Tidal Disruptions of Stars by Supermassive Black Holes
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Transcript of Tidal Disruptions of Stars by Supermassive Black Holes
Tidal Disruptions of Stars by Supermassive Black Holes
Suvi Gezari (Caltech)
Chris Martin & GALEX Team
Bruno Milliard (GALEX)Stephane Basa (SNLS)
• Probing the mass of dormant black holes in galaxies
• Tidal disruption theory
• Candidates discovered by ROSAT
• Search for flares with GALEX
• GALEX tidal disruption flare detections
• Future detections
Outline
• Direct dynamical measurement of MBH is possible when Rinf ≈ GMBH/2 is resolved.
Probing the Mass of DormantSupermassive Black Holes
Kormendy & Bender (1999)
Ghez+ (2005)
Milky Way
M31
• A dormant black hole will be revealed when a star approaches closer than RT≈Rstar(MBH/Mstar)1/3, and is tidally disrupted.
• This is a rare event in a galaxy, occurring only once every 103-105 yr depending on MBH and the nuclear density profile of the galaxy.
Probing the Mass of DormantSupermassive Black Holes
Rees (1988)
• L ≈ LEdd = 1.3x1044 (MBH/106 Msun) ergs s-1
• Blackbody spectrum: Teff=(LEdd/4RT
2)1/4.
• Start of flare: (t0-tD) k-3/2MBH
1/2
• Power-law decay: dM/dt (t-tD)-5/3.
• The temperature, luminosity, and decay of the flare can be used as a direct probe of MBH.
Probing the Mass of DormantSupermassive Black Holes
Evans & Kochanek (1989)
t-5/3
• The ROSAT All-sky survey in 1990-1991 sampled hundreds of thousands of galaxies in the soft X-ray band (0.1-2.4 keV).
• Detected a large amplitude soft X-ray flare from 3 galaxies which were classified as non-active from ground based spectra.
• Follow-up narrow-slit HST/STIS spectroscopy confirmed the ground-based classifications of 2 of the galaxies (Gezari+ 2003).
Previous Tidal Disruption Event Candidates
Halpern, Gezari, & Komossa (2004)
Lflare/L10yr = 240
Lflare/L10yr = 1000
Lflare/L10yr = 6000
HST Chandra
• 50 cm telescope with a 1.2 deg2 field of view.
• Simultaneous FUV/NUV imaging and grism spectra
• Data is time-tagged photon data (t=5ms) accumulated in 1.5 ks eclipses.
• Some deep fields are revisited over a baseline of 2-4 years to complete deep observations.
• Take advantage of the UV sensitivity, temporal sampling, and large survey volume of GALEX to search for flares.
Searching for Flares with GALEX
1350 Å 1750 Å 2800 Å | | |
• Assume L=LEdd, and Teff=2.5x105 (MBH/106 Msun)1/12 K.
• The large K correction makes flares detectable out to high z.
• Estimated attenuation by HI absorption for z>0.6 from Madau (1995)
• Contrast with host early type spirals and elliptical galaxies not a problem for detection in the UV.
Searching for Flares with GALEX
1350 Å 1750 Å 2800 Å | | |
Gezari+ (in prep)
5x107 Msun
1x106 Msun
• Estimate black hole mass function from Ferguson & Sandage (1991) luminosity function of E+S0 galaxies.
• Multiply by a factor of 2 for bulges in early-type spirals.
• Use MBH dependent event rate from Wang & Merritt (2004).
• Assume fraction of flares that radiate at LEdd from Ulmer (1999).
• Multiply by volume to which an LEdd flare can be detected in the FUV by a GALEX DIS exposure.
Searching for Flares with GALEX
1350 Å 1750 Å 2800 Å | | |
Gezari+ (in prep)
• Match UV sources that vary between yearly epochs at the 5 level with the CFHT Legacy Survey optical catalog.
• Rule out sources with optical hosts with the colors and morphology of a star or quasar.
• Follow up galaxy hosts that do not have an hard X-ray detection with optical spectroscopy to look for signs of an AGN.
• Trigger Chandra TOO X-ray observations of our best candidates.
Searching for Flares with GALEX
1350 Å 1750 Å 2800 Å | | |
Gezari+ (in prep)
stars
QSOs
galaxies
x : X-ray source
• AEGIS DEEP2 spectrum and ACS image of an early-type galaxy at z=0.3698.
• No evidence of Seyfert-like emission lines.
• No detection of hard X-rays.
• Archival Chandra observations during the flare detected a variable extremely soft X-ray source coincident with the galaxy.
Tidal Disruption Flare Detections
1350 Å 1750 Å 2800 Å | | |
Gezari+ (2006)
• TOO VLT spectrum and CFHTLS image of an early-type galaxy at z=0.326.
• No evidence of Seyfert-like emission lines.
• No detection of hard X-rays.
• First optical detection of a tidal disruption flare.
• Triggered a Chandra TOO observation which detected an extremely soft X-ray source coincident with the galaxy.
Tidal Disruption Flare Detections
1350 Å 1750 Å 2800 Å | | |
Gezari+ (in prep)
• Well described by a t-5/3 power-law decay.• MBH=k3[(t0-tD)/0.11]2 *106 Msun
Tidal Disruption Flare Detections
1350 Å 1750 Å 2800 Å | | |
Gezari+ (2006) Gezari+ (in prep)(t0-tD)/(1+z)=0.1-0.7 yr k3 (1-4)x107 Msun
(t0-tD)/(1+z)=0.45±0.4 yr k3 (1.7±0.3)x107 Msun
• TBB ≈ few x 105 K • RBB ≈ 1 x 1013 cm • RT= 1.5 x 1013 (MBH/107 Msun)1/3 cm• RSch= 3 x 1012 (MBH/107 Msun) cm
Tidal Disruption Flare Detections
Gezari+ (2006) Gezari+ (in prep)
Lbol = 6.5x1044 ergs s-1
Lbol > 1x1044 ergs s-1
• GALEX has proven to be successful in detecting tidal disruption flares.
• Goal is to measure the detailed properties and rate of the events to probe accretion physics, the mass of the black hole, and evolution of the tidal disruption rate.
• The next generation of optical synoptic surveys such as Pan-STARRs and LSST have the potential to detect hundreds of events.
• With a large sample we can probe the evolution of the black hole mass function, independent of studies of active galaxies.
Future Detections
Stay Tuned for More Flares!