Download Modeling the Spectral Energy Distributions and Variability

Gamma-Ray Bursts
• Short (sub-second to minutes) flashes of gammarays, for ~ 30 years not associated with any
counterparts in other wavelength bands
• First dedicated GRB detector: BATSE on the
Compton Gamma-Ray Observatory
GRB Light Curves
Long GRBs (duration > 2 s)
Short GRBs (duration < 1 s)
Possibly two different types of GRBs: Long and short bursts
General Properties
• Random distribution in the sky
• Approx. 1 GRB per day observed
• No repeating GRB sources
BeppoSAX
(1996 – 2003)
Wide-field Camera
and Narrow-Field
Instrument (NFI; X-ray
telescope) allowed
localization of GRBs
to arc-minute
accuracy
First identification of
X-ray and optical
afterglows of g-ray
bursts in 1997
Afterglows of GRBs
On the day of the GRB
3 days after the GRB
X-ray afterglow of GRB 970228
(GRBs are named by their date: Feb. 28, 1997)
Most GRBs have gradually decaying afterglows in X-rays,
some also in optical and radio.
1 day after GRB
2 days after GRB
Optical afterglow of GRB 990510 (May 10, 1999)
Optical afterglows of GRBs are
extremely difficult to localize:
Very faint (~ 18 – 20 mag.);
decaying within a few days.
Optical Afterglows of GRBs
Host Galaxy
Optical Afterglow
Optical afterglow of GRB 990123,
observed with Hubble Space
Telescope (HST/STIS)
Long GRBs are
often found in the
spiral arms (star
forming regions!)
of very faint host
galaxies
Energy Output of GRBs
Observed brightness
combined with large
distance implies huge
energy output of
GRBs, if they are
emitting isotropically:
E ~ 1054 erg
L ~ 1051 erg/s
… another one, observed Energy equivalent to the entire mass
by us with the MDM 1.3 m of the sun (E = mc2), converted into
gamma-rays in just a few seconds!
telescope on Kitt Peak!
Beaming
Evidence that GRBs are not
emitting isotropically (i.e. with
the same intensity in all
directions), but they are beamed:
E.g., achromatic breaks
in afterglow light curves.
GRB 990510
Models of GRBs (I)
There’s no consensus about what causes GRBs.
Several models have been suggested, e.g.:
Hypernova:
Supernova explosion of a
very massive (> 25 Msun) star
Iron core collapse
forming a black hole;
Material from the outer
shells accreting onto
the black hole
Accretion disk =>
Jets => GRB!
Models of GRBs (II)
Supranova:
If a neutron star is rotating
extremely rapidly, it could escape
collapse (for a few months) due to
centrifugal forces.
Neutron star will gradually lose
angular momentum, then
collapse into a black hole =>
collapse triggers the GRB
Results of the BeppoSAX Era
• During the BeppoSAX era, X-ray, optical,
radio afterglows were only found for long
GRBs.
• In afterglows and host galaxies, redshifts,
clustered around ~ 1, were measured;
unambiguously established the
cosmological origin of GRBs.
• Association with star-forming region,
similarities of some optical light curves and
spectra with type Ic supernovae provided
strong support for hypernova/collapsar
model for long GRBs
Swift
(launched 2004)
Dedicated GRB misssion with on-board soft g-ray,
X-ray, and optical telescopes;
Rapid automated
localization and electronic
distribution of GRBs with
arc-second precision
Localization of the first
short GRBs;
Some with significant
offsets from host galaxies,
favoring binary-compactobject merger models:
Models of GRBs (II)
Black-hole – neutron-star merger:
Black hole and neutron star (or
2 neutron stars) orbiting each
other in a binary system
Neutron star will be
destroyed by tidal effects;
neutron star matter accretes
onto black hole
=> Accretion disk
=> Jets => GRB!
Model works probably
only for short GRBs.
Most successful model: