Detection of Variable Absorption Lines in GRB Spectra

1
Detection of Variable Absorption Lines in GRB
Spectra
Heng Hao - CfA Kris Stanek - Ohio State Peter
Garnavich (Notre Dame), Tom Matheson
(NOAO), Maryam Modjaz (CfA), Adam Dobrzycki
(ESO), Chris Howk (Notre Dame), Misty Bentz (Ohio
State), Joanna Kuraszkiewicz (CfA), Guy Worthey
(WSU) Jedidiah Serven (WSU), Mike Calkins
(FLWO) astro-ph/0612409
2
Bright Sources Probe Gas Along
Line-of-Sight
GRB 021004 Matheson et al. 2002 z2.3

GRB QSO SN

z1.4
z1.6
3
The GRB Absorber Problem

There are far more MgII absorption systems seen
in GRB than expected from QSOs
Prochter et al. 2006
Prochter et al. found 14 MgII absorbers in 14
GRB sight-lines (EWgt1 Ang). Predict only 3.8
absorbers based on QSO systems 50000 QSOs
with 7000 MgII systems. Difference is
significant at the 99.9 level.

GRB
QSO
The results suggest that at least one of our
fundamental assumptions underpinning
extragalactic absorption line research is flawed
Absorber Redshift
4
Solutions to the Absorber Problem?
Prochter et al. 2006

X

• Absorbers are intrinsic to the GRB cold gas
  ejected at nearly c
• Absorber host galaxies gravitationally lens GRBs
  (but QSOs
• not so much)
• Dust in absorber hosts dim QSOs (but not GRBs so
  much)

X
X

Frank et al. 2007
?

• The absorber size is the same order as GRB beam
  size, but
• significantly smaller than QSO beam size

Should see some unsaturated lines- MgII doublet
ratio 21
Porciani et al. 2007
X

• No single explanation solves the Absorber
  problem, but all
• combined might

5
GRB 060206 a z4.05 Burst
Discovered by Swift and a bright optical
afterglow was identified by Fynbo et al.
Extensive optical photometry by RAPTOR (Wozniak
et al.) MDM (Stanek et al.) Liverpool (Monfardin
i et al.) Short time-scale variability on top of
power-law decline. Time-resolve spectra taken
at FLWO 1.5m.
GRB 060206 z4
FeII MgII _at_ z1.48
6
Fe II Variability
5? detection of variability


Observed Wavelength
7
Mg II Variability
3 probability of constant EW


Unsaturated 2796/2802 ratio 21
Observed Wavelength
8
Size Does Matter

A relativistic GRB afterglow appears as a filled
ring with a radius that increases as t5/8
(Waxman 1997 Loeb Perna 1998). For GRB
060206 at t5 hr R5x1015 cm Over 4 hours
Rgt50
R(t) 4.1x1015 (E52/n1)1/8 (1zs)-5/8 (t/hr)5/8
cm

Variations in absorption strength are largest
when the absorber and source are comparable in
size.
FeII
MgII variation is harder to explain - may require
hotspots or patchy emission on the afterglow gt
Light curve variations?
MgII
MgII
????arcsec
9
Conclusions

• MgII and FeII absorption systems can be about
  the
• size of early GRB afterglows gt causing
  variability in
• EW of lines unrelated to GRB.
• The solution to the GRB absorber mystery is
  likely
• to be a size difference between GRB and
  QSOs.
• We have now seen the unsaturated MgII ratio
  21
• Early time-resolved spectroscopy of GRB
• afterglows can be exciting and should be
  done
• when possible.