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Title: Revisiting Optical Tidal Disruption Events with iPTF16axa

Abstract

We report the discovery by the intermediate Palomar Transient Factory (iPTF) of a candidate tidal disruption event (TDE) iPTF16axa at z = 0.108 and present its broadband photometric and spectroscopic evolution from three months of follow-up observations with ground-based telescopes and Swift. The light curve is well fitted with a t -5/3 decay, and we constrain the rise time to peak to be <49 rest-frame days after disruption, which is roughly consistent with the fallback timescale expected for the ~5 × 10 6 M black hole inferred from the stellar velocity dispersion of the host galaxy. The UV and optical spectral energy distribution is well described by a constant blackbody temperature of T ~ 3 × 10 4 K over the monitoring period, with an observed peak luminosity of 1.1 × 10 44 erg s -1. The optical spectra are characterized by a strong blue continuum and broad He ii and Hα lines, which are characteristic of TDEs. We compare the photometric and spectroscopic signatures of iPTF16axa with 11 TDE candidates in the literature with well-sampled optical light curves. Based on a single-temperature fit to the optical and near-UV photometry, most of these TDE candidates have peak luminosities confined between log(L [erg s -1]) = 43.4–44.4, with constant temperatures of a few ×104 K during their power-law declines, implying blackbody radii on the order of 10 times the tidal disruption radius, that decrease monotonically with time. For TDE candidates with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. In conclusion, we find no correlation between the peak luminosity and the black hole mass, contrary to the expectations for TDEs to have $$\dot{M}\propto {M}_{\mathrm{BH}}^{-1/2}$$.

Authors:
ORCiD logo [1]; ORCiD logo [2];  [3]; ORCiD logo [2]; ORCiD logo [4]; ORCiD logo [3];  [5]; ORCiD logo [6]; ORCiD logo [6]; ORCiD logo [7]; ORCiD logo [3]; ORCiD logo [8]; ORCiD logo [9]; ORCiD logo [10]; ORCiD logo [11]
  1. Univ. of Maryland, College Park, MD (United States). Dept. of Astronomy
  2. Univ. of Maryland, College Park, MD (United States). Dept. of Astronomy; Univ. of Maryland, College Park, MD (United States). Joint Space-Science Inst.
  3. California Inst. of Technology (CalTech), Pasadena, CA (United States). Dept. of Astronomy
  4. Univ. of Maryland, College Park, MD (United States). Joint Space-Science Inst.; NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
  5. Hebrew Univ. of Jerusalem (Israel). Racah Inst. of Physics
  6. Univ. of California, Santa Barbara, CA (United States). Dept. of Physics; Las Cumbres Observatory, Goleta, CA (United States)
  7. California Inst. of Technology (CalTech), Pasadena, CA (United States). Caltech Optical Observatories, Cahill Center for Astronomy and Astrophysics; California Inst. of Technology (CalTech), Pasadena, CA (United States). Infrared Processing and Analysis Center
  8. Stockholm Univ. (Sweden). Oskar Klein Center, Dept. of Astronomy
  9. California Inst. of Technology (CalTech), Pasadena, CA (United States). Dept. of Astronomy; Univ. of Washington, Seattle, WA (United States). eScience Inst. and Astronomy Dept.
  10. Univ. of California, Berkeley, CA (United States). Dept. of Astronomy; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  11. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program
OSTI Identifier:
1412865
Report Number(s):
LA-UR-17-27705
Journal ID: ISSN 1538-4357; TRN: US1800380
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 842; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astronomy and Astrophysics; accretion; accretion disks; black hole physics; galaxies: nuclei; ultraviolet general

Citation Formats

Hung, T., Gezari, S., Blagorodnova, N., Roth, N., Cenko, S. B., Kulkarni, S. R., Horesh, A., Arcavi, I., McCully, C., Yan, Lin, Lunnan, R., Fremling, C., Cao, Y., Nugent, P. E., and Wozniak, Przemyslaw R. Revisiting Optical Tidal Disruption Events with iPTF16axa. United States: N. p., 2017. Web. doi:10.3847/1538-4357/aa7337.
Hung, T., Gezari, S., Blagorodnova, N., Roth, N., Cenko, S. B., Kulkarni, S. R., Horesh, A., Arcavi, I., McCully, C., Yan, Lin, Lunnan, R., Fremling, C., Cao, Y., Nugent, P. E., & Wozniak, Przemyslaw R. Revisiting Optical Tidal Disruption Events with iPTF16axa. United States. doi:10.3847/1538-4357/aa7337.
Hung, T., Gezari, S., Blagorodnova, N., Roth, N., Cenko, S. B., Kulkarni, S. R., Horesh, A., Arcavi, I., McCully, C., Yan, Lin, Lunnan, R., Fremling, C., Cao, Y., Nugent, P. E., and Wozniak, Przemyslaw R. 2017. "Revisiting Optical Tidal Disruption Events with iPTF16axa". United States. doi:10.3847/1538-4357/aa7337.
@article{osti_1412865,
title = {Revisiting Optical Tidal Disruption Events with iPTF16axa},
author = {Hung, T. and Gezari, S. and Blagorodnova, N. and Roth, N. and Cenko, S. B. and Kulkarni, S. R. and Horesh, A. and Arcavi, I. and McCully, C. and Yan, Lin and Lunnan, R. and Fremling, C. and Cao, Y. and Nugent, P. E. and Wozniak, Przemyslaw R.},
abstractNote = {We report the discovery by the intermediate Palomar Transient Factory (iPTF) of a candidate tidal disruption event (TDE) iPTF16axa at z = 0.108 and present its broadband photometric and spectroscopic evolution from three months of follow-up observations with ground-based telescopes and Swift. The light curve is well fitted with a t -5/3 decay, and we constrain the rise time to peak to be <49 rest-frame days after disruption, which is roughly consistent with the fallback timescale expected for the ~5 × 106 M ⊙ black hole inferred from the stellar velocity dispersion of the host galaxy. The UV and optical spectral energy distribution is well described by a constant blackbody temperature of T ~ 3 × 104 K over the monitoring period, with an observed peak luminosity of 1.1 × 1044 erg s-1. The optical spectra are characterized by a strong blue continuum and broad He ii and Hα lines, which are characteristic of TDEs. We compare the photometric and spectroscopic signatures of iPTF16axa with 11 TDE candidates in the literature with well-sampled optical light curves. Based on a single-temperature fit to the optical and near-UV photometry, most of these TDE candidates have peak luminosities confined between log(L [erg s-1]) = 43.4–44.4, with constant temperatures of a few ×104 K during their power-law declines, implying blackbody radii on the order of 10 times the tidal disruption radius, that decrease monotonically with time. For TDE candidates with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. In conclusion, we find no correlation between the peak luminosity and the black hole mass, contrary to the expectations for TDEs to have $\dot{M}\propto {M}_{\mathrm{BH}}^{-1/2}$.},
doi = {10.3847/1538-4357/aa7337},
journal = {The Astrophysical Journal (Online)},
number = 1,
volume = 842,
place = {United States},
year = 2017,
month = 6
}

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Cited by: 4works
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  • We examine whether disrupted binary stars can fuel black hole growth. In this mechanism, tidal disruption produces a single hypervelocity star (HVS) ejected at high velocity and a former companion star bound to the black hole. After a cluster of bound stars forms, orbital diffusion allows the black hole to accrete stars by tidal disruption at a rate comparable to the capture rate. In the Milky Way, HVSs and the S star cluster imply similar rates of 10{sup -5} to 10{sup -3} yr{sup -1} for binary disruption. These rates are consistent with estimates for the tidal disruption rate in nearbymore » galaxies and imply significant black hole growth from disrupted binaries on 10 Gyr timescales.« less
  • We present new observations with the Karl G. Jansky Very Large Array of seven X-ray-selected tidal disruption events (TDEs). The radio observations were carried out between 9 and 22 years after the initial X-ray discovery, and thus probe the late-time formation of relativistic jets and jet interactions with the interstellar medium in these systems. We detect a compact radio source in the nucleus of the galaxy IC 3599 and a compact radio source that is a possible counterpart to RX J1420.4+5334. We find no radio counterparts for five other sources with flux density upper limits between 51 and 200 {mu}Jymore » (3{sigma}). If the detections truly represent late radio emission associated with a TDE, then our results suggest that a fraction, {approx}> 10%, of X-ray-detected TDEs are accompanied by relativistic jets. We explore several models for producing late radio emission, including interaction of the jet with gas in the circumnuclear environment (blast wave model), and emission from the core of the jet itself. Upper limits on the radio flux density from archival observations suggest that the jet formation may have been delayed for years after the TDE, possibly triggered by the accretion rate dropping below a critical threshold of {approx}10{sup -2}-10{sup -3} M-dot {sub Edd}. The non-detections are also consistent with this scenario; deeper radio observations can determine whether relativistic jets are present in these systems. The emission from RX J1420.4+5334 is also consistent with the predictions of the blast wave model; however, the radio emission from IC 3599 is substantially underluminous, and its spectral slope is too flat, relative to the blast wave model expectations. Future radio monitoring of IC 3599 and RX J1420.4+5334 will help to better constrain the nature of the jets in these systems.« less
  • After the destruction of the star during a tidal disruption event (TDE), the cataclysmic encounter between a star and the supermassive black hole (SMBH) of a galaxy, approximately half of the original stellar debris falls back onto the hole at a rate that can initially exceed the Eddington limit by orders of magnitude. We argue that the angular momentum of this matter is too low to allow it to attain a disk-like configuration with accretion proceeding at a mildly super-Eddington rate, the excess energy being carried away by a combination of radiative losses and radially distributed winds. Instead, we proposemore » that the infalling gas traps accretion energy until it inflates into a weakly bound, quasi-spherical structure with gas extending nearly to the poles. We study the structure and evolution of such 'zero-Bernoulli accretion' flows as a model for the super-Eddington phase of TDEs. We argue that such flows cannot stop extremely super-Eddington accretion from occurring, and that once the envelope is maximally inflated, any excess accretion energy escapes through the poles in the form of powerful jets. We compare the predictions of our model to Swift J1644+57, the putative super-Eddington TDE, and show that it can qualitatively reproduce some of its observed features. Similar models, including self-gravity, could be applicable to gamma-ray bursts from collapsars and the growth of SMBH seeds inside quasi-stars.« less
  • During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates that may exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history or, at longer wavelengths, by its own emission. We model themore » evolution of TDE disk size and accretion rate by accounting for the time-dependent fallback rate, for the influence of wind losses in the early advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure-dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body because of the Lense-Thirring effect, and precession may manifest itself as a quasi-periodic modulation of the light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t = 500 days corresponding to a disk transition from the advective state to the gas-pressure-dominated state.« less
  • One of the puzzles associated with tidal disruption event candidates (TDEs) is that there is a dichotomy between the color temperatures of a few × 10{sup 4} K for TDEs discovered with optical and UV telescopes and the color temperatures of a few × 10{sup 5}–10{sup 6} K for TDEs discovered with X-ray satellites. Here, we propose that high-temperature TDEs are produced when the tidal debris of a disrupted star self-intersects relatively close to the supermassive black hole, in contrast to the more distant self-intersection that leads to lower color temperatures. In particular, we note from simple ballistic considerations thatmore » greater apsidal precession in an orbit is the key to closer self-intersection. Thus, larger values of β, the ratio of the tidal radius to the pericenter distance of the initial orbit, are more likely to lead to higher temperatures of more compact disks that are super-Eddington and geometrically and optically thick. For a given star and β, apsidal precession also increases for larger black hole masses, but larger black hole masses imply a lower temperature at the Eddington luminosity. Thus, the expected dependence of the temperature on the mass of the black hole is non-monotonic. We find that in order to produce a soft X-ray temperature TDE, a deep plunging stellar orbit with β > 3 is needed and a black hole mass of ≲5 × 10{sup 6}M{sub ⊙} is favored. Although observations of TDEs are comparatively scarce and are likely dominated by selection effects, it is encouraging that both expectations are consistent with current data.« less