Tidal Disruption of a Star By a Massive Black Hole
What: Tidal disruption occurs when a star on the orbit around a massive black hole makes a close approach and is pulled apart by the black hole tidal forces. Once stellar debris starts raining down on the black hole, the soft X-ray and UV radiation emerges from the innermost region of accreting debris. This radiation is intercepted by the debris and one fraction of it is reemitted as optical light.
We carried out smoothed particle hydrodynamics (SPH) simulations of tidal disruption of a solar type star by a million solar mass black hole in order to study characteristic emission signatures associated with it. We modeled the spectral signatures of light reemitted in hydrogen Balmer alpha transition as well as Balmer alpha and X-ray light curves from this event. See tidal disruption movies based on our SPH simulations.
Why: A tidal disruption event is signaled by a flare-up in the center of a previously quiescent galaxy, followed by the decrease in the soft X-ray luminosity, as ~t-5/3 at late times, months and years after the disruption took place. This signature has been utilized to select a number of tidal disruption candidates to date. It entails followup X-ray observations of the candidate, on timescale of years, in order to confirm the behavior of the X-ray light with time. We studied an additional optical signature, in form of the variable broad Balmer alpha emission line profiles, as a complementary signature to the X-ray emission, which could improve the confidence of a tidal disruption detection within a single epoch of observation. If observed, this signature could help make a more efficient selection of tidal disruption candidates on the sky, as well as give us additional insight into the physical properties of the region very close to a massive black hole.
How: We used SPH simulations of tidal disruption of a star by a black hole combined with photoionization calculations carried out with CLOUDY in order to characterize the emission from the evolving debris. We assumed that the accretion of the debris onto the central supermassive black hole gives rise to the UV and X-ray radiation that illuminates the debris and powers the optical emission. We calculated the response of the debris to this ionizing radiation with CLOUDY and followed the evolution of the modeled Balmer alpha emission line profiles on the time-scale of days after the disruption. We find that during this time the line profiles exhibit significant variability and look very distinct from double-peaked emission line profiles expected from circular and elliptical disk models. If such irregular and variable emission line profiles are observed subsequently after the X-ray or UV flares coming from the center of the same galaxy, they could be used to identify a tidal disruption event in the early phase.
This work was accomplished under supervision of my thesis advisors Michael Eracleous and Steinn Sigurdsson and in collaboration with Suvrath Mahadevan and Pablo Laguna. Find out more about it from our publication in Astrophysical Journal.
This research was supported by the National Science Foundation under cooperative agreement PHY-01-14375, Physics Frontier Center, Proj. Corr. L.S. Finn. Any opinions, findings and conclusions or recommendations expressed in this material are those of author(s) and do not necessarily reflect the views of the National Science Foundation.
 The animations are based on 3D smooth particle hydrodynamics calculations of tidal disruption of a star by a massive black hole. The star is initially placed on parabolic orbit around the black hole which is positioned in the coordinate beginning. The animations show the distribution of tidal debris in time for simulations with 5000 and 20,000 particles. The debris distribution is projected in xy-plane (plane of the debris). Particles gravitationally bound to the black hole are white, while the unbound particles are colored green or yellow. The time tags in each frame mark the current, real time in the animation with respect to the beginning of the simulation. The time span of each movie is as shown in the table.
** Animations by Michael Eracleous
5000 particles.
Time span: 0 to 22 days.
Full view.
5000 particles.
Time span: 34 to 94 days.
Zoom view near the black hole.
20000 particles.
Time span: 47 to 53 days.
Zoom view near the black hole.
Source: CXO