Transcript Document

Tidal Disruptions of Stars
by Supermassive Black Holes
Suvi Gezari
(Caltech)
Chris Martin
& GALEX Team
Bruno Milliard (GALEX)
Stephane Basa (SNLS)
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
Probing the Mass of Dormant
Supermassive Black Holes
Milky Way
• Direct dynamical measurement
of MBH is possible when Rinf ≈
GMBH/2 is resolved.
M31
Ghez+ (2005)
Kormendy & Bender (1999)
Probing the Mass of Dormant
Supermassive 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)
Probing the Mass of Dormant
Supermassive Black Holes
• L ≈ LEdd = 1.3x1044 (MBH/106
Msun) ergs s-1
t-5/3
• Blackbody spectrum:
Teff=(LEdd/4RT2)1/4.
• Start of flare:
(t0-tD)  k-3/2MBH1/2
Evans & Kochanek (1989)
• 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.
Previous Tidal Disruption
Event Candidates
HST
Chandra
Lflare/L10yr = 240
• 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).
Lflare/L10yr = 1000
• Detected a large amplitude
soft X-ray flare from 3
galaxies which were classified
as non-active from ground
based spectra.
Lflare/L10yr = 6000
• Follow-up narrow-slit
HST/STIS spectroscopy
confirmed the ground-based
classifications of 2 of the
galaxies (Gezari+ 2003).
Halpern, Gezari, & Komossa (2004)
Searching for Flares with GALEX
1350 Å 1750 Å
|
|
2800 Å
|
• 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
5x107 Msun
1x106 Msun
1350 Å 1750 Å
|
2800 Å
|
Gezari+ (in prep)
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• 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)
|
• 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
galaxies
QSOs
1350 Å 1750 Å
|
2800 Å
|
| x : X-ray source
stars
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.
Tidal Disruption Flare Detections
1350 Å 1750 Å
|
|
Gezari+ (2006)
2800 Å
|
• 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+ (in prep)
|
• TOO VLT spectrum and
CFHTLS image of an earlytype 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+ (2006)
(t0-tD)/(1+z)=0.1-0.7 yr
 k3 (1-4)x107 Msun
•
•
Gezari+ (in prep)
Well described by a t-5/3 power-law decay.
MBH=k3[(t0-tD)/0.11]2 *106 Msun
(t0-tD)/(1+z)=0.45±0.4 yr
 k3 (1.7±0.3)x107 Msun
Tidal Disruption Flare Detections
Lbol = 6.5x1044 ergs s-1
Lbol > 1x1044 ergs s-1
Gezari+ (2006)
•
•
•
•
105
TBB ≈ few x
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
Gezari+ (in prep)
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.
Stay Tuned for More Flares!