Transient X-ray Sources in the Magellanic-type Galaxy NGC 4449

TL;DR

This study identifies and characterizes seven transient X-ray sources in NGC 4449, revealing diverse types including supersoft sources and X-ray binaries, with detailed spectral and variability analysis over multiple years.

Contribution

First comprehensive analysis of transient X-ray sources in NGC 4449, including spectral, variability, and counterpart identification, expanding understanding of such sources in Magellanic-type galaxies.

Findings

Two supersoft X-ray sources consistent with accreting white dwarfs

Detection of short-term variability with a possible 2.3-hour period

X-ray properties similar to those in other Magellanic-type galaxies

Abstract

We report the identification of seven transient X-ray sources in the nearby Magellanic-type galaxy NGC 4449 using the archival multi-epoch X-ray observations conducted with {\it Chandra}, {\it XMM-Newton} and {\it Swift} telescopes over year 2001--2013. Among them, two sources are classified as supersoft X-ray sources (SSSs) because of their soft X-ray color and rest of the sources are X-ray binaries (XRBs). Transient SSSs spectra can be fitted with a blackbody of effective temperature $\sim 80-105$ eV and luminosities were $\simeq 10^{37} - 10^{38} {\rm~erg\ s}^{-1}$ in 0.3--8 keV. These properties are consistent with the widely accepted model for SSSs, an accreting white dwarf with the steady nuclear burning on its surface, while the SSS emission has also been observed in many post-nova systems. Detailed analysis of one sufficiently bright SSS revealed the strong short-term variability, possibly showing a 2.3 hour periodic modulation, and long-term variability, detectable over 23 years with different X-ray telescopes before year 2003. The X-ray properties of four other transients are consistent with neutron star or black hole binaries in their hard state, while the remaining source is most likely an XRB with a quasi-soft X-ray spectrum. Analysis of archival {\it Hubble Space Telescope} image data was also conducted, and multiple massive stars were found as possible counterparts. We conclude that the X-ray transient properties in NGC 4449 are similar to those in other Magellanic-type galaxies.