Novae occur in binary systems where a white dwarf and a companion star (such as a Sun-like star or its evolved form) orbit closely. The white dwarf’s intense gravity pulls matter from the companion star, accumulating it on the white dwarf’s surface.
As the matter piles up, the pressure and temperature increase, triggering a fusion reaction that causes a bright nova eruption. This eruption contributes to galactic chemical enrichment and offers insights into extreme physical conditions, such as shock mechanisms and thermonuclear processes.
An accretion disk around the white dwarf facilitates matter’s accretion, emitting radiation in the ultraviolet and blue regions of the spectrum.
Scientists from the Indian Institute of Astrophysics (IIA) in Bengaluru used data from the Ultraviolet Imaging Telescope (UVIT) aboard the AstroSat mission to study the Andromeda Galaxy. Initially, they aimed to investigate Far Ultraviolet (FUV) emission from novae during their dormant phases. However, in the process, they unexpectedly detected novae in the active eruption phase, providing new insights into these stellar events.
Astronomers have spotted far ultraviolet emissions from novae in the neighboring Andromeda galaxy for the first time.
This discovery allows scientists to study interacting binary star systems in the Andromeda Galaxy at various life cycle stages. Some of these systems accumulate matter from their companion stars, while others are expelling it into space.
Judhajeet Basu (IIA and Pondicherry University) said, “UVIT’s fine spatial resolution and a 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 consumes 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.”
The continuous accumulation of matter onto a white dwarf creates extreme temperature, pressure, and density conditions. This accumulating material forms a translucent shell that blocks some radiation from the white dwarf and the accretion disk. As a result, the system’s brightness diminishes, which serves as a clear signal that an eruption is imminent—like the calm before a storm.
Using data from the Ultraviolet Imaging Telescope (UVIT), built at the CREST campus and launched by ISRO, scientists discovered this phenomenon in two binary systems in the Andromeda Galaxy. This observation offers a glimpse into the early stages of nova eruptions, enhancing our understanding of these stellar processes.
Sudhanshu Barway (IIA) said, “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.”
“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 see are real sources and not bogus.”
“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.”
Journal Reference:
- Judhajeet Basu, Krishnendu S., Sudhanshu Barway, Shatakshi Chamoli, G.C. Anupama. Exploring the archives: A search for novae in UVIT snapshots of M31. The Astrophysical Journal. DOI: 10.48550/arXiv.2406.10006