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A supermassive black hole under the radar: repeating X-ray variability in a Seyfert galaxy

Matteo Imbrogno, Andrea Sacchi, Giovanni Miniutti, Francesco Tombesi, Gian Luca Israel, Enrico Piconcelli, Roberta Amato

TL;DR

The paper reports a repeating X-ray variability pattern in the Seyfert galaxy J1257, located in the Coma cluster, spanning ~20 years of Chandra and XMM-Newton data. The authors identify a modulating signal with a period of $P\approx 25.4\pm0.4$ ks (and a low-frequency component near $\nu\simeq3.3\times10^{-5}$ Hz) in the 2020 Chandra observations, accompanied by long-term spectral changes including a softening trend when brighter and episodic intrinsic absorption. They consider two interpretations for the variability: a quasi-periodic oscillation (QPO) tied to inner-disk dynamics or quasi-periodic eruptions (QPEs) akin to other low-mass SMBHs, noting that the available data provide only a few cycles and cannot yield a definitive classification. The findings place J1257 as a potential new member of the SMBH population exhibiting extreme, short-timescale X-ray variability, with implications for EMRI/IMRI physics and possible connections to future gravitational-wave observatories such as LISA. Further uninterrupted, long-baseline X-ray monitoring is required to confirm the nature of the variability and to refine timing-spectral correlations.

Abstract

In the last few years, a few supermassive black holes (SMBHs) have shown short-term (of the order of hours) X-ray variability. Given the limited size of the sample, every new addition to this class of SMBHs can bring invaluable information. Within the context of an automated search for X-ray sources showing flux variability in the \textit{Chandra} archive, we identified peculiar variability patterns in 2MASX J12571076+2724177 (J1257), a SMBH in the Coma cluster, during observations performed in 2020. We investigated the long-term evolution of the flux, together with the evolution of the spectral parameters throughout the \textit{Chandra} and \textit{XMM-Newton} observations, which cover a time span of approximately 20 years. We found that J1257 has repeatedly shown peculiar variability over the last 20 years, on typical timescales of $\simeq20-25$ ks. From our spectral analysis, we found hints of a softer-when-brighter behaviour and of two well-separated flux states. We suggest that J1257 might represent a new addition to the ever-growing size of relatively low mass SMBHs ($M\simeq10^6-10^7\mathrm{M}_\odot$) showing extreme, possibly quasi-periodic X-ray variability on short time scales. The available dataset does not allow for a definitive classification of the nature of the variability. However, given the observed properties, it could either represent a quasi-periodic oscillation at particularly low frequency or be associated with quasi-periodic eruptions in an AGN with peculiar spectral properties.

A supermassive black hole under the radar: repeating X-ray variability in a Seyfert galaxy

TL;DR

The paper reports a repeating X-ray variability pattern in the Seyfert galaxy J1257, located in the Coma cluster, spanning ~20 years of Chandra and XMM-Newton data. The authors identify a modulating signal with a period of ks (and a low-frequency component near Hz) in the 2020 Chandra observations, accompanied by long-term spectral changes including a softening trend when brighter and episodic intrinsic absorption. They consider two interpretations for the variability: a quasi-periodic oscillation (QPO) tied to inner-disk dynamics or quasi-periodic eruptions (QPEs) akin to other low-mass SMBHs, noting that the available data provide only a few cycles and cannot yield a definitive classification. The findings place J1257 as a potential new member of the SMBH population exhibiting extreme, short-timescale X-ray variability, with implications for EMRI/IMRI physics and possible connections to future gravitational-wave observatories such as LISA. Further uninterrupted, long-baseline X-ray monitoring is required to confirm the nature of the variability and to refine timing-spectral correlations.

Abstract

In the last few years, a few supermassive black holes (SMBHs) have shown short-term (of the order of hours) X-ray variability. Given the limited size of the sample, every new addition to this class of SMBHs can bring invaluable information. Within the context of an automated search for X-ray sources showing flux variability in the \textit{Chandra} archive, we identified peculiar variability patterns in 2MASX J12571076+2724177 (J1257), a SMBH in the Coma cluster, during observations performed in 2020. We investigated the long-term evolution of the flux, together with the evolution of the spectral parameters throughout the \textit{Chandra} and \textit{XMM-Newton} observations, which cover a time span of approximately 20 years. We found that J1257 has repeatedly shown peculiar variability over the last 20 years, on typical timescales of ks. From our spectral analysis, we found hints of a softer-when-brighter behaviour and of two well-separated flux states. We suggest that J1257 might represent a new addition to the ever-growing size of relatively low mass SMBHs () showing extreme, possibly quasi-periodic X-ray variability on short time scales. The available dataset does not allow for a definitive classification of the nature of the variability. However, given the observed properties, it could either represent a quasi-periodic oscillation at particularly low frequency or be associated with quasi-periodic eruptions in an AGN with peculiar spectral properties.

Paper Structure

This paper contains 15 sections, 38 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Chandra
  4. XMM-Newton
  5. Data analysis and results
  6. X-ray timing analysis
  7. Recurrent modulating behaviour
  8. Flare-like behaviour
  9. X-ray spectral analysis
  10. Discussion
  11. Quasi-periodic modulation candidate
  12. J1257 as a possible QPE candidate
  13. Conclusions
  14. Light curves
  15. Spectra

Figures (38)

  • Figure 1: Chandra light curve (black points) in the 0.5--7 keV band of ObsIDs 22648, 22649, and 23182. The bin time is 1500 s. The best-fit sinusoidal is shown in red. The dotted line shows the model's extrapolation during time intervals with no data.
  • Figure 2: LSPs of the C1 dataset. Top panel: LSP of observations 22648 and 22649. Bottom panel: LSP of the whole C1 dataset (bottom). The LSPs have been rebinned with a logarithmic factor of 1.14. The red dashed lines show the frequency of the peak found in the top LSP. At frequencies higher than $\nu\simeq0.01$ Hz, white noise dominates. Therefore, for visual purposes, we show the LSPs only up to 0.01 Hz.
  • Figure 3: Phase-folded profile of the Chandra's 2020 light curve. Top panel: folded light curve in the whole 0.5--7 keV band. Middle panel: folded light curve in the soft 0.5--2 keV band. Bottom panel: folded light curve in the hard 2--7 keV band. The input period in the three panels is the same as that of the best-fit sinusoid shown in Figure \ref{['fig:3BestChandraLC']} ($P=25.4\pm0.4$ ks). $A$ is the semi-amplitude of the modulation. Two cycles are shown in each panel for clarity.
  • Figure 4: Light curve of J1257 during the XMM-Newton observation 0691610201. Top panel: light curve in the soft (0.3--1 keV) and hard (1--10 keV) band. Bottom panel: hardness ratio of the hard count rate over the soft count rate. The red (dashed purple) line shows the result of the fit of the hardness ratio with a constant outside (within) the high-flux phase (highlighted in blue). The corresponding shaded region shows the 1$\sigma$ confidence interval. The bin time in both panels is 1500 s.
  • Figure 5: X-ray spectrum (upper panel) and residuals (lower panel) of the J1257, taken with XMM-Newton in June 2000 (ObsID 0124710101). In black EPIC/pn, in red EPIC/MOS1, and in magenta EPIC/MOS2 data. The best-fitting model is shown by a solid line, the two components, black body and power-law, are shown by the dotted black lines.
  • ...and 33 more figures