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Astronomers have spotted far ultra violet emissions from novae, a special class of transient astronomical event that causes the sudden appearance of a bright, apparently new star that slowly fades over weeks or months, during their outburst, for the first time in the neighbouring Andromeda galaxy.

All observed novae involve white dwarfs in close binary systems, but causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars.

A binary pair of stars comprising a White Dwarf, an earth-sized but very hot star, and a Sun-like (or its puffed-up evolved version) star are sometimes found orbiting each other in close proximity. In such systems, the White Dwarf’s intense gravitational force can deform the companion star and pull its matter onto the surface of the White Dwarf. The piling up of matter creates such intense densities that the fusion reaction is enhanced, giving off enormous amounts of light, which is seen as a nova eruption. Novae eruptions contribute towards galactic chemical enrichment, and hence they are important for study. These also provide laboratories to study extreme conditions of shock mechanisms, thermonuclear processes, and the binary evolution of stars.

This accretion process is streamlined through the presence of a disc-like structure around the White Dwarf, known as the accretion disk. These disks are very hot and emit electromagnetic waves in the UV and blue regions of the spectrum.

Scientists from Indian Institute of Astrophysics, Bengaluru, an autonomous institute of Department of Science and Technology, used Ultraviolet Imaging Telescope (UVIT/AstroSat) data of the Andromeda Galaxy from the public archives, to look for Far Ultra Violet (FUV) emission from novae during their dormancy. On the way, they stumbled upon novae around their eruption phase in a study published in the Astrophysical Journal.

The team consisting of Judhajeet Basu (IIA and Pondicherry University), Krishnendu S. (IIA and Amrita University), Sudhanshu Barway (IIA), Shatakshi Chamoli (IIA and Pondicherry University), and G. C. Anupama (IIA) discovered ultraviolet emission from 42 novae, a special class of stellar explosions, and even caught 4 of them in the act of outburst itself.

This could help the scientists study these interacting binary star systems in our nearest neighbor galaxy at different phases of their life, some piling up matter from their companion, while others spewing it into space.

“UVIT’s fine spatial resolution and unique capability to observe simultaneously in far UV and near UV helped us investigate the fluxes in different UV bands, which led to the detection of accretion disks in some of these systems, 2.5 million light years away. The brighter the disk, the more rapidly it is consuming its companion’s matter. We also studied how the flux from these discs changes with time, and as per our expectations, the accretion process was found to be stable in these systems.” said Basu, a PhD student at IIA who led the project. 

Continuous accumulation of matter onto the White Dwarf leads to extreme temperature, pressure, and density conditions. “This layer of material acts like a translucent shell, blocking off some of the radiation from the white dwarf and the accretion disc. Under these circumstances, the brightness of these systems diminishes, and it is a tell-tale signature of what’s going to come.

It’s like the calm before the storm, and this is exactly what we found in two of these systems in Andromeda Galaxy by using data from UVIT, a telescope built at our CREST campus and launched into space by ISRO,” Basu added.

Once the threshold temperature and densities are reached, all the accumulated hydrogen-rich matter undergoes a thermonuclear runaway reaction. “It is much like what happens in a fusion bomb, but on an “astronomical” scale. This explosion naturally leads to the brightening of the system by several orders of magnitude, hurling large quantities of material into the interstellar medium. We serendipitously found four systems caught in this act,” said Barway, a faculty at IIA.

However, it was not easy to detect all of these systems. “The central region of Andromeda is quite bright, encouraging us to use sophisticated image subtraction techniques to uncover more novae. We used two different techniques. Both yielded the same results, confirming what we are seeing are real sources and not bogus,” said Barway.

“Tracing these novae was possible only because of the Andromeda survey proposals taken up by AstroSat UVIT operated by ISRO. More such future missions, especially in UV and X-ray, can discover and follow up these systems, and could answer some of the missing puzzles of novae,” Basu pointed out.

Figure (1): Novae detected in the Andromeda in UVIT’s FUV/NUV filter.

Figure (2): Novae recovered from the bright central bulge of M31 by image subtraction.

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