[Paper Review] First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z<0.6

S. Sazonov, M. Gilfanov+23 more|arXiv (Cornell University)|Aug 5, 2021

Astrophysical Phenomena and Observations4 citations

TL;DR

This study presents the first 13 tidal disruption events (TDEs) discovered by the SRG/eROSITA all-sky survey, analyzing their X-ray and optical properties. It finds a TDE X-ray luminosity function with a power-law slope of −0.6±0.2 and a total rate of (1.1 ± 0.5) × 10⁻⁵ TDEs per galaxy per year—significantly lower than optical surveys—suggesting X-ray-bright TDEs are a minority, likely viewed along the accretion disk axis.

ABSTRACT

We present the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected in the 0<l<180 deg hemisphere by eROSITA during its second sky survey (10 June - 14 December 2020) and confirmed by optical follow-up observations. The most distant event occurred at z=0.581. One TDE continued to brighten at least 6 months. The X-ray spectra are consistent with nearly critical accretion onto black holes of a few 10^3 to 10^8 M_Sun, although supercritical accretion is possibly taking place. In two TDEs, a spectral hardening is observed 6 months after the discovery. Four TDEs showed an optical brightening apart from the X-ray outburst. The other 9 TDEs demonstrate no optical activity. All 13 TDEs are optically faint, with Lopt/Lx<0.3 (Lopt and Lx being the g-band and 0.2-6 keV luminosity, respectively). We have constructed a TDE X-ray luminosity function, which can be fit by a power law with a slope of -0.6+/-0.2, similar to the trend observed for optically selected TDEs. The total rate is estimated at (1.1+/-0.5)10^-5 TDEs per galaxy per year, an order of magnitude lower than inferred from optical studies. This suggests that X-ray bright events constitute a minority of TDEs, consistent with models predicting that X-rays can only be observed from directions close to the axis of a thick accretion disk formed from the stellar debris. Our TDE detection threshold can be lowered by a factor of ~2, which should allow a detection of ~700 TDEs by the end of the SRG survey.

Motivation & Objective

• To identify and characterize the first tidal disruption events (TDEs) discovered by the SRG/eROSITA all-sky survey.
• To investigate the X-ray and optical properties of these TDEs, including their luminosity function and spectral evolution.
• To compare the observed TDE rate from X-ray surveys with those from optical surveys to understand selection biases.
• To test models of TDE emission, particularly those involving orientation-dependent obscuration by thick accretion disks.
• To assess the detectability threshold of eROSITA and project the total number of TDEs expected by the end of the survey.

Proposed method

• Identified X-ray transients in the 0 < l < 180° hemisphere during eROSITA’s second sky survey (June–December 2020).
• Confirmed TDE candidates via multi-wavelength optical follow-up observations using ground-based telescopes (BTA, AZT-33IK, RTT150, CMO, etc.).
• Analyzed X-ray light curves and spectra in the 0.3–6 keV band to determine luminosity, spectral shape, and evolution.
• Conducted spectroscopy and photometry in g-band and other optical filters to assess optical activity and host galaxy properties.
• Fitted host galaxy SEDs using FSPS and Prospector packages to estimate stellar population parameters and host luminosity.
• Constructed a TDE X-ray luminosity function and derived the total TDE rate per galaxy per year using the observed sample.

Experimental results

Research questions

• RQ1 What are the X-ray and optical properties of the first TDEs discovered by eROSITA during the SRG all-sky survey?
• RQ2 How does the TDE X-ray luminosity function derived from this X-ray-selected sample compare to those from optical surveys?
• RQ3 Why is the observed TDE rate in X-rays significantly lower than in optical surveys, and what does this imply about TDE selection effects?
• RQ4 Do spectral hardening or optical brightening events occur in X-ray-selected TDEs, and what do they indicate about accretion physics?
• RQ5 What is the expected detection yield of TDEs by eROSITA over the full survey, and how does the current detection threshold compare to the theoretical limit?

Key findings

• Thirteen TDEs were discovered by eROSITA during its second sky survey, with redshifts up to z = 0.581, and one event brightened in X-rays for at least six months.
• X-ray spectra are consistent with near-critical accretion onto black holes of masses between a few ×10³ and 10⁸ M⊙, with possible supercritical accretion in some cases.
• Spectral hardening was observed in two TDEs approximately six months after discovery, indicating evolving accretion conditions.
• Four TDEs showed optical brightening coincident with X-ray outbursts, while nine remained optically faint, with Lg/LX < 0.3.
• The TDE X-ray luminosity function is well-fit by a power law with a slope of −0.6 ± 0.2, similar to optical samples, but the total rate is (1.1 ± 0.5) × 10⁻⁵ TDEs per galaxy per year—about an order of magnitude lower than optical estimates.
• The low X-ray rate suggests that X-ray-bright TDEs are a minority, likely viewed close to the accretion disk axis, consistent with orientation-dependent obscuration models; the detection threshold could be improved by a factor of ~2, enabling detection of ~700 TDEs by the end of the survey.