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The Faintest, Extremely Variable X-ray Tidal Disruption Event from a Supermassive Black Hole Binary?

Mengqiu Huang, Yongquan Xue, Shuo Li, Fukun Liu, Shifu Zhu, Jin-Hong Chen, Rong-Feng Shen, Yibo Wang, Yi Yang, Ning Jiang, Franz Erik Bauer, Cristian Vignali, Fan Zou, Jialai Wang, Alexei V. Filippenko, Bin Luo, Chen Qin, Jonathan Quirola-Vásquez, Jun-Xian Wang, Lulu Fan, Mouyuan Sun, Qingwen Wu, Qingling Ni, Thomas G. Brink, Tinggui Wang, Weikang Zheng, Xinwen Shu, Xuechen Zheng, Xiaozhi Lin, Xu Kong, Yijun Wang, Yibin Luo, Zheyu Lin

Abstract

Tidal disruption events (TDEs), which occur when stars enter the tidal radii of supermassive black holes (SMBHs) and are subsequently torn apart by their tidal forces, represent intriguing phenomena that stimulate growing research interest and pose an increasing number of puzzles in the era of time-domain astronomy. Here we report an unusual X-ray transient, XID 935, discovered in the 7 Ms Chandra Deep Field-South, the deepest X-ray survey ever. XID 935 experienced an overall X-ray dimming by a factor of more than 40 between 1999 and 2016. Not monotonically decreasing during this period, its X-ray luminosity increased by a factor $> 27$ within 2 months, from $L_{\rm 0.5-7\ keV}<10^{40.87}$ erg s$^{-1}$ (10 October 2014 -- 4 January 2015) to $L_{\rm 0.5-7\ keV}=10^{42.31\pm 0.20}$ erg s$^{-1}$ (16 March 2015). The X-ray position of XID 935 is located at the center of its host galaxy with a spectroscopic redshift of 0.251, whose optical spectra do not display emission characteristics associated with an active galactic nucleus. The peak 0.5--2.0 keV flux is the faintest among all the X-ray-selected TDE candidates to date. Thanks to a total exposure of $\sim 9.5$ Ms in the X-ray bands, we manage to secure relatively well-sampled, 20-year-long X-ray light curves of this deepest X-ray-selected TDE candidate. We find that a partial TDE model could not explain the main declining trend. An SMBH binary TDE model is in acceptable accordance with the light curves of XID 935; however, it fails to match short-timescale fluctuations exactly. Therefore, the exceptional observational features of XID 935 provide a key benchmark for refining quantitative TDE models and simulations.

The Faintest, Extremely Variable X-ray Tidal Disruption Event from a Supermassive Black Hole Binary?

Abstract

Tidal disruption events (TDEs), which occur when stars enter the tidal radii of supermassive black holes (SMBHs) and are subsequently torn apart by their tidal forces, represent intriguing phenomena that stimulate growing research interest and pose an increasing number of puzzles in the era of time-domain astronomy. Here we report an unusual X-ray transient, XID 935, discovered in the 7 Ms Chandra Deep Field-South, the deepest X-ray survey ever. XID 935 experienced an overall X-ray dimming by a factor of more than 40 between 1999 and 2016. Not monotonically decreasing during this period, its X-ray luminosity increased by a factor within 2 months, from erg s (10 October 2014 -- 4 January 2015) to erg s (16 March 2015). The X-ray position of XID 935 is located at the center of its host galaxy with a spectroscopic redshift of 0.251, whose optical spectra do not display emission characteristics associated with an active galactic nucleus. The peak 0.5--2.0 keV flux is the faintest among all the X-ray-selected TDE candidates to date. Thanks to a total exposure of Ms in the X-ray bands, we manage to secure relatively well-sampled, 20-year-long X-ray light curves of this deepest X-ray-selected TDE candidate. We find that a partial TDE model could not explain the main declining trend. An SMBH binary TDE model is in acceptable accordance with the light curves of XID 935; however, it fails to match short-timescale fluctuations exactly. Therefore, the exceptional observational features of XID 935 provide a key benchmark for refining quantitative TDE models and simulations.

Paper Structure

This paper contains 29 sections, 1 equation, 6 figures, 1 table.

Table of Contents

  1. GRAPHICAL ABSTRACT
  2. PUBLIC SUMMARY
  3. ABSTRACT
  4. KEYWORDS
  5. INTRODUCTION
  6. MATERIALS AND METHODS
  7. RESULTS AND DISCUSSIONS
  8. CONCLUSIONS
  9. RESOURCE AVAILABILITY
  10. FUNDING AND ACKNOWLEDGMENTS
  11. AUTHOR CONTRIBUTIONS
  12. DECLARATION OF INTERESTS
  13. SUPPLEMENTAL INFORMATION
  14. LEAD CONTACT WEBSITE

Figures (6)

  • Figure 1: Adaptive-bin X-ray light curves of XID 935. (a) Light curve in the rest-frame 0.5--7.0 keV band (full band, containing $768.9\pm43.3$ and $905.0\pm84.0$ Chandra and XMM-Newton net counts, respectively). (b) Light curve in the rest-frame 0.5--2.0 keV band (soft band, containing $595.9\pm28.5$ and $680.7\pm62.6$ Chandra and XMM-Newton net counts, respectively). Red, dark blue, green, black, and orange data are from Chandra, XMM-Newton, Swift, ROSAT, and eROSITA, respectively. Grey data represent individual observations of Chandra and XMM-Newton. Horizontal bars indicate ranges of time bins. Arrows indicate 90%-confidence-level upper limits. The data gaps between 2002–2004 and 2005–2007 indicate no X-ray observational coverage. Dashed lines denote the observation times of optical spectra.
  • Figure 2: SED fitting result for XID 935 using CIGALE. The stellar mass $M_{\star}=(3.1\pm0.6)\times10^{10}\ M_{\odot}$ and the star-formation rate SFR$<1.3\times10^{-2} M_{\odot}$/year (90% confidence level) were obtained. Note that an AGN component is not required in the fitting.
  • Figure 3: Optical spectra of XID 935. The spectral analysis of the Keck/LRIS spectrum (blue), performed on 8 September 2021, shows that the host galaxy of XID 935 has no significant AGN characteristics. The flux upper limit of the [O iii@] emission line is $10^{-16.68}$ erg s$^{-1}$ cm$^{-2}$ and the stellar velocity dispersion is $138\pm{31}$ km/s. The VLT/VIMOS spectrum (green) was obtained on 30 October 2006 (Balestra et al. 2010). The VLT/FORS spectrum (orange) was observed on 18-20 September 2001 (Szokoly et al. 2004).
  • Figure 4: Multi-wavelength images of XID 935. (a) 7 Ms CDF-S image in the observed 0.5--7.0 keV band, with total net counts of $768.9\pm43.3$. The green polygon is the 90% encircled-energy fraction region, centered at the source position (green plus; with a 1-$\sigma$ positional accuracy of 0.3 arcsec, indicated by the green circle, corresponding to 0.99 kpc at $z=0.251$). (b) GOODS z band image. (c) JWST-NIRCam F227W band image. The green pluses and circles in Panels (b) and (c) are the same as that in Panel (a). The position of XID 935 is consistent with the nucleus of its host galaxy.
  • Figure 5: X-ray-selected TDEs. (a) Distribution of absorption-corrected peak 0.5--2.0 keV fluxes of X-ray-selected TDEs, with the vertical red dashed line highlighting XID 935. (b) Absorption-corrected peak 0.5--2.0 keV luminosities as a function of redshift, with XID 935 marked with a star symbol and other X-ray selected TDEs represented by blue points. The purple dashed curve is the 5-sigma detection limit of EP/WXT in an individual observation (i.e., 1.2-ks exposure).
  • ...and 1 more figures