grond observations of grb afterglows
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Jochen Greiner - 1Kyoto, 19-23.4.2010
GROND observations of GRB Afterglows
Jochen Greiner Max-Planck-Institute for extraterrestrial Physics
Garching, Germany
• Color changes in afterglow light curves
• Dark bursts
• High-z perspectives
Jochen Greiner - 2Kyoto, 19-23.4.2010
GROND=GRB Optical/NIR Detector
K
J
H
g
r
iz
Imaging in 7 channels simultaneously
Jochen Greiner - 3Kyoto, 19-23.4.2010
GROND: General Design
7 bands: Sloan g’, r’, i’, z’ and J, H, KOne 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 hrgr 21.5 mag 24.5 mag
iz 20.5 mag 23.5 magJ/H/K (AB) 18.5/17.5/16.5 mag
21.5/20.5/19.5 mag
Jochen Greiner - 4Kyoto, 19-23.4.2010
GROND @ 2.2m MPI/ESO telescope
GROND
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
Jochen Greiner - 5Kyoto, 19-23.4.2010
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
Jochen Greiner - 6Kyoto, 19-23.4.2010
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
Krühler et al. 2008
GRB 070802: flares
Jochen Greiner - 7Kyoto, 19-23.4.2010
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)
Jochen Greiner - 8Kyoto, 19-23.4.2010
Two GRBs with 2-component jets…
Krühler et al. 2009
GRB 080710 GRB 080413B
Filgas et al. 2010
I II III IV
Kyoto, 19-23.4.2010
…, but different SEDs
GRB 080710
Clear difference in spectral evolution, yet (nearly) similar interpretation:
GRB 080413B
Two component jet viewed off-axis: fits light curve and SED; also consistent with low Eγ
iso 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
Jochen Greiner - 10Kyoto, 19-23.4.2010
2. The “dark burst” issue
• 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)
Jakobsson et al. 2004…2009
Potential causes for optical darkness:
Dark bursts: ßox < 0.5
Jochen Greiner - 11Kyoto, 19-23.4.2010
DARKBURSTS
GROND GROND at the 2.2m at the 2.2m MPI/ESO MPI/ESO telescopetelescope
4 min4 min10 min10 min
1 hr1 hrFirst exposure starts within 2-10 min
UVOT/Swift
pre-Swift “dark” GRBs vs. UVOT vs. GROND
UVOT probes ~5 mag deeper at earlier times; yet large fraction of “dark” bursts high-z, extinction
GROND probes GROND probes another ~2 mag another ~2 mag deeper, +NIRdeeper, +NIR
Figure adapted fromFox et al. 2004
Kyoto, 19-23.4.2010
The sample
• 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)
Selection criterion: Swift/XRT detection
GR
ON
D li
mit
in 2
ks
Jochen Greiner - 13Kyoto, 19-23.4.2010
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%
Kyoto, 19-23.4.2010
GRB 080710Only 1 GRB with X-ray and
optical/NIR on same powl
ßox sensitive to extinction AV AND SED shape
GRB 070802GRB 080129
XRT-GROND SEDs
1 keV
R band
KHJ zirg
ßox
Kyoto, 19-23.4.2010
GRB 080913
… both have nearly ßox ~ 0:
GRB 080805
2 examples with flat SEDs
AV ~ 1.2 z=6.7
Kyoto, 19-23.4.2010
Dark bursts in our sample
5 dark bursts≡ 25%(with another 4 just at borderline)
Consistent with earlier numbers
Kyoto, 19-23.4.2010
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
Kyoto, 19-23.4.2010
Intrinsic AV distribution in GRBs
10% (Kann et al. 2009)
25%
• 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
Jochen Greiner - 19Kyoto, 19-23.4.2010
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
Kyoto, 19-23.4.2010
Redshift distribution
• Has completeness of 95%
• Is flatter than previous distributions (which had smaller completeness levels)
• Statistics not yet great
Kyoto, 19-23.4.2010
The 2 non-detected GRBs
GRB 090429B
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
Jochen Greiner - 22Kyoto, 19-23.4.2010
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)
GRBs at redshift >5
060522 5.11071025 5.2060927 5.47050814 5.77050904 6.29080913 6.7090423 8.2
But:
Severe bias against z~4-6 bursts with moderate extinction
Obs
erve
d r’
-ban
d ex
tinct
ion
Kyoto, 19-23.4.2010
GROND is sensitive up to z~13
• 7 filter bands cover Lyman-α for large z-range: 3<z<13 (assuming K-band detection)
z=10z=13
Jochen Greiner - 24Kyoto, 19-23.4.2010
Example of GROND sensitivity
Advantage of separate detector per filter: can add sensibilization for spectral band: [email protected] detector/filter is 4x more sensitive than FORS2@VLT total efficiency only 4x less than FORS2, not 16 (8.22/2.22)
• 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
Kyoto, 19-23.4.2010
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…
Jochen Greiner - 26Kyoto, 19-23.4.2010
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