A Suppressed Volumetric Rate of High-Luminosity Mid-Infrared Selected Tidal Disruption Events

Technical Summary: A Suppressed Volumetric Rate of High-Luminosity Mid-Infrared Selected Tidal Disruption Events

Problem Statement
Tidal Disruption Events (TDEs) serve as critical probes for the population of supermassive black holes (SMBHs) in galactic centers. While optical and X-ray surveys have identified approximately 100 TDEs, these searches are biased toward lower-luminosity events and local volumes. Previous infrared (IR) studies, such as Masterson et al. (2024), have successfully identified dust-obscured TDEs but were limited to the nearby universe ($z \lesssim 0.05$), thereby missing rare, highly luminous events. Furthermore, distinguishing TDEs from flaring Active Galactic Nuclei (AGN) in the IR remains a challenge. This work addresses the need to extend the census of IR-selected TDEs to the high-luminosity regime to investigate potential rate suppressions at high black hole masses and to refine selection criteria for obscured transients.

Methodology
The authors performed a systematic search of the NEOWISE archive using the unWISE project's time-resolved co-added images.

• Initial Selection: A customized pipeline based on the ZOGY algorithm identified large-amplitude transients. Candidates were cross-matched with the GLADE+ galaxy catalog (complete to $z \sim 0.2$) and restricted to nuclear transients (within 2" of a galaxy center).
• Photometric Filtering:
  • Light Curve Morphology: Selected events lasting $\ge 2$ years with smooth rise and decay profiles, excluding sources peaking in the final two years of the survey to minimize supernova contamination.
  • Luminosity Cut: Focused on sources with peak absolute Vega magnitude $M < -24$ in the WISE W2 band ($L_{peak, W2} \gtrsim 8 \times 10^{43}$ erg s$^{-1}$), reducing the sample to the 60 brightest sources.
  • Pre-flare Color: Excluded sources with pre-flare $W1-W2 > 0.8$, a signature of persistent AGN activity.
  • Color Evolution: Implemented a novel criterion based on the evolution of the $W1-W2$ color. True TDEs were required to show a stable pre-flare color, a significant reddening (increase in $W1-W2$) during the flare onset, and a subsequent return to bluer colors as the transient faded. This behavior distinguishes them from AGN, which often show high, variable pre-flare colors or different evolutionary patterns.
• Spectroscopic Follow-up: Spectra for the remaining 13 candidates were obtained using Magellan/Clay (LDSS-3), Magellan/Baade (IMACS), and MDM (OSMOS).
  • Continuum Fitting: Stellar continua were removed using pPXF.
  • Line Diagnostics: Gaussian fits to emission lines ($H\alpha$, $H\beta$, $[O III]$, etc.) were used to calculate flux ratios for BPT diagrams to separate AGN from star-forming galaxies.
  • Broad Line Rejection: Sources with broad lines ($\sigma \ge 500$ km s$^{-1}$), indicative of AGN, were excluded.
• Rate Calculation: The volumetric rate was calculated using the classical $1/V_{max}$ method (Schmidt 1968), corrected for survey completeness based on the GLADE+ catalog and Poisson statistics.

Key Results

• Sample Identification: The selection process yielded a final sample of 10 highly luminous IR-bright TDE candidates with peak luminosities $L_{peak, W2} \gtrsim 3 \times 10^{43}$ erg s$^{-1}$.
• Multi-wavelength Counterparts: Only two sources (WTP17aanbho and WTP17aaiyxe) had reported optical counterparts. In both cases, optical activity preceded the IR flare, consistent with IR emission arising from reprocessed optical/UV/X-ray light. No X-ray counterparts were detected in eROSITA, XMM-Newton, Swift, or NuSTAR data, likely due to the late-time nature of the observations or distance.
• Luminosity Function and Rate Suppression: The calculated volumetric rate for this high-luminosity sample is $1.2^{+0.5}_{-0.4} \times 10^{-10}$ Mpc$^{-3}$ yr$^{-1}$. When compared to the luminosity function of lower-luminosity IR TDEs (Masterson et al. 2024) and optical/X-ray samples, a significant "turn-over" or suppression in the event rate is observed at $\log(L_{peak, W2}/\text{erg s}^{-1}) \simeq 43.4$.
• Black Hole Mass Connection: The mean estimated black hole mass for the sample is $\log M_{BH} \approx 8.01 \pm 0.13 M_\odot$. This is consistent with the theoretical limit where the tidal radius falls within the event horizon for non-spinning SMBHs ($\sim 10^8 M_\odot$), explaining the rate suppression.

Significance and Claims
The paper claims that the observed suppression of high-luminosity events provides unambiguous evidence that the detected flares are indeed TDEs rather than AGN flares, as the suppression aligns with the physical limit of tidal disruption around massive black holes.

• Methodological Contribution: The study introduces a robust selection criterion based on the full temporal evolution of mid-IR colors ($W1-W2$). This method achieves a high purity ($\approx 80\%$) in distinguishing TDEs from AGN without requiring immediate spectroscopic follow-up, which is vital for future large-scale IR surveys (e.g., Roman Space Telescope, SPHEREx).
• Physical Insight: The results confirm that the TDE population is suppressed at high black hole masses, a phenomenon previously hinted at in optical and X-ray bands but now clearly demonstrated in the mid-IR. The authors suggest that the specific turn-over luminosity is influenced by dust reprocessing physics and bolometric corrections.
• Future Outlook: The authors state that these findings motivate further investigation into the SMBH mass and spin distributions using multi-wavelength TDE samples, though they defer a detailed flux-limited analysis to a future publication.