs to the special class of objects called Be high mass X-ray binaries. The bigger blue star in this case is a special type of giant blue star called a Be star, which indicates that, in the spectrum of the star, it's possible to see very bright line-like features at specific wavelengths that are produced by an extended cloud of ionized gas surrounding the star, which is kind of weird by itself. In A 0535+26 specifically and Be high mass X-ray binaries more generally, the gas surrounding the Be star lies within a disk. The orbit of the neutron star or black hole in these specific systems is actually very elongated and elliptical and can take a relatively long amount of time to orbit the bigger blue star. In A 0535+26 specifically, the smaller object is a pulsar, which is a rapidly rotating version of a neutron star, and the pulsar completes an orbit around the bigger blue star once every 111 days [4, 5, 6]. Most of the time, the pulsar is too far away from the giant blue star to really affect the blue star's outer gas layers at all. However, the pulsar will occasionally pass through the disk of ionized gas surrounding the larger blue star, and when it does that, part of the gas disk falls onto the pulsar, gets really compressed, and produces X-ray emission. Since it takes a short amount of time for the pulsar to pass through the gas disk, it looks like the star system is producing a relatively short burst of X-ray emission rather than continuously emitting X-rays.;
This variable X-ray emission has now been studied intensely for five decades. However, the X-ray bursts have not always been consistent. In 1995, the X-ray bursts ceased, and A 0535+26 effectively went on hiatus [7]. I guess, sort of like webcomic creators, YouTubers, and podcasters, A 0535+26 needed to take a break, and sometimes when people go on hiatus, their webcomics and podcasts never come back, but other times they come back after a few months or a few years. Well, A 0535+26 returned from hiatus in 2005 [5]. It seemed like the gas disk surrounding the giant blue star in the system disappeared for a few years and needed to be regenerated. The system also did not just make a comeback; it came back even stronger than before. In 2020, A 0535+26 produced one the brightest X-ray outbursts ever measured from the system [8].;
This object was definitely one of the first Be high mass X-ray binaries ever identified, and it has been instrumental in helping to define this entire class of abnormal X-ray binary star systems. As long as it doesn't go on hiatus again and never come back, astronomers will continue to observe this star to unlock the secrets of how this class of stars works. ;