grond observations of grb afterglows

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


GROND=GRB Optical/NIR Detector

K

J

H

g

r

iz

Imaging in 7 channels simultaneously


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


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


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


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


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)


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


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


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


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


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


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


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…


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

grond observations of grb afterglows

Documents