Early results in the search for extreme coronal line emitters with the Dark Energy Spectroscopic Instrument

TL;DR

This study reports a DESI Early Data Release search for extreme coronal line emitters (ECLEs), identifying three TDE-linked ECLEs and over 200 AGN-linked CrLs, aided by the SLEIPNIR real-time identification pipeline. By combining DESI spectroscopy with extensive multi-wavelength photometry and crossmatching to Transient Name Server and literature TDE catalogs, the authors quantify ECLE rates in three flavors: galaxy-normalized ($R_G = 5^{+5}_{-3} \times 10^{-6}\ \mathrm{galaxy}^{-1}\ \mathrm{yr}^{-1}$), mass-normalized ($R_M = 1.1^{+1.2}_{-0.6} \times 10^{-16}\ \mathrm{M}_{\odot}^{-1}\ \mathrm{yr}^{-1}$), and volumetric ($R_V = 2.3^{+10.7}_{-1.6} \times 10^{-8}\ \mathrm{Mpc}^{-3}\ \mathrm{yr}^{-1}$). The results align with prior SDSS/BOSS work and suggest that roughly 5–50% of TDEs may produce ECLE signatures, though not all ECLEs arise from TDEs. DESI’s large, nearby galaxy sample promises ~35–50 additional TDE-ECLE discoveries, while the broader CrL-AGN set highlights substantial contaminants requiring long-term, multi-wavelength follow-up to disentangle transient nuclear activity. The work also demonstrates MIR colour-luminosity relationships in CrL-TDEs and showcases the utility of automated, real-time pipelines for rare transient discoveries in large spectroscopic surveys.

Abstract

Here we present the results of our search through the Early Data Release (EDR) of the Dark Energy Spectroscopic Instrument (DESI) for extreme coronal line emitters (ECLEs) - a rare classification of galaxies displaying strong, high-ionization iron coronal emission lines within their spectra. With the requirement of a strong X-ray continuum to generate the coronal emission, ECLEs have been linked to both active galactic nuclei (AGNs) and tidal disruption events (TDEs). We focus our search on identifying TDE-linked ECLEs. We identify three such objects within the EDR sample, highlighting DESI's effectiveness for discovering new nuclear transients, and determine a galaxy-normalized TDE-linked ECLE rate of $R_\mathrm{G}=5~^{+5}_{-3}\times10^{-6}~\mathrm{galaxy}^{-1}~\mathrm{yr}^{-1}$ at a median redshift of z = 0.2 - broadly consistent with previous works. Additionally, we also identify more than 200 AGNs displaying coronal emission lines, which serve as the primary astrophysical contaminants in searches for TDE-related events. We also include an outline of the custom python code developed for this search.

Figures (34)

• Figure 1: Summary of the workflow involved in the processing and classification of the spectra from DESI EDR. Note $^1$ prior to this point in the workflow, individual spectra are treated independently, following this point all spectra obtained for a single galaxy are used to provide a single classification.
• Figure 2: Upper left:The redshift distribution of the SLEIPNIR input object sample including a per observational program breakdown highlighting the numeric dominance of the DESI-BGS sample. Note: Each targeted galaxy can be included in the selection for more than one program and as such the full sample redshift distribution is not a simple numeric sum of the individual program redshift distributions. BGS selected targets dominate the sample with 85.4% of objects being included in the BGS selection. Upper right: Breakdown of the number of observational configurations (and hence spectra) per galaxy in the SLEIPNIR input object. 23.63% of the sample have been observed in multiple configurations. Bottom: Breakdown of the DESI observational program distribution of the SLEIPNIR object input sample. 13.44% of the SLEIPNIR input object sample are part of multiple observational programs.
• Figure 3: Left: Comparison of the flagging criteria between the number of spectra flagged by SLEIPNIR before and following full classification. The highest fractions of false positives are seen in the those spectra flagged on general score and single strong features. With this being the result of a combination of low SNR, skyline contamination and the effect of single serendipitously located bright artefacts. Right: Comparison of the ECLE detection score between the number of spectra flagged by SLEIPNIR before and following full classification. As expected, those objects flagged with lower scores (i.e., fewer confident Fe CrL detections) are more likely to be assessed as false positives.
• Figure 4: Comparison of the redshift distributions of the DESI input sample and the SLEIPNIR flagged sample of candidate coronal line galaxies (solid black and dot-dashed purple curves, respectively). We also include the distributions of the coronal line galaxies selected from searches of SDSS Legacy and BOSS LOWZ galaxy samples callow_2024_rateextremecoronalcallow_2025_rateextremecoronal. The EDR coronal line galaxies are under-represented compared to the overall EDR sample between 0.11 < z < 0.22 and over-represented between 0.28 < z < 0.44. Note: As in callow_2025_rateextremecoronal, the comparison BOSS LOWZ sample has been filtered to remove galaxies at the narrow redshift bands most heavily contaminated by skyline emission features at the position of the Fe-CrLs.
• Figure 5: Comparison of the mass distributions of the DESI input sample and the SLEIPNIR flagged sample of candidate coronal line galaxies (solid black and dot-dashed purple curves, respectively). We also include the distributions of the coronal line galaxies selected from searches of SDSS Legacy and BOSS LOWZ galaxy samples callow_2024_rateextremecoronalcallow_2025_rateextremecoronal. The vertical green line marks the theoretical galaxy stellar mass limit above which a Sun-like star would fall directly into the galaxy’s SMBH instead of being disrupted as a TDE rees_1988_Tidaldisruptionstars. The relation between the stellar mass of a galaxy and the mass of its SMBH used in this calculation is from reines_2015_RELATIONSCENTRALBLACK. The peak of the EDR coronal line galaxy sample is lower than the overall EDR sample, but very few galaxies with masses below $10^9~\hbox{M$_{\odot}$}$ are detected as CrL galaxies.