The universe is teeming with various astronomical objects, including stars, young stellar objects, binary systems, and compact object systems. Each of these categories emits prominently at different wavebands, necessitating the use of distinct technologies to explore the physics governing these sources. To examine X-ray emissions from astronomical objects, one must go beyond Earth's atmosphere, as X-rays are absorbed by it.

The era of X-ray astronomy started with the discovery of  X-ray emission from the first extra-solar ( outside our solar system)  source Sco X-1, located in the constellation scorpius in 1966, which was accomplished using a sounding rocket experiment. Uhuru, launched by NASA on December 12, 1970, was the first satellite dedicated to X-ray astronomy. Over the past 50-60 years, advancements in detector technology have resolved many long-standing mysteries related to high-energy emission processes from astronomical objects; however, numerous new questions have emerged. Telescopes such as Uhuru, Chandra, XMM-Newton, NICER, NuSTAR, and Astrosat have been designed for pointed observations with long exposure times on specific sources. In contrast, telescopes like ROSAT and eROSITA are tailored for all-sky surveys in X-rays, allowing them to cover a significant portion of the sky, albeit at the cost of reduced exposure time on each individual source.

Different types of X-ray emitting objects
Stars:
While sun-like stars are not particularly very active in X-rays, cooler and less massive stars exhibit higher levels of X-ray activity, primarily due to intense magnetic phenomena. Magnetic reconnection events are believed to be the main driving force behind X-ray emissions in these stars. This magnetic activity is even more pronounced in young stellar objects (YSOs), making them significantly brighter in X-rays. In contrast, stars that are more massive than the sun tend to be X-ray dark, as they exhibit not so pronounced magnetic activity. 

Compact object systems
Cataclysmic variables (CVs)
Cataclysmic variables are binary star systems in which a white dwarf pulls gas from its companion star through a process known as accretion. This accreted material creates a disk around the white dwarf due to the conservation of angular momentum. In cataclysmic variables, the accretion disk primarily emits bright optical light, while the region close to the white dwarf is responsible for the emission of X-rays.

X-ray binaries (XRB)

X-ray binaries are binary systems in which a black hole/neutron star accretes matter from the companion star. The accretion disk in these systems is hot enough to emit X-rays.

Detection of Galactic transients in the eROSITA catalog
eROSITA is an X-ray detector aboard the SRG satellite that conducted the first all-sky survey from 2019 to 2020 (i.e. eRASS1), resulting in the detection of approximately one million X-ray sources . Another X-ray detector, ROSAT, performed a similar all-sky survey from 1990 to 1991 (2RXS). In this study, we analyze Galactic sources that exhibit significant variability in flux between the 2RXS and eRASS1 catalogs. 

Histogram of ROSAT (2RXS) to eROSITA (eRASS1) fluxes, fitted with gaussian function.

Sources on the right side of the blue line and on the left side of the black line show significant flux variability between the two epochs. These sources along with the ones, which are detected only in eRASS1 but not in 2RXS, are called transients in our analysis.

To select only galactic sources, we cross-matched these transients with Gaia DR2 catalog.

In total,  we detected 738 transients and classified them into different categories like:
SS/IB (Single star or interacting binary)

YSO ( Young stellar object)

LPV ( Long period variable)

IB/ACO ( Interacting binary/ accreting compact object)

IB ( Interacting binary)

CV (Cataclysmic variables)

XRB (X-ray binary)

WR (Wolf-Rayet)

PSR (Pulsar)

Classification was done on the basis of their existing Simbad classification, X-ray spectra, position of the source in HR diagram, orbital or spin period information and their X-ray luminosities.

We have classified most of the CVs purely on the basis of their position in the HR diagram, as they occupy a very special place between white dwarfs and main sequence stars in the HR diagram.

Photometric data of ~ 70% of the transients was available in Gaia DR3. We used Lomb-Scargle periodogram to detect periodicity signatures in the transients. We report the period information of 130 transients, out of which 50 have been classified as IB/ACO. These transients have periods less than 8 hours. Typical CVs have orbital time period of this order, as also depicted in the figure below. 

Major results: 

1.All main sequence stars, including our Sun, lie on this slanted straight line, where most of the sources from the SS/IB category are found. When stars exhaust the hydrogen in their cores, their outer layers expand, causing them to become giants and move to the right off this line. We have observed that many of these giant stars are very luminous in X-rays. While Sun-like stars emit X-rays due to magnetic activity, this principle cannot be applied to giants, as they are not intrinsically very magnetically active. We suspect that these X-ray bright giant stars are either part of binary star systems, and the interaction between the components is responsible for their X-ray emission or they have some anomalous magnetic fields, whose origin and mechanism is not very well understood yet.

2. Detection of 9 new CVs: We report the detection of 9 new CVs in our transients search, which were either wrongly classified or not classified in simbad. 

X-ray spectra of all CVs in the sample, spectral fits using blackbody, thermal bremsstrahlung and APEC models (thick lines) are overplotted. Newly detected CVs data points are marked with asterisk (*).

We observed that CVs exhibit two different kinds of X-ray spectrum, as shown above.

We suspect that this distinction is associated with the magnetic nature of the white dwarf in the systems. Spectral fitting results and X-ray to optical flux ratio favors transients in the lower panel being magnetic systems. While those in the upper panel are non-magnetic systems.

3. CVs are most variable between 2RXS and eRASS1 catalogs, while LPVs are least variable.