Host galaxy properties of quasi-periodically erupting X-ray sources

Abstract

Context. Quasi-periodic X-ray eruptions (QPEs) are a recently discovered phenomenon, the nature of which remains unclear. Based on their discovery in active galactic nuclei (AGNs), explanations related to an AGN accretion disk or potentially a stellar tidal disruption event (TDE) have been put forward. Alternatives, including highly unequal mass compact object binaries, have also been proposed to explain their properties. Aims. We perform a systematic study of the five known QPE host galaxies with the aim of providing new insights as to their nature. Methods. We analysed new and archival medium resolution optical spectroscopy of the QPE hosts. We measured emission (and absorption) line fluxes, their ratios, and equivalent widths (EWs) to locate the QPE hosts on diagnostic diagrams. We also measured the velocity dispersion of the stellar absorption lines to estimate their black hole masses. Results. All QPE host galaxies show emission lines in their optical spectra. Based on their ratios and EWs, we find evidence for the presence of an AGN in all sources, including those previously reported as passive. We measure velocity dispersions between 36 and 90 km s−1, implying the presence of low mass (105−6.7 M⊙) black holes, consistent with literature findings. Finally, we find a significant over-representation (two out of the five sources, or a factor of 13−10+13) of quiescent Balmer strong (post-starburst) galaxies among QPE hosts. Conclusions. The presence of a narrow line region consistent with an AGN in all QPE host galaxies implies that a long-lived accretion flow likely plays an integral part in the QPE phenomenon. The strong over-representation of quiescent Balmer strong galaxies among QPE hosts can be naturally explained in both the TDE and interacting extreme mass ratio inspiral hypotheses.

The complex time and energy evolution of quasi-periodic eruptions in eRO-QPE1

“…This mass range is consistent with QPE hosts (Wevers et al 2022), and is also where nuclear star clusters are nearly ubiquitous (e.g., Neumayer et al 2020, for a review), which was confirmed for at least GSN069 (M19; Sheng et al 2021) and RX J1301.9+2747 (Shu et al 2017), and which enhances interactions between the nuclear black hole and stellarmass objects (Rauch 1999;Babak et al 2017;Neumayer et al 2020). At least (but see Wevers et al 2022) the QPE source RX J1301.9+2747 is a young post-starburst galaxy (Caldwell et al 1999), a type in which an enhanced rate of TDEs is observed (e.g., Arcavi et al 2014;French et al 2016) due to higher stellar density (Stone & van Velzen 2016;Stone et al 2018;Law-Smith et al 2017;Hammerstein et al 2021) compared to other much more common types of quiescent galaxies. GSN 069 shows UV line-ratios consistent with a past TDE (Sheng et al 2021), which is also supported by the long-term X-ray emission previous to the QPEs discovery (Shu et al 2018;M19).…”

“…QPE have so far been identified in nearby low-mass galaxies (with stellar masses of M star ≈ 1 − few × 10 9 M ; A21). These galaxies host black holes of M BH ≈ 10 5 − few × 10 6 M , as inferred -with the usual large uncertainties -from methods which are both independent (Shu et al 2017;M19;G20;Wevers et al 2022) and dependent on the stellar mass (Strotjohann et al 2016;A21). Given their peak X-ray luminosity (L 0.5−2.0 keV ≈ 10 42 − 10 43 erg s −1 ), the above black hole mass estimates suggest that QPEs peaks are close to being Eddington limited.…”

“…Studies of the QPEs' host galaxies have found them to be both active (Esquej et al 2007; M19; G20) and, quite surprisingly, seemingly inactive galaxies (A21). However, recent studies performing a careful subtraction of the host galaxy's stellar population have found evidence of an ionizing source in addition to star formation for the narrow lines in all five QPEs (Wevers et al 2022). Nevertheless, the absence of broad lines in the optical spectra and of infrared photometry excess associated with the dusty environment typical of active galactic nuclei (AGN), intriguingly suggests that a pre-existing canonical AGN-like accretion flow is not a pre-requisite for QPEs.…”

A Model for the Possible Connection Between a Tidal Disruption Event and Quasi-periodic Eruption in GSN 069

“…In addition, the fifth QPE candidate XMMSL1 J024916.6-041244 showed two QPE-like flares in soft X-rays in 2006 (Chakraborty et al 2021). These QPEs have a coincidence with the galactic nuclei, where all host galaxies are found to be dwarf galaxies with a total stellar mass of around m å ∼ 10 9 M e (Arcodia et al 2021) and black hole (BH) mass of ∼ 10 5−6 M e (e.g., Wevers et al 2022). Interestingly, historical data for GSN 069 shows that it was a quiescent galaxy in 1989, while its X-ray emission was brightened by a factor of at least 240 in 2010 (Miniutti et al 2013;Shu et al 2018).…”

Modified models of radiation pressure instability in application to 10, 10$^5$, and 10$^7$ $M_{\odot}$ accreting black holes

“…For those 4 sources the lack of broad emission lines in the optical band is also characteristic, and this feature rules out the possibility to estimate black hole masses independently using broad line diagnostics. As Wevers et al (2022) point out, emission lines noticed in those sources are typical of star-forming or accreting galaxies.…”