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The Search for Tidal Disruption Flares with with GALEX

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

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The Search for Tidal Disruption Flares with with GALEX

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  1. The Search for Tidal Disruption Flares withwith 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

  2. Outline Capability of GALEX to Study Variability Tidal Disruption Flares: Theory and Observations Searching for Flares with GALEX Tidal Disruption Flare Discovered Future Dedicated Time Domain Survey

  3. 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 GALEX SDSS

  4. 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.

  5. 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. t-5/3 Evans & Kochanek (1989)

  6. 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.

  7. 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) NLSy1 Sy1.9 non- active Properties of a tidal disruption event!

  8. Halpern, Gezari, & Komossa (2004) HST Chandra STIS Lflare/L10yr = 240 Follow-up narrow-slit HST/STIS spectra confirmed the galaxies as non-active (Gezari et al. 2003). Lflare/L10yr = 1000 STIS Lflare/L10yr = 6000

  9. NGC 5905 Halpern, Gezari, & Komossa (2004) Gezari et al. (2003) • 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.

  10. 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.

  11. Selection of Variable Sources is a measure of the dispersion in mag due to photometric errors

  12. Identify the Host of the Flare Followed-up with TOO optical spectroscopy • Optically unresolved oOptically resolved • x Xray detection • Optically variable Spectroscopic AGN Stars Quasars Galaxies CFHTLS Colors & Morphology The most convincing candidates have inactive galaxy hosts!

  13. 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.

  14. Confirmed Inactive Galaxy Host AEGIS DEEP2 Keck Spectrum Gezari et al. 2006, in press Early-type galaxy with no evidence of Seyfert activity!

  15. UV Flare with t-5/3 Decay The delay between tD and the peak of the flare implies MBH > 108 M. tD t-5/3 Gezari et al. 2006, in press

  16. Simultaneous SED of the Flare GALEX Properties of the Flare: T = 1 to 5 x 105 K L = 1044 to 1046 erg s-1 Macc > 0.3 Msun In excellent agreement with theoretical predictions for a stellar disruption flare. Chandra (AEGIS) CFHT SNLS Gezari et al. 2006, in press Strongest empirical evidence for a stellar disruption event to date!

  17. 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!

  18. New Confirmed Flare from Inactive Galaxy TOO VLT Spectrum courtesy of Stephane Basa z = 0.32 2004.6 2004.9 No Seyfert emission lines! Awaiting results from Chandra TOO!

  19. 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

  20. Stay Tuned….

  21. Candidate in D4 2003 2005

  22. CFHT SNLS courtesy of Stephane Basa Galaxy Host Le Phare photo-z fits an elliptical galaxy at z=0.56 FUVflare=22.9 2004.4 2006.0 • 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. Flare SED T = 5 x 105 K

  23. Image Subtraction 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. Transient sources!

  24. Transient Source

  25. CFHT SNLS courtesy of Stephane Basa 2005.0 2005.6 Optically unresolved quasar!

  26. Detection Rate with GALEX 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. Volume to which flares can be detected in a 10 ks DIS exposure 6.5x10-4yr-1 (MBH/106 M)-.25 (WM 2004) E+S0 luminosity function MBH = 8.1x10-5 (Lbulge/L )0.18 (FS 1991, MT 1991, MF 2001) Yields 5 events yr-1 sq.deg-1(z ≤ 1)

  27. Transient Source

  28. tmax≈2005.12 CFHT SNLS courtesy of Stephane Basa 2005.0 2005.6 Optically resolved galaxy!

  29. 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.

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