AstroNote 2024-99

AT 2024eff was reported as a newly detected brightening event coincident with the center of the galaxy LEDA 858238 / 2MASS J05514189-1913061. The discovery was based on the significant brightening detected in the Gaia lightcurve (Gaia24axr). The event was classified as a TDE based on an NTT/EFOSC2 classification spectrum obtained on March 14, 2024 (by the ePESSTO+ team). That classification report (AstroNote 2024-74) mentions a possible alternative interpretation of “a type I Seyfert AGN”, likely based on the broad Balmer emission lines clearly seen in the classification spectrum.

Here we argue that AT 2024eff is a Changing Look/State AGN, based on analysis of archival and newly obtained data.

An optical spectrum of the system was obtained as part of the 6 degree Field Survey  (6dFGS, Jones et al. 2004;  MNRAS, 355, 747), where the source is designated as 6dF J0551419-191306, on November 5, 2004. This spectrum, publicly available through the NASA Extragalactic Database (NED), shows only narrow emission lines, specifically strong [OIII]5007,4959 and [NII]6584 emission lines, with very weak (visually nearly absent) Hbeta emission. We performed a detailed analysis of the key narrow emission lines in this archival spectrum and found that the archival spectrum would classify the galaxy as an AGN based on strong emission line ratio diagnostics (Baldwin et al. 1981; PASP, 93, 5; Kewley et al. 2006; MNRAS, 372, 961).

Second, there are archival detections of the source in both the UV (with GALEX; 2006 January), X-rays (with ROSAT; 1990 August) and in the radio (with NVSS; 1993 October). 

The soft X-ray flux in the 0.1-2.4 keV band is F(0.1-2.4 keV) = (1.64 +/- 0.40)e-12 erg/s/cm^2, which at the redshift of the source (z = 0.059538) translates to L(0.1-2.4 keV) = 1.4e43 erg/s. Assuming an X-ray photon index of Gamma = 2, this translates to fluxes of F(0.5-2 keV) = (6.9+/-1.7)e-13 erg/s/cm^2 and F(2-10 keV) = 8.0 +/- 2.0 e-13 erg/s/cm^2, and luminosities of L(0.5-2 keV) = 5.9e+42 erg/s  and  L(0.5-2 keV) = 6.8e+42 erg/s. These are consistent with X-ray luminosities of (low-redshift) AGN, and far exceed what is seen in inactive, star-forming galaxies (implied SFR would be >1000 M_sol/yr; Ranalli et al. 2003, A&A 399, 39). 

The radio flux density in the 1.4GHz band is F_nu(1.4GHz) = 3.8 +/- 0.6 mJy, which translates to L_nu(1.4 GHz) = 3.2e22 W/Hz or nu*L_nu(1.4 GHz) = 2.7e37 erg/s . If this radio emission originates from star formation processes, the implied SFR would be ~18 M_sol/yr (Hopkins et al. 2003, ApJ 599, 971), which is rather high, but not uncommon, for low-redshift star-forming galaxies, and could perhaps again point towards some AGN activity. The NUV GALEX measurement, at NUV = 20.29 +/- 0.12 (AB mag) corresponds to nu*L_nu (NUV) = 3.2e+42 erg/s. This level of UV emission can be accounted for by star formation (e.g., Salim et al., 2007; ApJS, 173, 267).

We conclude that the archival data in hand strongly support the host system of AT 2024eff being a narrow-line AGN, as late as 2004.

As noted, the optical classification spectrum shows broad Balmer emission lines, as well as a blue continuum - typical of unobscured AGN. In addition to the EFOSC2 classification spectrum, we obtained two FLOYDS spectra with the Las Cumbres Observatory Faulkes Telescope South at Siding Spring Observatory, on 2024 Mar 14 and 26. Both spectra are highly consistent with each other, and confirm the blue continuum emission & prominent, broad Balmer emission lines seen in the EFOSC2 spectrum. Likewise, there’s no evidence for a prominent and (extremely) broad emission feature near HeII 4681, commonly seen in the spectra of (UV/optical) TDEs. All three spectra (EFOSC2 and two FLOYDS) are thus consistent with a broad-line AGN interpretation. 

A preliminary analysis of the EFOSC2 classification spectrum (2024 Mar 14) and the 1st FLOYDS spectrum (2024 Mar 14) with the PyQSOFit package (Guo et al. 2018; http://ascl.net/1809.008) suggests Balmer line widths of roughly 10,000 km/s, and monochromatic optical continuum luminosity of “L_5100” = lambda*L_lambda(5100A) = 2.0e43 erg/s for  EFOSC2 and 3.2e43  erg/s  for FLOYDS. Combining these with “virial” prescription of black hole mass (e.g., those of Mejia-Restrepo et al. 2022; ApJS 261, 5) implies log(M_BH / M_sun)  in the range 8-8.5. Assuming a bolometric luminosity of L_bol = 9*L_5100 = 1.8e44 erg/s for  EFOSC2 and 2.8e44 for  FLOYDS implies an Eddington ratio of L/L_Edd of order 0.01.

We conclude that all data in hand indicate that AT 2024eff is a (dramatic) brightening event in an AGN, classifying the AGN itself as part of the “changing look/state” class. The origin of the event remains unknown, but could hopefully be elucidated with multi-wavelength follow-up observations.