We hereby report the discovery of ATLAS17jrp as an extraordinary tidal disruption event (TDE) in the star-forming galaxy SDSS J162034.99+240726.5 in our recent sample of mid-infrared outbursts in nearby galaxies. Its optical/UV light curves rise to a peak luminosity of ∼1.06 × 1044 erg s−1 in about a month and then decay as t −5/3 with a roughly constant temperature around 19,000 K, and the optical spectra show a blue continuum and very broad Balmer lines with FWHM ∼ 15,000 km s−1, which gradually narrowed to 1400 km s−1 within 4 yr, all agreeing well with other optical TDEs. A delayed and rapidly rising X-ray flare with a peak luminosity of ∼1.27 × 1043 erg s−1 was detected ∼170 days after the optical peak. The high MIR luminosity of ATLAS17jrp (∼2 × 1043 erg s−1) has revealed a distinctive dusty environment with a covering factor as high as ∼0.2, which is comparable to that of a torus in active galactic nuclei but at least one order of magnitude higher than normal optical TDEs. Therefore, ATLAS17jrp turns out to be one of the rare unambiguous TDEs found in star-forming galaxies, and its high dust-covering factor implies that dust extinction could play an important role in the absence of optical TDEs in star-forming galaxies.
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.
Optical time-domain survey has been the dominant means of hunting for rare tidal disruption events (TDEs) in the past decade and remarkably advanced the TDE study. Particularly, the Zwicky Transient Facility (ZTF) has opened the era of population studies and the upcoming Large Synoptic Survey Telescope (LSST) at the Vera Rubin Observatory (VRO) is believed to further revolutionize the field soon. Here we present the prospects of finding TDEs with another powerful survey to be performed by 2.5-metre Wide-Field Survey Telescope (WFST). The WFST, located in western China, will be the most advanced facility dedicated to optical time-domain surveys in the northern hemisphere once commissioning. We choose to assess its TDE detectability on the basis of mock observations, which is hitherto closest to reality by taking into consideration of site conditions, telescope parameters, survey strategy and transient searching pipeline. Our mock observations on 440 deg2 field (CosmoDC2 catalogue) show that 29 ± 6 TDEs can be robustly found per year if observed at u, g, r, i bands with 30-second exposure every 10 days, in which a discovery is defined as ≥10 epochal detections in at least two filters. If the WFST survey is fully optimized for discovering TDE, we would expect to identify 392 ± 74 of TDEs every year, with the redshift up to z ∼ 0.8, which poses a huge challenge to follow-up resources.
Wide-field Infrared Survey Explorer all-sky survey has discovered a new population of hot dust-obscured galaxies (Hot DOGs), which have been confirmed to be dusty quasars. Previous statistical studies have found significant overdensities of submillimeter and mid-IR-selected galaxies around Hot DOGs, indicating they may reside in dense regions. Here we present the near-infrared (J and K s bands) observations over a 7 .′ 5 × 7 .′ 5 field centered on a Hot DOG W1835+4355 at z ∼ 2.3 using the wide-field infrared camera on the Palomar 200 inch telescope. We use the color criterion J − K s > 2.3 for objects with K S < 20 , to select distant red galaxies (DRGs). We find a significant excess of number density of DRGs in the W1835+4355 field compared to three control fields, by a factor of about two. The overdensity of red galaxies around W1835+4355 is consistent with the multiwavelength environment of Hot DOGs, suggesting that Hot DOGs may be a good tracer for dense regions at high redshift. We find that W1835+4355 does not reside in the densest region of the dense environment traced by itself. A possible scenario is that W1835+4355 is undergoing a merging process, which lowers the local number density of galaxies in its surrounding region.
Kilonovae are approximately thermal transients, produced by the mergers of binary neutron stars (BNSs) and neutron star (NS)–black hole binaries. As the optical counterpart of the gravitational-wave event GW170817, AT2017gfo is the first kilonova detected with smoking-gun evidence. Its observation offers vital information for constraining the Hubble constant, the sources of cosmic r-process enrichment, and the equation of state of NSs. The 2.5 m Wide-Field Survey Telescope (WFST) operates in six bands (u, g, r, i, z, w), spanning from 320 to 925 nm. It will be completed in the first half of 2023, and with a field-of-view diameter of 3°, aims to detect kilonovae in the near future. In this article, considering the influence of the host galaxies and sky brightness, we generate simulated images to investigate WFST’s ability to detect AT2017gfo-like kilonovae. Due to their spectra, host galaxies can significantly impact kilonova detection at longer wavelengths. When kilonovae are at peak luminosity, we find that WFST performs better in the g and r bands and can detect 90% (50%) of kilonovae at a luminosity distance of 248 Mpc (338 Mpc) with 30 s exposures. Furthermore, to reflect the actual efficiency under target-of-opportunity observations, we calculate the total time of follow up under various localization areas and distances. We find that if the localization areas of most BNS events detected during the fourth observing (O4) run of LIGO and Virgo are hundreds of deg2, WFST is expected to find ∼30% of kilonovae in the first two nights following the detection of a GW event produced by a BNS during the O4 period.
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