A star that wanders too close to a massive black hole (BH) is shredded by the BH's tidal gravity. Stellar gas falls back to the BH at a rate initially exceeding the Eddington rate, releasing a flare ...of energy. In anticipation of upcoming transient surveys, we predict the light curves and spectra of tidal flares as a function of time, highlighting the unique signatures of tidal flares at optical and near-infrared wavelengths. A reasonable fraction of the gas initially bound to the BH is likely blown away when the fallback rate is super-Eddington at early times. This outflow produces an optical luminosity comparable to that of a supernova; such events have durations of ∼10 d and may have been missed in supernova searches that exclude the nuclear regions of galaxies. When the fallback rate subsides below Eddington, the gas accretes onto the BH via a thin disc whose emission peaks in the ultraviolet to soft X-rays. Some of this emission is reprocessed by the unbound stellar debris, producing a spectrum of very broad emission lines (with no corresponding narrow forbidden lines). These lines are the strongest for BHs with MBH∼ 105–106 M⊙ and thus optical surveys are particularly sensitive to the lowest mass BHs in galactic nuclei. Calibrating our models to ROSAT and Galaxy Evolution Explorer (GALEX) observations, we predict detection rates for Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Palomar Transient Factory (PTF) and the Large Synoptic Survey Telescope (LSST) and highlight some of the observational challenges associated with studying tidal disruption events in the optical. Upcoming surveys such as Pan-STARRS should detect at least several events per year, and may detect many more if current models of outflows during super-Eddington accretion are reasonably accurate. These surveys will significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of intermediate mass BHs and the role of tidal disruption in the growth of massive BHs.
AU Microscopii is a 12 Myr old M dwarf that harbors an optically thin, edge-on disk of dust. The scattered light surface brightness falls with projected distance b from the star as b super(-a); ...within b = 43 AU, a- 1-2, while outside 43 AU, a-4-5. We devise a theory to explain this profile. At a stellocentric distance r = r sub(BR) = 43 AU, we posit a ring of parent bodies on circular orbits: the "birth ring," wherein micron-sized grains are born from the collisional attrition of parent bodies. The "inner disk" at r <r sub(BR) contains grains that migrate inward by corpuscular and Poynting-Robertson (CPR) drag. The "outer disk" at r> r sub(BR) comprises grains just large enough to remain bound to the star, on orbits rendered highly eccentric by stellar wind and radiation pressure. How the vertical optical depth t scales with r depends on the fraction of grains that migrate inward by CPR drag without suffering a collision. If this fraction is large, the inner disk and birth ring share the same optical depth, and t8r super(-5/2) in the outer disk. By contrast, under collision-dominated conditions, the inner disk is empty, and t8r super(- 3/2) outside. These scaling relations, which we derive analytically and confirm numerically, are robust against uncertainties in the grain size distribution. By simultaneously modeling the surface brightness and thermal spectrum, we break model degeneracies to establish that the AU Mic system is collision dominated and that its narrow birth ring contains a lunar mass of decimeter-sized bodies. The inner disk is devoid of micron-sized grains; the surface brightness at b<43 AU arises from light forward scattered by the birth ring. Inside b = 43 AU, the disk's V-H color should not vary with b; outside, the disk must become bluer as ever smaller grains are probed.
Was PS1-10jh, an optical/ultraviolet transient discovered by the Pan-STARRS Medium Deep Survey, the tidal disruption of a star by a massive black hole (BH)? We address two aspects of the problem: the ...composition of the putative disrupted object (using the spectroscopic data), and the energetics of the observed gas and radiation (using the photometric data). We perform photoionization calculations and compare with the observed lower limit of the line ratio
$L_{\rm He\,\small {II}\,\, 4686}/L_{\rm H\alpha }>5$
to argue that this event was not the disruption of a solar-type star, and instead was likely the disruption of a helium core (as first proposed by Gezari et al.). Disruption of such a dense object requires a relatively small central BH, M
BH ≲ 2 × 105 M⊙. We use the photometric data to infer that PS1-10jh comprised an outflow of ∼0.01 M⊙ of gas, escaping from the BH at ∼1000 km s−1, and we propose that this outflow was driven primarily by radiation pressure trapped by Thomson and resonance line scattering. The large ratio of radiated energy to kinetic energy, E
rad/E
K ∼ 104, together with the large value of E
rad ∼ 2 × 1051 erg, suggests that the outflow was shocked at large radius (perhaps similar to superluminous supernovae or the internal shock model for gamma-ray bursts). We describe puzzles in the physics of PS1-10jh, and discuss how this event may help us understand future tidal disruptions and super-Eddington accretion events as well.
During the tidal disruption of a main-sequence star by a massive black hole (BH) having mass M
BH≲ 107 M⊙, the stellar debris is expected to fall back to the BH at a rate well above the Eddington ...rate. Some fraction of this gas is predicted to be blown away from the BH, producing an optically bright flare of radiation. We predict the spectra and spectral evolution of tidal disruption events, focusing on the signatures produced by photoionized gas outside the photosphere of this super-Eddington outflow. We show that the spectrum of such an outflow should show absorption lines that are strongly blueshifted relative to the host galaxy, are typically very broad (0.01-0.1c) and are most prominent at ultraviolet wavelengths (e.g. C iv, Lyman α, O vi) at early times (≲1 month for a ∼106 M⊙ BH). There may also be optical absorption lines of hydrogen and He ii if there is a lower velocity component to the outflow (≲0.01c). At later times, the outflow falls out of thermal equilibrium and the continuum emission likely hardens - the absorption lines will then primarily be in the soft X-rays.
Supernovae in galactic nuclei are a significant source of confusion in optical surveys for tidal disruption events: we estimate that nuclear Type Ia supernovae are two orders of magnitude more common than tidal disruption events at z∼ 0.1 for ground-based surveys. Nuclear Type II supernovae occur at a comparable rate but can be excluded by pre-selecting red galaxies. The contamination from nuclear supernovae can be reduced to a manageable level by using high-resolution follow-up imaging with adaptive optics or the Hubble Space Telescope. Our predictions should help optical transient surveys capitalize on their potential for discovering tidal disruption events.
Gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is ...torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 10 6 to 10 7 solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar.
Using a 1D height integrated model, we calculate the evolution of an unequal mass binary black hole with a coplanar gas disc that contains a gap due to the presence of the secondary black hole. ...Viscous evolution of the outer circumbinary disc initially hardens the binary, while the inner disc drains on to the primary (central) black hole. As long as the inner disc remains cool and thin at low (rather than becoming hot and geometrically thick), the mass of the inner disc reaches an asymptotic mass typically ∼10−3–10−4 M⊙. Once the semimajor axis shrinks below a critical value, angular momentum losses from gravitational waves dominate over viscous transport in hardening the binary. The inner disc then no longer responds viscously to the inspiraling black holes. Instead, tidal interactions with the secondary rapidly drive the inner disc into the primary. Tidal and viscous dissipation in the inner disc lead to a late time brightening in luminosity, L∝t−5/4minus, where tminus is the time prior to the final merger. This late time brightening peaks ∼1 d prior to the final merger at ∼0.1LEdd. This behaviour is relatively robust because of self-regulation in the coupled viscous–gravitational evolution of such binary systems. It constitutes a unique electromagnetic signature of a binary supermassive black hole merger and may allow the host galaxy to be identified if used in conjunction with the Laser Interferometric Space Antenna localization.
ABSTRACT We present a Hubble Space Telescope Space Telescope Imaging Spectrograph spectrum of ASASSN-14li, the first rest-frame ultraviolet (UV) spectrum of a tidal disruption flare (TDF). The ...underlying continuum is well fit by a blackbody with K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Superimposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a high-velocity cloud), and narrow absorption and broad (∼2000-8000 km s−1) emission lines at or near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by Δv = −(250-400) km s−1. Due both to this velocity offset and the lack of common low-ionization features (Mg ii, Fe ii), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N iii, N iv, and He ii are quite prominent, while the common quasar emission lines of C iii and Mg ii are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission processes at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars.
The purpose of this case study was to investigate the perception and experience of ten new faculty member participants in paired teaching. Along with providing evidence for paired teaching as an ...avenue for professional development in higher education, the study provides experiential narratives of paired teaching relationships as an illustration of the process. Key outcomes of the study include: the value of peer-to-peer engagement for prompting reflection on practice and facilitation of instructor use of active learning approaches within the classroom.
Much work in physics education research (PER) characterizes faculty teaching practice in terms of whether faculty use specific named PER-based teaching methods, either with fidelity or with ...adaptation; we call this research paradigm the "teaching-method-centered paradigm." However, most faculty do not frame their teaching in terms of which particular named methods they use, but rather in terms of their own ideas and values, suggesting that the teaching-method-centered paradigm misses key features of faculty teaching. These key features include the productive ideas that faculty have about student learning and faculty agency around teaching. We present three case studies of faculty talking about their teaching, and analyze them in terms of two theoretical frameworks: a framework of teaching principles (How Learning Works) and a framework of faculty agency (self-determination theory). We show that these frameworks well characterize key features of faculty teaching practices and agency, and can be combined in a new paradigm for modeling faculty teaching which we call an "asset-based agentic paradigm." We therefore encourage physics education researchers to move beyond the teaching-method-centered paradigm and think about faculty teaching using an asset-based agentic paradigm.
We present the discovery and characterization of PTF10iya, a short-lived (Δt≈ 10 d, with an optical decay rate of ∼0.3 mag d−1), luminous (
mag) transient source found by the Palomar Transient ...Factory. The ultraviolet/optical spectral energy distribution is reasonably well fitted by a blackbody with T≈ (1-2) × 104 K and peak bolometric luminosity L
BB≈ (1-5) × 1044 erg s−1 (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z= 0.224 05 ± 0.000 06) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic 'big blue bump' seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early 'super-Eddington' phase of a solar-type star being disrupted by a ∼107 M⊙ black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence.
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