Tidal Disruptions of Stars by Supermassive Black Holes

1. Tidal Disruptions of Stars by Supermassive Black Holes Suvi Gezari (Caltech) Chris Martin & GALEX Team Bruno Milliard (GALEX) Stephane Basa (SNLS)

2. Outline • 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

3. Probing the Mass of DormantSupermassive Black Holes Milky Way • Direct dynamical measurement of MBH is possible when Rinf ≈ GMBH/2 is resolved. M31 Ghez+ (2005) Kormendy & Bender (1999)

4. Probing the Mass of DormantSupermassive Black Holes • 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. Rees (1988)

5. Probing the Mass of DormantSupermassive Black Holes • L ≈ LEdd = 1.3x1044 (MBH/106 Msun) ergs s-1 • Blackbody spectrum: Teff=(LEdd/4RT2)1/4. • Start of flare: (t0-tD)  k-3/2MBH1/2 • 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. t-5/3 Evans & Kochanek (1989)

6. Previous Tidal Disruption Event Candidates HST Chandra • 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). Lflare/L10yr = 240 Lflare/L10yr = 1000 Lflare/L10yr = 6000 Halpern, Gezari, & Komossa (2004)

7. Searching for Flares with GALEX • 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. 1350 Å 1750 Å 2800 Å | | |

8. Searching for Flares with GALEX • 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. 5x107 Msun 1x106 Msun 1350 Å 1750 Å 2800 Å | | | Gezari+ (in prep)

9. Searching for Flares with GALEX • 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. 1350 Å 1750 Å 2800 Å | | | Gezari+ (in prep)

10. Searching for Flares with GALEX • 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. galaxies QSOs 1350 Å 1750 Å 2800 Å | | | x : X-ray source stars Gezari+ (in prep)

11. Tidal Disruption Flare Detections • 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. 1350 Å 1750 Å 2800 Å | | | Gezari+ (2006)

12. Tidal Disruption Flare Detections • 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. 1350 Å 1750 Å 2800 Å | | | Gezari+ (in prep)

13. Tidal Disruption Flare Detections • Well described by a t-5/3 power-law decay. • MBH=k3[(t0-tD)/0.11]2 *106 Msun 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

14. Tidal Disruption Flare Detections • 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 Lbol = 6.5x1044 ergs s-1 Lbol > 1x1044 ergs s-1 Gezari+ (2006) Gezari+ (in prep)

15. Future Detections • 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.

16. Stay Tuned for More Flares!