The Tidal Disruption Event AT 2018hyz II: Light-curve modelling of a partially disrupted star

Sebastian Gomez, Matt Nicholl, Philip Short, Raffaella Margutti, Kate D. Alexander, Peter K. Blanchard, Edo Berger, Tarraneh Eftekhari, Steve Schulze, Joseph Anderson, Iair Arcavi, Ryan Chornock, Philip S. Cowperthwaite, Lluis Galbany, Laura J. Herzog, Daichi Hiramatsu, Griffin Hosseinzadeh, Tanmoy Laskar, Tomás E. Müller Bravo, Locke PattonGiacomo Terreran

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


AT 2018hyz (= ASASSN-18zj) is a tidal disruption event (TDE) located in the nucleus of a quiescent E+A galaxy at a redshift of z = 0.04573, first detected by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present optical+UV photometry of the transient, as well as an X-ray spectrum and radio upper limits. The bolometric light curve of AT 2018hyz is comparable to other known TDEs and declines at a rate consistent with a t-5/3 at early times, emitting a total radiated energy of E = 9 × 1050 erg. An excess bump appears in the UV light curve about 50 d after bolometric peak, followed by a flattening beyond 250 d. We detect a constant X-ray source present for at least 86 d. The X-ray spectrum shows a total unabsorbed flux of ~4 × 10-14 erg cm -2 s-1 and is best fit by a blackbody plus power-law model with a photon index of r = 0.8. A thermal X-ray model is unable to account for photons > 1 keV, while a radio non-detection favours inverse-Compton scattering rather than a jet for the non-thermal component. We model the optical and UV light curves using the Modular Open-Source Fitter for Transients (MOSFiT) and find a best fit for a black hole of 5.2 × 106 M0 disrupting a 0.1 M0 star; the model suggests the star was likely only partially disrupted, based on the derived impact parameter of ß = 0.6. The low optical depth implied by the small debris mass may explain how we are able to see hydrogen emission with disc-like line profiles in the spectra of AT2018hyz (see our companion paper).

Original languageEnglish
Pages (from-to)1925-1934
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Publication statusPublished - 1 Jul 2020

Bibliographical note

Funding Information:
2IRAF is written and supported by the National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation. 3 Figure 2. Optical and UV light curves of AT 2018hyz, host-subtracted and corrected for galactic extinction. The black lines mark the times for which we have optical spectra (Short et al. 2020). The photometry shown here is available on the online version of this journal.

Funding Information:
We thank B. Mockler for useful discussions regarding the MOSFiT TDE model and an anonymous referee for comments towards the improvement of this paper. The Berger Time-Domain Group at Harvard is supported in part by NSF under grant AST-1714498 and by NASA under grant NNX15AE50G. S. Gomez is partly supported by an NSF Graduate Research Fellowship. MN is supported by a Royal Astronomical Society Research Fellowship. KDA acknowledges support provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51403.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. IA is a CIFAR Azrieli Global Scholar in the Gravity and the Extreme Universe Program and acknowledges support from that program, from the Israel Science Foundation (grant numbers 2108/18 and 2752/19), from the United States – Israel Binational Science Foundation (BSF), and from the Israeli Council for Higher Education Alon Fellowship. Operation of the Pan-STARRS1 telescope is supported by the National Aeronautics and Space Administration under grant No. NNX12AR65G and grant No. NNX14AM74G issued through the NEO Observation Program. This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, https: // Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. This research has made use of NASA’s Astrophysics Data System. This research has made use of the SIMBAD data base, operated at CDS, Strasbourg, France. Additional software used for this paper: ASTROPY (Astropy Collaboration 2018), PYRAF (Science Software Branch at STScI 2012), SAOIMAGE DS9 (Smithsonian Astrophysical Observatory 2000), MATPLOTLIB (Hunter 2007), NUMPY (Oliphant 2007), EXTINCTION (Barbary 2016) PYPHOT ( uesneau/pyphot).

Publisher Copyright:
© 2020 The Author(s).


  • black hole physics
  • galaxies: nuclei
  • Astrophysics - High Energy Astrophysical Phenomena


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