the search for tidal disruption flares with with galex
DESCRIPTION
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. Outline Capability of GALEX to Study Variability - PowerPoint PPT PresentationTRANSCRIPT
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Suvi Gezari
California Institute of Technology
& The GALEX Team
California Institute of Technology,
Laboratoire Astrophysique de Marseille
Xi’an AGN 2006 -- October 21, 2006
The Search for Tidal Disruption Flares withThe Search for Tidal Disruption Flares with with GALEXwith GALEX
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OutlineOutline
i. 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
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Capabilities of GALEXCapabilities 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
SDSSGALEX
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A star will be disrupted when:Rp < RT ≈ R(MBH/M)1/3
Evans & Kochanek (1989)
Tidal Disruption EventsTidal Disruption Events
The bound fraction (< 0.5) of the stellar debris falls back onto the black hole, resulting in a luminous accretion flare.
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Evans & Kochanek (1989)
t-5/3
Properties of a Tidal Disruption FlareProperties of a Tidal Disruption Flare
• For 106 - 108 M black holes, the stellar debris accretes in a thick disk (Ulmer 1999)
• Lflare≈LEdd=1.31045 M7 erg s-1
• Teff≈(LEdd/4RT2)1/4=3105 M7
1/12 K
• L(t)=(dM/dt)c2t-5/3
• dN/dt7/2MBH-1MBH
-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.
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Why Search For Tidal Disruption Events?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.
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Flares Detected by ROSATFlares 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!
NLSy1
Sy1.9
non-active
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Halpern, Gezari, & Komossa (2004)
Follow-up narrow-slit HST/STIS spectra confirmed the galaxies as non-active (Gezari et al. 2003).
Lflare/L10yr = 240
Lflare/L10yr = 6000
Lflare/L10yr = 1000
HST Chandra
STIS
STIS
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• 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.
Halpern, Gezari, & Komossa (2004)
NGC 5905
Gezari et al. (2003)
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Search for Flares in the Deep Imaging SurveySearch 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.
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Selection of Variable SourcesSelection of Variable Sources
is a measure of the dispersion in mag due to photometric errors
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Identify the Host of the FlareIdentify the Host of the Flare
StarsQuasarsGalaxies
CFHTLS Colors & Morphology
Followed-up with TOO optical
spectroscopy
The most convincing candidates have inactive galaxy hosts!
Optically unresolvedo Optically resolvedx Xray detection Optically variable
Spectroscopic AGN
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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.
Jan 2006 MDM 2.4m spectrumL([O III]) = 9.4x1040 ergs s-1
Seyfert at z = 0.355!Some hosts show AGN activitySome hosts show AGN activity
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AEGIS DEEP2 Keck Spectrum
Early-type galaxy with no evidence of Seyfert activity!
Confirmed Inactive Galaxy HostConfirmed Inactive Galaxy Host
Gezari et al. 2006, in press
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UV Flare with tUV Flare with t-5/3-5/3 Decay Decay
t-5/3 The delay between tD and the peak of the flare implies MBH > 108 M.
tD
Gezari et al. 2006, in press
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Properties of the Flare:T = 1 to 5 x 105 KL = 1044 to 1046 erg s-1
Macc > 0.3 Msun
In excellent agreement with theoretical predictions for a stellar disruption flare.
Simultaneous SED of the FlareSimultaneous SED of the Flare
CFHT SNLS
GALEX
Chandra(AEGIS)
Strongest empirical evidence for a stellar disruption event to date!
Gezari et al. 2006, in press
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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 cmRms = 6Rg (no spin)Rms Rg (with spin)
If MBH > 108 Msun then RBB < 6Rg, and the black hole must have spin!
Disruption of a Star by a Disruption of a Star by a SpinningSpinning Black Hole Black Hole
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2004.6 2004.9
NewNew Confirmed Flare from Inactive Galaxy Confirmed Flare from Inactive Galaxy
TOO VLT Spectrumcourtesy of Stephane Basa
Awaiting results from Chandra TOO!
z = 0.32
No Seyfert emission lines!
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Dedicated Time Domain SurveyDedicated 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 circulars• 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
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Stay Tuned….Stay Tuned….
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Candidate in D4Candidate in D42003 2005
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2004.4 2006.0
Le Phare photo-z fits an elliptical galaxy at z=0.56
FUVflare=22.9
CFHT SNLScourtesy of Stephane Basa
Flare SED
Galaxy Host
• Chandra TOO X-ray observation in May 8, 2006 does not have a soft blackbody temperature.
• Follow-up spectrum of a ROSAT candidate also showed hard X-ray spectrum.
• Awaiting results from VLT optical spectrum.
T = 5 x 105 K
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Image SubtractionImage Subtraction
Transient sources!
Method:1) Register images using a list of point source positions in both images.2) Subtract second image from reference image after polynomial spatial warping.3) Search difference image for sources above a threshold value with a correlation with the PSF of > 0.5.
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Transient SourceTransient Source
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Optically unresolved quasar!
CFHT SNLScourtesy of Stephane Basa
2005.0 2005.6
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GALEX FUV band is sensitive to Rayleigh Jean’s tail of the soft X-ray 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)
E+S0 luminosity functionMBH = 8.1x10-5 (Lbulge/ L )0.18
(FS 1991, MT 1991, MF 2001)
Volume to which flares can be detected in a 10 ks DIS exposure
Yields 5 events yr-1 sq.deg-1(z ≤ 1)
Detection Rate with GALEXDetection Rate with GALEX
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Transient SourceTransient Source
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Optically resolved galaxy!
tmax≈2005.12
CFHT SNLScourtesy of Stephane Basa
2005.0 2005.6
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SummarySummary
• 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.