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GROND observations of GRB Afterglows

GROND observations of GRB Afterglows. Color changes in afterglow light curves Dark bursts High-z perspectives. Jochen Greiner Max-Planck-Institute for extraterrestrial Physics Garching, Germany. GROND = GR B O ptical/ N IR D etector. r. g. z. i. H. K. J.

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GROND observations of GRB Afterglows

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  1. GROND observations of GRB Afterglows Color changes in afterglow light curves Dark bursts High-z perspectives Jochen GreinerMax-Planck-Institute for extraterrestrial Physics Garching, Germany

  2. GROND=GRBOptical/NIRDetector r g z i H K J Imaging in 7 channels simultaneously

  3. GROND: General Design • 7 bands: Sloan g’, r’, i’, z’ and J, H, K One detector for one filter band (no movable filters!) 3 HAWAII 1K*1K Arrays + 4 E2V 2K*2K CCDs • Field-of-view: Visual: 5.4´x5.4´ (0.16´´/pixel) NIR: 10´x10´ (0.59´´/pixel) • Dichroics tuned to minimize intrinsic polarization effects • 2 shutters, i.e. g’r’ and i’z’ pairs of CCDs have same exposure • Combined telescope and intrinsic mirror (K-band) dithering • Sensitivity: 4 min 1 hr gr 21.5 mag 24.5 mag iz 20.5 mag 23.5 mag J/H/K (AB) 18.5/17.5/16.5 mag 21.5/20.5/19.5 mag

  4. GROND @ 2.2m MPI/ESO telescope History: First light: Apr 30, 2007 First GRB: May 21, 2007 Photometric calibration: Jul 2007 Routine observations: since Sep 2007 fastest response time: 2 min GROND

  5. 1. Color changes in GRB light curves GRB 070802: Sum of multiple flares dominates total flux: mimics spectral break because flares are harder than AG emission GRB 080129: flare of 3 mag amplitude and 20 min duration: SEDs on few minutes timescale shows changes throughout the flare GRB 080913B: two-component jet seen on-axis, with different α, ß, p GRB 080710: two-component jet seen off-axis

  6. GRB 070802: flares Krühler et al. 2008 Decay steeper after flares + each following flare has even steeper decay  thus no simple energy ejection Flares have harder emission than afterglow if not resolved, looks like spectral break

  7. Changing SEDs GRB 080129 Greiner et al. 2009 • spectral changes clearly visible, down to timescales of ~5 min • Likely cause: Residual collision a la Li & Waxman (2008)

  8. Two GRBs with 2-component jets… Krühler et al. 2009 Filgas et al. 2010 GRB 080710 GRB 080413B I II III IV

  9. …, but different SEDs GRB 080710 GRB 080413B Clear difference in spectral evolution, yet (nearly) similar interpretation: Two component jet viewed off-axis: fits light curve and SED; also consistent with low Eγiso and soft BAT spectrum (low Epeak) 2nd jet: Γo~50, θjet>10o Two component jet viewed on-axis: fits light curve and SED evolution(no cooling break due to flattening);jets have different α, ß, p2nd jet: Γo~20, θjet>9o

  10. 2. The “dark burst” issue Potential causes for optical darkness: Intrinsically faint (e.g. Fynbo et al. 2001) High redshift (e.g. Lamb & Reichart 2000) Large extinction (e.g. Rhoads 1999, Fynbo et al. 2001) Dark bursts: ßox < 0.5 Jakobsson et al. 2004…2009

  11. DARK BURSTS pre-Swift “dark” GRBs vs. UVOT vs. GROND Figure adapted fromFox et al. 2004 UVOT probes ~5 mag deeper at earlier times; yet large fraction of “dark” bursts high-z, extinction UVOT/Swift          GROND at the 2.2m MPI/ESO telescope 4 min GROND probes another ~2 mag deeper, +NIR 10 min 1 hr First exposure starts within 2-10 min

  12. The sample Selection criterion: Swift/XRT detection 20 GRBs with GROND start-of-observation within 30 min 2 not detected with GROND: GRB 090429B (but Gemini; z~9.3 candidate (Tanvir talk) ) GRB 080915 (very faint X-ray afterglow, but consistent with picture as described in the following) GROND limit in 2 ks

  13. Data handling • All bursts: fit simultaneous Swift/XRT and GROND SED  determine ßo, ßx, AV • z-distribution (all 18 have z) • AV-distribution Completeness level: 18/20 ≡ 90%

  14. XRT-GROND SEDs KHJ zirg GRB 080710 Only 1 GRB with X-ray and optical/NIR on same powl ßox sensitive to extinction AV AND SED shape ßox R band 1 keV GRB 080129 GRB 070802

  15. 2 examples with flat SEDs … both have nearly ßox ~ 0: AV ~ 1.2 z=6.7 GRB 080805 GRB 080913

  16. Dark bursts in our sample 5 dark bursts≡ 25%(with another 4 just at borderline) Consistent with earlier numbers

  17. SED slope distribution Majority has break, but break energy not well defined Break is consistent with 0.5 in all but 1 case Implies optical brightness up to ~4 mag fainter than without break, if break energy near 0.1 keV

  18. Intrinsic AV distribution in GRBs We see substantially more afterglows with solid AV detection 25% of GRBs have AV~0.5 mag; ~10% have AV > 1 mag Within statistics similar to AV distribution of core-collapse SN-IIP 10% (Kann et al. 2009) 25%

  19. Dark bursts revealed Optical darkness well explained by combination of • up to 4 mag due to break between X-ray and optical SED • combination of moderate redshift and moderate AV “Dark” bursts are not a reservoir of high-z bursts

  20. Redshift distribution Has completeness of 95% Is flatter than previous distributions (which had smaller completeness levels) Statistics not yet great

  21. The 2 non-detected GRBs Extrapolating X-ray spectrum with Δß=0.5, and comparing with GROND upper limits: • GRB 080915  consistent with even low z and AV=0 • GRB 090429B  consistent with z>6 & AV=0, or z~5 & AV~1 Effectively not only 18 detected, but all GRBs consistent with above picture GRB 090429B

  22. GRBs at redshift >5 060522 5.11071025 5.2060927 5.47050814 5.77050904 6.29080913 6.7090423 8.2 3. High-z capabilities of GROND GROND statistics (May 2007 - Apr 2010): 181 GRBs (south of δ=+30o) observed out of 258 95 detections out of 146 GRBs with XRT 31 with Ly-alpha affecting g’-band (z~2.5-3.5) 3 with 2175 Å bump (a la GRB 070802 @ z=2.45) 5 with (only) g’-band drop out (z=3.5-5) 2 high-z bursts (080913, 090423) But: Severe bias against z~4-6 bursts with moderate extinction Observed r’-band extinction

  23. z=10 z=13 GROND is sensitive up to z~13 7 filter bands cover Lyman-α for large z-range: 3<z<13 (assuming K-band detection)

  24. Example of GROND sensitivity 7 filter bands cover Lyman-α for large z-range: 3<z<13 GROND has proper z-band sensitivity to define drop-out for z~8 Advantage of separate detector per filter: can add sensibilization for spectral band: GROND@2.2m detector/filter is 4x more sensitive than FORS2@VLT  total efficiency only 4x less than FORS2, not 16 (8.22/2.22)

  25. Recognize ‘thermal’ transients 7 filter bands cover Lyman-α for large z-range: 3<z<13 GROND has proper z-band sensitivity to define drop-out for z~8 Allows to distinguish other types of transients, e.g. accretion-disk dominated galactic transients “GRB” 100331A likely galactic transient Only drawback: it’s only a 2m telescope…

  26. Summary Multi-color data allow spectral characterisation of flares/bumps/breaks in afterglow light curves Dark bursts: sample of 20 GRBs observed with GROND within 30 min has detection completeness of 90%  darkness due to (i) spectral break (~4 mag) (ii) combination of moderate AV and moderate z (1-5 mag) dark bursts are not a reservoir of high-z bursts! If GRB at z~8-13 are not substantially fainter than 080913/090423 (and δ< +30o) GROND is likely to detect those (crossing fingers for long life of Swift!)(GRB 090429 at z~9.3 (Tanvir talk) was ~1.5 mag fainter and missed) simulated GROND redshift error

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