XMM-Newton and Swift Unveil Another X-Ray Transient in NGC 4945, XMM J130514.64-493311.27

Ryan W. Pfeifle, Kimberly A. Weaver, Jenna M. Cann, Murray Brightman, Miranda McCarthy

First listed 2026-07-24 | Last updated 2026-07-23

Abstract

NGC 4945 hosts a Compton-thick, low luminosity AGN and a nuclear starburst in its core and has been the subject of many previous X-ray investigations of the AGN emission structure, nuclear starburst properties, and ultraluminous X-ray (ULX) source populations. Indeed, NGC 4945 has proven to be a rich system for ULX and X-ray transients, with at least four X-ray transients being reported over the last 15 years. Here we report the detection of an X-ray transient source, XMM J130514.64-493311.27, in NGC 4945 in the latest and deepest XMM-Newton observation of NGC 4945 to-date. The source lies $\sim5.5'$ south of the nucleus and was not detected in any previous XMM-Newton or Chandra imaging, but the source was detected by Swift-XRT in 2008, 2019, and in 2022, the latter of which coincides roughly with our new XMM-Newton imaging; we are therefore tracing the most recent outburst of this object with XMM-Newton. The source is soft with $Γ\approx3$, and our spectroscopic analysis suggests the source is best characterized by a multicolor disk + power law model or, alternatively, a multicolor disk + thermal plasma model, with a 0.3-10 keV X-ray luminosity of $\sim2.2-2.3\times10^{38}$. We find no evidence for a counterpart in the optical, near-infrared, ultraviolet, or in the radio, but we identify a candidate counterpart in NEOWISE mid-IR light curves. We propose that this X-ray source is an X-ray binary within NGC 4945 caught during its most recent active phase.

Short digest

Pfeifle et al. report XMM J130514.64-493311.27, a newly recognized off-nuclear X-ray transient 5.5 arcmin south of the Compton-thick AGN in NGC 4945, found in the deepest 2022 XMM-Newton imaging after being absent from earlier XMM-Newton and Chandra data. Its soft X-ray spectrum (Gamma approximately 3) is well described by either a multicolor disk plus power law or multicolor disk plus thermal-plasma model, with a 0.3-10 keV luminosity of about 2.2-2.3 x 10^38 erg s^-1 and no obvious flaring during the XMM-Newton exposure. Swift-XRT detections in 2008, 2019, and 2022 trace recurrent activity, while the lack of optical through radio counterparts and a possible NEOWISE mid-IR counterpart lead the authors to identify the source as an X-ray binary caught in an active phase, underscoring the transient-rich compact-object population around this nearby obscured AGN host.

Key figures to inspect

• Figure 1. This field-level 2022 XMM-Newton image locates the new transient within NGC 4945 relative to the previously cataloged ULX and X-ray-binary population, establishing that the target is an off-nuclear source rather than nuclear AGN structure.
• Figure 2. The multi-epoch XMM-Newton and stacked Chandra comparison is the clearest visual evidence for transience: the source is absent in 2001, 2004, and the 2000-2021 Chandra mosaic but prominent in 2022.
• Figure 4. The favored XMM-Newton spectral fits directly support the paper's physical interpretation, showing the soft multicolor-disk-plus-power-law model and the disk-plus-thermal-plasma alternative that yield the reported X-ray-binary-like luminosity.
• Figure 5. The Swift spectrum and long-baseline light curve place the 2022 XMM-Newton detection in a recurrent-outburst context, documenting detections in multiple epochs as well as intervening upper limits and the source's soft character.