Observations investigate ultraluminous X-ray sources in the galaxy NGC 5813
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Using NASA's Chandra and ESA's XMM-Newton space observatories, Indian astronomers have explored the population of ultraluminous X-ray sources in the galaxy NGC 5813, which resulted in the detection of a new source of this type. Results of the observational campaign were August 7 on the pre-print server arXiv.
Ultraluminous X-ray sources (ULXs) are extragalactic point-like sources extremely bright in X-rays—emitting more radiation than one million suns emit at all wavelengths. Although they have been proposed to host neutron stars or stellar-mass black holes, their true nature still remains elusive.
NGC 5813 is a giant elliptical galaxy with regular morphology, at a distance of some 105 million light years. It is a member of the NGC 5846 group of galaxies in the Virgo supercluster.
Previous observations have found that NGC 5813 hosts a supermassive black hole with a mass of around 280 million solar masses and an associated active galactic nucleus (AGN) that is a known radio source. It harbors a kinematically distinct core and hosts both red and blue globular cluster populations.
NGC 5813 is assumed to contain an unusually high number of ULXs and to date eight such sources have been identified in this galaxy. The population of ULXs in this galaxy has recently been investigated by the team of astronomers led by T. R. Rajalakshmi of the Mahatma Gandhi University in Kerala, India.
"It has been reported that NGC 5813, the central dominant (cD) galaxy in the NGC 5846 group of galaxies, which shows signs of a possible recent merger event, has an unusually high number of ULXs. We have undertaken a multi-epoch spectral study of the persistent ULXs in the galaxy using Chandra and XMM-Newton observations," the researchers wrote in the paper.
Chandra and XMM-Newton allowed Rajalakshmi's team to re-identify four known ULXs in NGC 5813 and to find that two other previously reported ULXs are eclipsing binaries and foreground sources. Most importantly, the observations resulted in the detection of a new ULX in this galaxy, which received designation CXOJ150101.11+014119.80 (S4).
According to the study, CXOJ150101.11+014119.80 (S4) was detected approximately 2.65 arcminutes from the center of NGC 5813. The re-identified ULXs were found at distances between 0.43 and 1.97.
The most luminous ULX out of the five investigated in the study turned out to be one of those reported by previous observations, designated CXOJ150116.555+014133.97 (S5), with an average luminosity of 14.5 duodecillion erg/s. The other four have average luminosities within the range of 1.5-3.82 duodecillion erg/s.
The observations found that none of the ULXs in NGC 5813 show intra-observational variability. However, CXOJ150116.555+014133.97 (S5) exhibits an inter-observational fractional variability of approximately 15.1%. The researchers noted that there is no unambiguous evidence of longer-term (over years) variability for the investigated ULXs.
The study also found that one of the inspected ULXs in NGC 5813, known as CXOJ150104.927+014136.02 (S6), has an average photon index of less than 1.0. This suggests that it may host a neutron star; therefore, the authors of the paper have classified this source as a possible candidate ULX pulsar.
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More information: T. R. Rajalakshmi et al, Ultraluminous X-ray sources in the group-centric elliptical galaxy NGC 5813, arXiv (2025).
Journal information: arXiv
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