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!