The Luminosity Function of Tidal Disruption Flares for the ZTF-I Survey

Abstract

The high-cadence survey of the Zwicky Transient Facility (ZTF) has completely dominated the discovery of tidal disruption events (TDEs) in the past few years and resulted in the largest sample of TDEs with optical/UV light curves well sampled around their peaks, providing us an excellent opportunity to construct a peak luminosity function (LF) of tidal disruption flares (TDFs). The new construction is necessary particularly considering that the most updated LF reported in literature has been inferred from only 13 sources from five different surveys. Here we present the optical and blackbody LFs calculated by 33 TDFs discovered in the ZTF-I survey. The optical LF can be described by both a power-law profile dN / dL g ∝ L g − 2.3 ± 0.2 , and a Schechter-like function. The blackbody LF can be described by a power-law profile dN / dL bb ∝ L bb − 2.2 ± 0.2 , shallower than the LF made of the previous van Velzen (2018) sample. A possible low-luminosity turnover in the optical LF supports an Eddington-limited emission scenario. The drop of the volumetric rate at high luminosity suggests a rate suppression due to direct captures of the black hole. The total volumetric rate is 1 order of magnitude lower than the previous estimation, which is probably not simply caused by the high fraction postpeak sources (7/13) in the previous sample. Instead, the normalization step during the previous LF construction to reconcile various surveys might adversely amplify the influence of serendipitous discoveries. Therefore, TDFs selected from ongoing and upcoming uniform surveys like ZTF, Vera Rubin Observatory, and the Wide-Field Survey Telescope should yield more accurate LFs.

The Luminosity Function of Tidal Disruption Events from Fallback-powered Emission: Implications for the Black Hole Mass Function

“…This cutoff in the luminosity function arises from the direct capture radius of an SMBH and the inverse relationship between the fallback luminosity and the black hole mass-since larger SMBH masses correspond to smaller luminosities according to the fallback rate, the direct capture of stars suppresses TDEs at the low-luminosity end of the distribution. This suppression was found observationally in Lin et al (2022), but those authors attributed this cutoff to the Eddington luminosity of the black hole.…”

“…Recent samples of TDEs have been used to constrain this function: van Velzen (2018) used a set of 12 TDEs and found that the luminosity function of TDEs in the g band was µ -  dN dL L TDE 2.6 0.2 . More recently, Lin et al (2022) used a sample of 33 TDEs from the ZTF-I survey and found µ -  dN dL L TDE 2.3 0.2 for g-band luminosities, or ∝L −2.2±0.2 in terms of the bolometric luminosity-slightly shallower than the power-law slope inferred by van Velzen (2018). Charalampopoulos et al (2022) recovered a g-band luminosity function of µ - dN dL L TDE 2.4 from a set of 30 TDEs from the ZTF catalog in Hammerstein et al (2023), andYao et al (2023) used 33 ZTF-obtained TDEs, finding that the g-band luminosity function favored ∝L −3 when modeled as a single power law, while the bolometric luminosity scaled as ∝L −2.4 .…”

AT 2023clx: The Faintest and Closest Optical Tidal Disruption Event Discovered in Nearby Star-forming Galaxy NGC 3799

“…Although only a handful of TDEs have been discovered to be faint (M g > − 18), the luminosity functions (LFs) of optical TDEs suggest that their real number should be large as all of these LFs show a rising trend toward the low end (van Velzen 2018; Lin et al 2022b;Charalampopoulos et al 2023;Yao et al 2023). However, an accurate LF profile calls for more precise constraints, particularly when discovering more faint TDEs.…”

“…As one of the faintest TDEs discovered by the ZTF survey, AT 2023clx could provide the first data point at L g < 10 42.4 erg s −1 . We also used the "  1 max" method described in Lin et al (2022b) to estimate the volumetric rate and considered the latest TDE selection criteria from the ZTF survey (Yao et al 2023). The  max is defined as: For the survey duration (τ survey ), we set a starting date of 2018 October 1 UT and an end date of 2023 May 1 UT.…”

“…The  max is defined as: For the survey duration (τ survey ), we set a starting date of 2018 October 1 UT and an end date of 2023 May 1 UT. Consistent with Lin et al (2022b), the effective survey area is set to A ≈ 15000, dg 2 . We binned the two faintest ZTF TDEs (AT 2020wey and AT 2023clx) to estimate the volumetric rate in the faint end (L g < 10 42.5 erg s −1 ) and compared it with known optical TDE LFs (see Figure 6).…”