Graduate Student Research Forum, Tuesday, March 14, 2006, Philips Auditorium
Andrew Friedman
Title: “The Promise and Limitations of Gamma-Ray Burst Standard Candles"
Abstract:
I will briefly discuss the evolution of the field of Gamma-Ray Burst (GRB) cosmology, including my own work
with Joshua Bloom on turning GRBs into standard candles to measure the cosmological parameters, in parallel
with Type Ia supernovae. I will address some of the recent excitement and controversy in the GRB cosmology
field, highlighted at the past AAS meeting, and attempt to clear up some potential misunderstandings about the
current practical utility of GRBs to contrain the dark energy or its time evolution. I will conclude with some
thoughts on ongoing work using data from the NASA Swift and HETE II satellites and a world wide network of
ground based telescopes, and discuss the role of possible future GRB cosmology satellites that have been
suggested.
Story at the AAS. Brad Schaeffer press release before paper. New GRB COSMOLOGY OUTLINE
York times. Was Einstein wrong after all? GRBs favor time variation
of dark energy, a universe inconsistent with a cosmological constant.
1. GOOD
No GRBs are not overturning cosmology, and I hope to explain why.
2. BAD
3. GRB HUBBLE DIAGRAM
Skytel article.
4. FUTURE
Questions at any time. I may not stick exactly to the abstract, but this is informal, so no worries.
As you know, GRBs are associated with core collapse of massive stars and Type Ib/c SNe. The signal BH
formation and paired relativistic jets are shot out along the spin axis of the system. GRBs are beamed
explosions, far brighter in gamma-rays than supernovae. The jet slams into the ISM, producing a multiwavelength afterglow. Spectra of the afterglow allow us to measure z of host galaxy. Light curve of afterglow
allows us to measure beaming size of jet, total beaming-corrected energy and luminosity of the explosion.
Draw picture.
1. THE GOOD
Big Picture motivations for using GRBs for
cosmology vs. SNe Ia
Brighter than Ia
A. HIGH-z
(z = 6.3 max so far)
(z~1.7 for SNe Ia)
Gamma-rays unaffected
B. DUST
-but afterglow affected
More tractable kC. SIMPLER
corrections
SPECTRA
W/ Swift, can do high-z
D. TIMESCALE
cosmology now vs.
JDEM?
i.e. luminosity evolution
E. DIFFERENT
vs. Ia
SYSTEMATICS
F. z-DISTRIBUTION Complementary to Ia
2. THE BAD
The drawbacks and current problems faced by GRBs as
standard candles
?which one, if any, is
A. MANY METHODS
optimal
G. ΩM
G. SELECTION
EFFECTS
H. SENSITIVITY TO
MODEL
ASSUMPTIONS
H. 0 < z < 2
Prior big help to global
study of dark energy
Region still interesting
for dark energy,
transition redshift
B. GRB HUBBLE
ANALYSIS
C. SMALL NUMBER
STATISTICS
D. DANGEROUS TO
COMBINE ALL
METHODS
E. LOW-z
CALIBRATION
F. OUTLIERS
I. LUMINOSITY
EVOLUTION vs. z
Still controversy over how
to do the analysis properly
20 GRBs best method
50 all methods
Schaeffer et. al. 2006
Few nearby GRBs – follow
SFR. No Ia training set
Several GRBs are outliers
to standard candle
correlations
Standard candle relations
may by bogus
-density profile – WIND
vs. ISM
-jet model, flat vs.
structured
Most hairy systematic error
(more on this later)
BAD outweighs the good. Went all the way up to I!
3. THE GRB HUBBLE DIAGRAM
Explain the current best method for making a GRB Hubble diagram.
HANDOUT
-what standard candle should you consider?
-standard vs. standardizable candles
For SNe Ia it’s the intrinsic optical luminosity, corrected for scatter with empirical corrections between the
light curve shape and the luminosity
For GRBs it’s the beaming corrected gamma-ray energy (or luminosity), corrected for scatter with empirical
correlations between the energy (or luminosity) and the properties of the gamma-ray spectrum
Eiso, Egamma, Egamma,cor, LZ
4. THE FUTURE
Ongoing work and future prospects with Swift, HETE-II and other new space satellites of the future.
A. Swift [15,150] keV
B. HETE-II
C. Integral
I. Swift XRT data
?New correlations