Quintin and colleagues reported the discovery of NGC 7793 ULX-4, a new transient ultraluminous X-ray source (ULX) in NGC 7793, a spiral galaxy already well known for harbouring several ULXs. Previously, NGC 7793 P13, a pulsating ULX also discovered in same galaxy. Their study recently appeared on Journal Monthly Notices of the Royal Astronomical Society.
Ultraluminous X-ray sources (ULXs) are extragalactic non-nuclear sources whose luminosity exceed the Eddington limit for a stellar mass compact object (∼ 3 × 1039 erg.s¯1), while their non-nuclear position excludes AGNs. They were first discovered using the Einstein Laboratory.
Quintin and colleagues studied NGC 7793 ULX-4, using a wealth of multi-wavelength data from XMM-Newton, the Neil Gehrels Swift Observatory (hereby Swift), and Chandra in X-rays and from the HST and the VLT Multi Unit Spectroscopic Explorer (hereby MUSE) in the optical.
This new source underwent an outburst in 2012, when it was detected by XMM-Newton and the Swift X-ray telescope. The outburst reached a peak luminosity of 3.4× 1039 erg s¯1 and lasted for about 8 months, after which the source went below a luminosity of 1037 erg s¯1; previous Chandra observations constrain the low-state luminosity below ∼ 2×1036 erg s¯1, implying a variability of at least a factor 1000.
Quintin and colleagues proposed four possible optical counterparts, found in archival HST observations of the galaxy and found that the pulse period of NGC 7793 ULX-4 is surprisingly close to that of P13 (2.4 Hz, Israel et al. 2017b), but the spin-up (¤𝑓) values are extremely different (2.2 × 10¯10 Hz.s¯1 for P13 compared to 3.5 × 10¯8 Hz.s¯1 for ULX-4), which excludes a possible contamination of the observation by P13. Finally, the fact that P13 was in a low–luminosity state at the time of the peak luminosity for ULX-4 (see Featured image), and that the two ULXs are separated by 155″, further excludes the possibility of a contamination by P13.
Assuming that the pulsation detected for ULX-4 is confirmed, NGC 7793 is the first known galaxy hosting two pulsating ULXs.
Other observations of NGC 7793 ULX-4 during an outburst are needed to confirm the pulse period and determine the first and second period derivatives. Further spectral studies over the outburst would help to confirm the physical nature of the processes behind the evolution of luminosity, and would also allow to rigorously study the presence emission lines and their possible blueshift. Finally, optical observations during the outburst would be useful to identify which of the possible counterparts is the true counterpart.
“This source has been quiescent since its first and only detected outburst, in May 2012; the constant monitoring of the nearby ULX NGC 7793 P13 will however undoubtedly allow us to detect a potential new high activity period in ULX-4.”— told Quintin, first author of the study
Featured image: A view from the XMM-Newton observation 0693760101. The green circle indicates the newly discovered ULX in its highest detected state. This 40″ radius circular region was used to extract the photons in the analysis. The blue circles correspond to the known ULXs in NGC 7793, respectively P9 on the top left and P13 on the bottom right, which was the target of the observation and in a low state at this time. The field of view is about 360″×560″, and roughly covers the optical extent of the galaxy © Quintin et al.
Reference: E Quintin, N A Webb, A Gúrpide, M Bachetti, F Fürst, A new candidate pulsating ULX in NGC 7793, Monthly Notices of the Royal Astronomical Society, 2021;, stab814, https://doi.org/10.1093/mnras/stab814
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