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Register a new userThe Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution
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Nadejda Blagorodnova
Publication date
2018
fields
Physics
and research's language is
English
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We present multi-wavelength observations of the tidal disruption event (TDE) iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF) survey at redshift $z=0.07897$. The optical and ultraviolet (UV) light curves of the transient show a slow decay over five months, in agreement with previous optically discovered TDEs. It also has a comparable black-body peak luminosity of $L_{rm{peak}} approx 1.5 times 10^{44}$ erg/s. The inferred temperature from the optical and UV data shows a value of (3$-$5) $times 10^4$ K. The transient is not detected in X-rays up to $L_X < 3 times 10^{42}$erg/s within the first five months after discovery. The optical spectra exhibit two distinct broad emission lines in the He II region, and at later times also H$alpha$ emission. Additionally, emission from [N III] and [O III] is detected, likely produced by the Bowen fluorescence effect. UV spectra reveal broad emission and absorption lines associated with high-ionization states of N V, C IV, Si IV, and possibly P V. These features, analogous to those of broad absorption line quasars (BAL QSOs), require an absorber with column densities $N_{rm{H}} > 10^{23}$ cm$^{-2}$. This optically thick gas would also explain the non-detection in soft X-rays. The profile of the absorption lines with the highest column density material at the largest velocity is opposite that of BAL QSOs. We suggest that radiation pressure generated by the TDE flare at early times could have provided the initial acceleration mechanism for this gas. Spectral UV line monitoring of future TDEs could test this proposal.
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The existence of optical-ultraviolet Tidal Disruption Events (TDEs) could be considered surprising because their electromagnetic output was originally predicted to be dominated by X-ray emission from an accretion disk. Yet over the last decade, the growth of optical transient surveys has led to the identification of a new class of optical transients occurring exclusively in galaxy centers, many of which are considered to be TDEs. Here we review the observed properties of these events, identified based on a shared set of both photometric and spectroscopic properties. We present a homogeneous analysis of 33 sources that we classify as robust TDEs, and which we divide into classes. The criteria used here to classify TDEs will possibly get updated as new samples are collected and potential additional diversity of TDEs is revealed. We also summarize current measurements of the optical-ultraviolet TDE rate, as well as the mass function and luminosity function. Many open questions exist regarding the current sample of events. We anticipate that the search for answers will unlock new insights in a variety of fields, from accretion physics to galaxy evolution.
We report the discovery of non-stellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05$c$) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr ($z=0.071$). In the HST UV spectra, absorption of high- and low-ionization lines are also present at this velocity, making AT2018zr resemble a low-ionization broad absorption line (LoBAL) QSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped H$alpha$ emission in a spherically expanding medium, without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow ($sim$1000~km~s$^{-1}$) NIII$lambda$4640, HeII$lambda4686$, H$alpha$, and H$beta$, emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late time as observed by Swift. Including AT2018zr, we find a high association rate (3 out of 4) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and may be less sensitive to viewing angle effects compared to QSO outflows.
We survey the properties of stars destroyed in TDEs as a function of BH mass, stellar mass and evolutionary state, star formation history and redshift. For Mbh<10^7Msun, the typical TDE is due to a M*~0.3Msun M-dwarf, although the mass function is relatively flat for $M*<Msun. The contribution from older main sequence stars and sub-giants is small but not negligible. From Mbh~10^7.5-10^8.5Msun, the balance rapidly shifts to higher mass stars and a larger contribution from evolved stars, and is ultimately dominated by evolved stars at higher BH masses. The star formation history has little effect until the rates are dominated by evolved stars. TDE rates should decline very rapidly towards higher redshifts. The volumetric rate of TDEs is very high because the BH mass function diverges for low masses. However, any emission mechanism which is largely Eddington-limited for low BH masses suppresses this divergence in any observed sample and leads to TDE samples dominated by Mbh~10^6.0-10^7.5Msun BHs with roughly Eddington peak accretion rates. The typical fall back time is relatively long, with 16% having Tfb<10^(-1) years (37 days), and 84% having longer time scales. Many residual rate discrepancies can be explained if surveys are biased against TDEs with these longer Tfb, which seems very plausible if Tfb has any relation to the transient rise time. For almost any BH mass function, systematic searches for fainter, faster time scale TDEs in smaller galaxies, and longer time scale TDEs in more massive galaxies are likely to be rewarded.
We present the results of a large multi-wavelength follow-up campaign of the Tidal Disruption Event (TDE) dsg, focusing on low to high resolution optical spectroscopy, X-ray, and radio observations. The galaxy hosts a super massive black hole of mass $rm (5.4pm3.2)times10^6,M_odot$ and careful analysis finds no evidence for the presence of an Active Galactic Nucleus, instead the TDE host galaxy shows narrow optical emission lines that likely arise from star formation activity. The transient is luminous in the X-rays, radio, UV and optical. The X-ray emission becomes undetected after $sim$125 days, and the radio luminosity density starts to decay at frequencies above 5.4 GHz by $sim$180 days. Optical emission line signatures of the TDE are present up to $sim$250 days after the discovery of the transient. The medium to high resolution spectra show traces of absorption lines that we propose originate in the self-gravitating debris streams. At late times, after $sim$200 days, narrow Fe lines appear in the spectra. The TDE was previously classified as N-strong, but after careful subtraction of the host galaxys stellar contribution, we find no evidence for these N lines in the TDE spectrum, even though O Bowen lines are detected. The observed properties of the X-ray emission are fully consistent with the detection of the inner regions of a cooling accretion disc. The optical and radio properties are consistent with this central engine seen at a low inclination (i.e., seen from the poles).
We present late-time optical spectroscopy and X-ray, UV, and optical photometry of the nearby ($d=214$ Mpc, $z=0.0479$) tidal disruption event (TDE) ASASSN-15oi. The optical spectra span 450 days after discovery and show little remaining transient emission or evolution after roughly 3 months. In contrast, the Swift and XMM-Newton observations indicate the presence of evolving X-ray emission and lingering thermal UV emission that is still present 600 days after discovery. The thermal component of the X-ray emission shows a unique, slow brightening by roughly an order of magnitude to become the dominant source of emission from the TDE at later times, while the hard component of the X-ray emission remains weak and relatively constant throughout the flare. The TDE radiated $(1.32pm0.06)times10^{51}$ ergs across all wavelengths, and the UV and optical emission is consistent with a power law decline and potentially indicative of a late-time shift in the power-law index that could be caused by a transition in the dominant emission mechanism.
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