An Obscured Tidal Disruption Event Uncovered by Its Mid- and Near-Infrared Dust Echo in a Star-Forming Galaxy

Technical Summary: An Obscured Tidal Disruption Event Uncovered by Its Mid- and Near-Infrared Dust Echo in a Star-Forming Galaxy

Problem Statement
Optical time-domain surveys have discovered over 200 tidal disruption events (TDEs), yet these events exhibit a puzzling host galaxy distribution: they are overrepresented in post-starburst galaxies but nearly absent in normal star-forming galaxies. This discrepancy suggests a potential selection bias where TDEs in star-forming galaxies are obscured by dust, rendering them invisible to optical surveys. While mid-infrared (MIR) surveys have identified potential obscured TDE candidates via dust echoes, the nature of the vast majority of these candidates remains unverified due to a lack of optical counterparts and multi-wavelength validation. This paper addresses the need to verify the TDE nature of such IR flares and develop tools to infer their intrinsic UV properties to correct the observed TDE population bias.

Methodology
The study focuses on SDSS J010320.39+140152.5 (J0103+1401), a nearby ($z=0.04181$) star-forming galaxy selected from the MIR Outbursts in Nearby Galaxies (MIRONG) sample. The authors employed a comprehensive multi-wavelength approach:

• Data Collection: They utilized archival data from WISE/NEOWISE (MIR), ASASSN, CRTS, ZTF, and ATLAS (optical), GALEX, SDSS, Pan-STARRS, and UKIDSS (SED), and VLA/VLASS/RACS (radio).
• Follow-up Observations: New observations were conducted using the CFHT (NIR imaging in J, H, K bands), Palomar 5m (optical spectroscopy), and Swift (X-ray).
• Analysis Techniques:
  • Image Subtraction: Used HOTPANTS to isolate the transient flux from the host galaxy in MIR and NIR bands.
  • Spectral Analysis: Fitted optical spectra with stellar and narrow emission line components using PPXF to classify the host galaxy and search for broad lines.
  • SED Modeling: Modeled the pre-flare SED with CIGALE to derive host properties (stellar mass, star formation rate, black hole mass).
  • Dust Echo Modeling: Applied a dust radiative transfer model (Lu et al. 2016) to the MIR/NIR light curves and SEDs to infer the properties of the primary UV source, accounting for dust sublimation and reprocessing.

Key Results

• Transient Characteristics: The MIR luminosity of J0103+1401 rose rapidly to a peak of $\sim 5.4 \times 10^{43}$ erg s$^{-1}$, remained high for $\sim$1 year, and then declined. No optical variability was detected in ASASSN, ZTF, or ATLAS data over a 20-year baseline.
• Localization: Near-IR follow-up pinpointed the flare to the galactic nucleus with a positional offset of $\lesssim 100$ pc ($3\sigma$ upper limit).
• Physical Properties:
  • The IR spectral energy distribution (SED) is consistent with thermal dust emission at temperatures of $\sim 900$ K (peaking) declining to $\sim 700$ K.
  • The host galaxy is a star-forming disk with a star formation rate (SFR) of $\sim 5-6$ M$_\odot$ yr$^{-1}$ and a central supermassive black hole (SMBH) mass of $\sim 10^7$ M$_\odot$.
  • Radio observations show no transient emission; the radio SED is consistent with star formation. X-ray observations yielded only upper limits ($L_X < 2.4 \times 10^{41}$ erg s$^{-1}$).
• Inferred Primary Event: Using the dust radiative transfer model, the authors inferred a primary UV flare with a peak luminosity of $\sim (4-10) \times 10^{44}$ erg s$^{-1}$, a duration of $\sim 200$ days, and a total energy release of $\sim (0.9-2) \times 10^{52}$ erg. The total IR energy derived from the light curve integration is $\sim 5.1 \times 10^{51}$ erg.

Nature of the Transient
The authors systematically ruled out alternative explanations:

• Supernova (SN): Ruled out due to the nucleus-centered position (most SNe are off-center), the total energy exceeding that of typical SNe (though comparable to rare super-luminous SNe, the host properties are inconsistent), and the MIR peak luminosity exceeding known SNe by an order of magnitude.
• Changing-Look AGN (CLAGN): Disfavored because the host showed no pre-flare AGN signatures (no broad lines, blue MIR colors), the flare amplitude ($\Delta W2 \approx 1.9$ mag) exceeds typical CLAGN variations, and the rise time ($<0.5$ yr) is faster than typical CLAGN flares.
• TDE: The event is identified as an obscured TDE. The timescales, luminosity, temperature evolution, and total energy are consistent with known IR-bright TDEs (e.g., Arp 299-B AT1). The lack of optical detection implies significant dust extinction ($A_V \gtrsim 2-5$ magnitudes).

Significance and Claims
The paper presents J0103+1401 as one of the most compelling cases to date for the emerging class of dust-obscured TDEs. The authors claim that:

1. Selection Bias: The rarity of optical TDE detections in star-forming galaxies is likely a selection bias caused by dust obscuration, rather than a physical difference in TDE rates.
2. Population Correction: IR-selected samples are crucial for uncovering the "missing" TDE population in star-forming environments. The estimated event rate for these obscured TDEs ($\sim 3 \times 10^{-5}$ gal$^{-1}$ yr$^{-1}$) is comparable to optical and X-ray TDE rates, suggesting the intrinsic TDE rate in star-forming galaxies may not be lower than in post-starburst galaxies.
3. Methodological Utility: The study demonstrates a robust method for identifying obscured TDEs via IR echoes and inferring the intrinsic UV properties of the primary flare, which is essential for constructing an unbiased UV luminosity function for TDEs.
4. Environmental Insight: The inferred dust mass ($\sim 10-140$ M$_\odot$) and covering factor at the sub-parsec scale suggest a drastic evolution of the nuclear environment in star-forming galaxies compared to passive galaxies, potentially driven by feedback from star formation or galaxy mergers.

The authors conclude that while J0103+1401 provides strong evidence for obscured TDEs, further analysis of similar IR flares is required to fully determine the TDE rate in star-forming galaxies and understand the mechanisms driving TDE production.