Presentation on theme: "Suvi Gezari California Institute of Technology & The GALEX Team California Institute of Technology, Laboratoire Astrophysique de Marseille Xi’an AGN 2006."— Presentation transcript:
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 with with GALEX
Outline Outline 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
Capabilities of GALEX Deep Imaging Survey, 80 deg 2, >30 ksec (m lim ~ 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
A star will be disrupted when: R p < R T ≈ R (M BH /M ) 1/3 Evans & Kochanek (1989) Tidal Disruption Events The bound fraction (< 0.5) of the stellar debris falls back onto the black hole, resulting in a luminous accretion flare.
Evans & Kochanek (1989) t -5/3 Properties of a Tidal Disruption Flare For 10 6 - 10 8 M black holes, the stellar debris accretes in a thick disk (Ulmer 1999) L flare ≈L Edd =1.3 10 45 M 7 erg s -1 T eff ≈(L Edd /4 R T 2 ) 1/4 =3 10 5 M 7 1/12 K L(t)= (dM/dt)c 2 t -5/3 dN/dt 7/2 M BH -1 M BH -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.
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 3x10 5 galaxies in the soft X-ray band (0.1 - 2.4 keV). T bb = 6 - 12 x 10 5 K L x = 10 42 - 10 44 ergs s -1 t flare ~ months Event rate ≈ 1 x 10 -5 yr -1 (Donley et al. 2002) Properties of a tidal disruption event! NLSy1 Sy1.9 non- active
Halpern, Gezari, & Komossa (2004) Follow-up narrow-slit HST/STIS spectra confirmed the galaxies as non-active (Gezari et al. 2003). L flare /L 10yr = 240 L flare /L 10yr = 6000 L flare /L 10yr = 1000 HSTChandra STIS
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 L x from L(H ) for LLAGNs, of ~ 9 x 10 38 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)
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 Stars Quasars Galaxies CFHTLS Colors & Morphology Followed-up with TOO optical spectroscopy The most convincing candidates have inactive galaxy hosts! Optically unresolved o Optically resolved x Xray detection Optically variable Spectroscopic AGN
Archival Chandra ACIS detection in April 2002 with L x ≈ 9.3x10 42 ergs s -1 Presence of persistent Seyfert activity makes the tidal disruption scenario difficult to prove. Jan 2006 MDM 2.4m spectrum L([O III]) = 9.4x10 40 ergs s -1 Seyfert at z = 0.355! Some hosts show AGN activity
AEGIS DEEP2 Keck Spectrum Early-type galaxy with no evidence of Seyfert activity! Confirmed Inactive Galaxy Host Gezari et al. 2006, in press
UV Flare with t -5/3 Decay t -5/3 The delay between t D and the peak of the flare implies M BH > 10 8 M . tDtD Gezari et al. 2006, in press
Properties of the Flare: T = 1 to 5 x 10 5 K L = 10 44 to 10 46 erg s -1 M acc > 0.3 M sun In excellent agreement with theoretical predictions for a stellar disruption flare. Simultaneous 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
Constraints on M BH : M BH (t 0 -t D ) > 1 x 10 8 M sun M BH ( ) ~ 1 +1 -.5 x 10 9 M sun Critical upper limit on M BH for R T > R s : M crit = 1 x 10 8 M sun (no spin) M crit = 8 x 10 8 M sun (O5 star) M crit = 8 x 10 8 M sun (with spin) Upper limit on M BH for R BB > R ms : R BB < 4 x 10 13 cm R ms = 6R g (no spin) R ms R g (with spin) If M BH > 10 8 M sun then R BB < 6R g, and the black hole must have spin! Disruption of a Star by a Spinning Black Hole
2004.6 2004.9 New Confirmed Flare from Inactive Galaxy TOO VLT Spectrum courtesy of Stephane Basa Awaiting results from Chandra TOO! z = 0.32 No Seyfert emission lines!
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 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
2004.4 2006.0 Le Phare photo-z fits an elliptical galaxy at z=0.56 FUV flare =22.9 CFHT SNLS courtesy 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 10 5 K
Image 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.
Optically unresolved quasar! CFHT SNLS courtesy of Stephane Basa 2005.0 2005.6
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 (M BH /10 6 M ) -.25 (WM 2004) E+S0 luminosity function M BH = 8.1x10 -5 (L bulge / 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 GALEX
Optically resolved galaxy! t max ≈2005.12 CFHT SNLS courtesy of Stephane Basa 2005.0 2005.6
Summary 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.