Results from the search for tidal disruption flares in the GALEX Deep

Download Report

Transcript Results from the search for tidal disruption flares in the GALEX Deep

The Search for Tidal Disruption Flares with
with GALEX
Suvi Gezari
California Institute of Technology
& The GALEX Team
California Institute of Technology,
Laboratoire Astrophysique de Marseille
Xi’an AGN 2006 -- October 21, 2006
Capability of GALEX to Study Variability
ii. Tidal Disruption Flares: Theory and Observations
iii. Searching for Flares with GALEX
iv. Tidal Disruption Flare Discovered
v. Future Dedicated Time Domain Survey
Capabilities of GALEX
Deep Imaging Survey, 80 deg2, >30 ksec (mlim ~ 25)
Observations obtained in 1.5 ksec eclipses
Time-tagged photon data (time resolution of 5 msec)
Large field of view (1.2 sq. deg.) and a large survey volume
Low sky background (source detection with 10 photons)
Simultaneous NUV (1750 - 2750 Å) and FUV (1350 - 1750 Å) imaging
Simultaneous R=100/200 NUV/FUV spectroscopic grism data
Tidal Disruption Events
A star will be disrupted when:
Rp < RT ≈ R(MBH/M)1/3
Evans & Kochanek (1989)
The bound fraction (< 0.5) of the stellar debris
falls back onto the black hole, resulting in a
luminous accretion flare.
Properties of a Tidal Disruption Flare
• For 106 - 108 M black holes, the stellar debris
accretes in a thick disk (Ulmer 1999)
• Lflare≈LEdd=1.31045 M7 erg s-1
• Teff≈(LEdd/4RT2)1/4=3105 M71/12 K
• L(t)=(dM/dt)c2t-5/3
• dN/dt7/2MBH-1MBH-1/4≈10-4 yr-1
(Wang & Merritt 2004)
Tidal disruption theory predicts
rare but luminous flares that
peak in the UV/X-ray domain,
with decay timescales ~ months.
Evans & Kochanek (1989)
Why Search For Tidal Disruption Events?
• They are an unambiguous probe for supermassive black
holes lurking in the nuclei of normal galaxies.
• The luminosity, temperature, and decay of the flare is
dependent on the mass and spin of the black hole.
• They may contribute to black hole growth over cosmic
times, and the faint end of the AGN luminosity function.
• Tidal disruption rates are sensitive to the structure and
dynamics of the stellar galaxy nucleus.
Flares Detected by ROSAT
The ROSAT All-Sky Survey (RASS) conducted in 1990-1991 was
an excellent experiment to detect TDEs since it sampled 3x105
galaxies in the soft X-ray band (0.1 - 2.4 keV).
Tbb = 6 - 12 x 105 K
Lx = 1042 - 1044 ergs s-1
tflare ~ months
Event rate ≈ 1 x 10-5 yr-1 (Donley et al. 2002)
Properties of a tidal
disruption event!
Halpern, Gezari, & Komossa (2004)
Lflare/L10yr = 240
Follow-up narrow-slit HST/STIS
spectra confirmed the galaxies as
non-active (Gezari et al. 2003).
Lflare/L10yr = 1000
Lflare/L10yr = 6000
NGC 5905
Gezari et al. (2003)
Halpern, Gezari, & Komossa (2004)
• Narrow-line
emission requires excitation by a
persistent Seyfert nucleus.
• Seyfert nucleus in it inner 0.”1 was previously
masked by H II regions in ground-based spectra
• The Chandra upper-limit on the nuclear X-ray
luminosity is consistent with the predicted Lx from
L(H) for LLAGNs, of ~ 9 x 1038 ergs s-1.
Li et al. (2002)
modeled the event as
the partial stripping of
a low-mass star, or the
disruption of a brown
dwarf or giant planet.
Search for Flares in the Deep Imaging Survey
TOO observations with
Chandra and Keck will
probe the early-phase
of decay of TDEs for
the first time!
We take advantage of the UV sensitivity, temporal
sampling, and large survey volume of GALEX.
Selection of Variable Sources
 is a measure of the dispersion in
mag due to photometric errors
Identify the Host of the Flare
with TOO
 Optically unresolved
o Optically resolved
x Xray detection
 Optically variable
 Spectroscopic AGN
CFHTLS Colors & Morphology
The most convincing candidates have inactive galaxy hosts!
Some hosts show AGN activity
Seyfert at z = 0.355!
Jan 2006 MDM 2.4m spectrum
L([O III]) = 9.4x1040 ergs s-1
Archival Chandra ACIS detection in
April 2002 with Lx ≈ 9.3x1042 ergs s-1
Presence of persistent Seyfert activity makes the tidal
disruption scenario difficult to prove.
Confirmed Inactive Galaxy Host
AEGIS DEEP2 Keck Spectrum
Gezari et al. 2006, in press
Early-type galaxy with no evidence of Seyfert activity!
UV Flare with t-5/3 Decay
Gezari et al. 2006, in press
The delay between
tD and the peak of
the flare implies
MBH > 108 M.
Simultaneous SED of the Flare
Properties of the Flare:
Chandra T = 1 to 5 x 105 K
(AEGIS) L = 1044 to 1046 erg s-1
Macc > 0.3 Msun
In excellent agreement with
theoretical predictions for a
stellar disruption flare.
Gezari et al. 2006, in press
Strongest empirical evidence for a stellar disruption event to date!
Disruption of a Star by a Spinning Black Hole
Constraints on MBH:
MBH(t0-tD) > 1 x 108 Msun
MBH() ~ 1+1-.5 x 109 Msun
Critical upper limit on MBH for RT > Rs:
Mcrit = 1 x 108 Msun (no spin)
Mcrit = 8 x 108 Msun (O5 star)
Mcrit = 8 x 108 Msun (with spin)
Upper limit on MBH for RBB > Rms:
RBB < 4 x 1013 cm
Rms = 6Rg (no spin)
Rms  Rg (with spin)
If MBH > 108 Msun then RBB < 6Rg,
and the black hole must have spin!
New Confirmed Flare from Inactive Galaxy
TOO VLT Spectrum
courtesy of Stephane Basa
2004.6 z
= 0.32
No Seyfert emission lines!
Awaiting results from
Chandra TOO!
Dedicated Time Domain Survey
• First time domain survey in UV
• Designed to complement future ground-based TDS (PanStarrs, LSST)
• All time-domain products, notably variable object alerts, will be
immediately made public for community follow-up
• Produce automated pipeline triggers to generate IAU and/or GCN
• Prevalence of UV-bright early evolution of flaring objects
• The TDS may detect: supernovae, gamma-ray faint bursts, novae,
macronovae, magnetic degenerate binaries, low mass x-ray binaries,
chromospherically active stars, QSOs and AGNs, pulsating degenerates,
luminous blue variables
Stay Tuned….
Candidate in D4
2003 2005
Galaxy Host
courtesy of Stephane Basa
Le Phare photo-z
fits an elliptical
galaxy at z=0.56
Flare SED
T = 5 x 105 K
• Chandra TOO X-ray
observation in May 8, 2006 does
not have a soft blackbody
• Follow-up spectrum of a
ROSAT candidate also showed
hard X-ray spectrum.
• Awaiting results from VLT
optical spectrum.
Image Subtraction
1) Register images using
a list of point source
positions in both images.
2) Subtract second image
from reference image
after polynomial spatial
3) Search difference
image for sources above
a threshold value with a
correlation with the PSF
of > 0.5.
Transient sources!
Transient Source
courtesy of Stephane Basa
Optically unresolved quasar!
Detection Rate with GALEX
GALEX FUV band is sensitive to
Rayleigh Jean’s tail of the soft Xray blackbody emission.
A large K correction makes
unextincted flare flux detectable
by DIS out to high z.
6.5x10-4 yr-1 (MBH/106 M)-.25
(WM 2004)
Volume to which
flares can be
E+S0 luminosity function
detected in a 10 ks
MBH = 8.1x10-5 (Lbulge/ L )0.18 DIS exposure
(FS 1991, MT 1991, MF 2001)
Yields 5 events yr-1 sq.deg-1(z ≤ 1)
Transient Source
courtesy of Stephane Basa
Optically resolved galaxy!
• Rich data set in spectral and temporal coverage when you
combine GALEX + CFHT SNLS.
• Optical spectroscopy is necessary to confirm that flaring
galaxies do not have an AGN, and we follow up our best
candidates with Chandra TOO imaging.
• Legacy fields with multiwavelength coverage seem to be the
most promising for identifying interesting variable sources.
• Can measure the distribution of masses and spins of dormant
black holes.
• We have a new candidate with an inactive galaxy host
confirmed by VLT TOO optical spectroscopy. We are awaiting the
Chandra TOO observation results.