| Abstract : | Ultraluminous X-ray Sources (ULXs) are non nuclear X-ray point sources whose luminosity exceeds 1039 erg/s (Makishima et al. 2000). There are persistent as well as transient ULXs where persistent ULXs have luminosities always above 10^39 erg/s and they can be explained by binary systems which host a stellar mass compact object with super Eddington accretion rate. But transient ULXs have luminosities sometimes above 10^39 erg/s, sometimes below 10^39 erg/s and go undetected. The duration of outburst varies from a few days (M 101 - ULX 1) to a few years (NGC 3628 – ULX). Kalogera et al. (2004) suggest that mass transfer from massive stars to Black Holes produces unstable disks and thus a transient X-ray source. Hameury and Lasota (2020) suggest that the thermal–viscous instability is known to drive outbursts of dwarf novae and low-mass X-ray binary transient sources. There are ULXs that are highly variable (both persistent and transient) that are found to be hosting magnetized neutron stars. So, transient ULXs are very interesting sources.
We report detection of a transient ULX in NGC 5813, a central dominant giant elliptical galaxy where ULXs are not very common. We used Chandra ACIS observations and XMM-Newton EPIC observations of NGC 5813 carried out in 2005, 2008, 2009, and 2011. In Chandra ACIS observation of 2005, the source was not detected and in 2008, the source luminosity was below 10^39 erg/s. In 2011, the source luminosity exceeded 10^39 erg/s. The spectrum of this ULX can be fitted with an absorbed powerlaw or an absorbed multicolor disk black body. We could not find any counterparts for this source in FUV and visible wavelengths. We search for the intra-observational and inter-observational variability for the source and also try to estimate the duty cycle. |
