Gamma-ray bursts

1. Gamma-ray bursts
Volodymyr Savchenko
ISDC, Geneve
21.11.12

2. Outline
● What do we know about GRBs?
● What there is more to learn about GRBs?
● How are the GRBs useful?

3. GRBs are bright
Fluence reaching 10-3 erg/cm2 hugely dominating the gamma-ray sky
Numerous detectors may serve unintentionally as GRB detectors: as long as they
are out of atmosphere
In fact the first one to observe them was military satellite searching for the nuclear
explosions in 1967.

4. Early theories
First decades after the
discovery, a number of
theories were suggested
Most of them appeared to
be applicable in other
objects
Essential to keep in mind nowadays!

5. First indication of the luminosity scale
● CGRO/BATSE: big dedicated detector (1991-2000)
~2500 GRBs: No correlation with the galactic plane or any local structures, suggesting
extragalactic origin for the bulk of the events

6. Afterglows: the breakthrough
Dedicated instrument had to be build to react to the prompt emission
by rotating the X-ray instrument.
First implemented in Beppo-SAX immediately led to discovery of X-
ray emission following GRB (Costa et al 1997), opening the whole
new field.
Redshifted lines were
observed in the afterglow,
firmly establishing
cosmological nature of the
GRBs
Lasting sometimes for
months

7. Afterglows: the breakthrough
Dedicated instrument had to be build to react to the prompt emission
by rotating the X-ray instrument.
First implemented in Beppo-SAX immediately led to discovery of X-
ray emission following GRB (Costa et al 1997), opening the whole
new field.
Redshifted lines were
The luminosity of the order of 1054 erg
observed in the afterglow,
on the time scale of about some seconds
firmly establishing
would suggest underlying
cosmological nature of the
gravitational source of energy
GRBs
for the bulk of the events
Lasting sometimes for
months

8. Early spectra
Peaks at ~1MeV
Powerlaw (i.e. certainly non-
thermal) both above and
below the peak
Phenomenological “Band model”
is used to describe:
Spectra of the bulk of the GRBs still contain roughly same amount of information

9. Compatness problem
Fast variability suggests small region - ~<10ms
High luminosity in small emission region would cause pair production and
thermalize the particles
Very large optical depth
But the observed spectrum is non-thermal!

10. Compatness problem
The situation can be saved assuming the emission region is moving relativistically
Gamma-factor at least 100 is required
The most relativistic outflow known.

11. Beaming
●
The isotropic equivalent of 1054 erg solely in gamma-rays would
be hard to explain: probably the emission is beamed.
Characteristic achromatic break in the
afterglow light curve is a signature of
beaming
Another confirmation comes from the observation of late time radio scintilations
Beaming of the order of 1-10 degrees is usually inferred
It is possible that there are two components, differently beamed

12. Emission mechanism
● Thermal: expected, but observed non-thermal,
can be a contribution
● Electron synchrotron: requires non-thermal
population of electrons
● Electron Inverse Compton: requires target field
● Proton synchrotron, pion decay: requires proton-
loaded outflow

13. The fireball model
The most radiatively efficient process is the electron synchrotron
Non-thermal population would have to be re-accelerated in the shocks
Rapid and violent “internal” shocks are responsible for the prompt
emission
More regular external shock accounts for the afterglow

14. Challenge to the fireball model
● Prediction of the “synchrotron deathline”
BATSE
Preece at al 2000
Low-energy asymptote can not be
harder than that of a single electron
But it is.
Swift/BAT Savchenko et al 2008

15. Challenge to the fireball model
● Prediction of the “synchrotron deathline”
Preece at al 2000
A set of models were proposed to
address the problem (modified synchrotron,
inverse compton, thermal contribution), all with
considerable issues
The measurement itself was not
considered quite reliable due to lack
of systematically high precision at the most
Low-energy asymptote can not be
important low energy part of the spectrum
harder than that of a single electron
But it is.
Savchenko et al 2008

16. Not the true spectrum
Another complication is that the spectrum is highly variable
Evolution of spectral parameters of GRB 090902B Evolution of spectral parameters of GRB 080319B
low-energy slope
peak energy
seconds
The measured spectra are averaged on the time scale larger then variability

17. The true spectrum
Another complication is that the spectrum is highly variable
Evolution of spectral parameters of GRB 090902B Evolution of spectral parameters of GRB 080319B
low-energy slope
peak energy
Big detector is required to measure
spectra below the variability scale.
To access 1 ms one needs
10 m2 at 1 MeV
10.000 kg
seconds
The measured spectra are averaged on the time scale larger then variability

18. Polarization of the MeV emission
Polarization of sub-MeV photons can be measured by measuring direction of
Compton-scattered electron
Requires dedicated instrument or very careful analysis
IKAROS: Solar sail with a GRB detector
Strong and variable
polarization?
INTEGRAL
IBIS
Yonetoku 2012
Gotz 2004
Would indicate ordered magnetic field in the emission region and non-thermal
emission process. But further measurements are required.

19. Polarization of the MeV emission
Polarization of sub-MeV photons can be measured by measuring direction of
Compton-scattered electron
Requires dedicated instrument or very careful analysis
IKAROS: Solar sail with a GRB detector
Strong and variable
polarization?
INTEGRAL
IBIS
only 3-4 sigma results
Would indicate ordered magnetic field in the emission region and non-thermal
emission process. But further measurements are required.

20. Polarization of the MeV emission
Polarization of sub-MeV photons can be measured by measuring direction of
Compton-scattered electron
Requires dedicated instrument or very careful analysis
IKAROS: Solar sail with a GRB detector
Strong and variable
Dedicated instrument: POLAR
polarization?
INTEGRAL
IBIS
In space soon
Would indicate ordered magnetic field in the emission region and non-thermal
emission process. But further measurements are required.

21. Extension of the energy range: GeV
● First observed only in a handful of cases by CGRO/EGRET
● Fermi/LAT since 2008 has dramatically improved the quality of the measurements
The emission correlates with the prompt at first but then extends for decades
longer

22. Extension of the energy range: GeV
● First observed only in a handful of cases by CGRO/EGRET
● Fermi/LAT detected 30 GeV-loud bursts in 4 years
It's not yet clear what fraction of the bursts have GeV emission. The number of the bursts in LAT
is less then expected, but no HE cut off was so far observed, putting extreme limit of
>1000 on the Lorentz factor.

23. Extension of the energy range: GeV to TeV
Current generation Cherenkov telescopes are barely able to perform GRB observations
MAGIC specificity was designed light – rapid – to follow GRBs. But no bright enough burst
was in the FoV.
CTA will be major improvement. Might measure the cutoff due to pair production,
study the decay of the emission in greater detail

24. Extension of the energy range: keV
The additional component extends also below the peak
It modifies measurements of the low-energy slopes during the prompt phase
In one case instead an inexplicable suppression is measured
Very few quality measurements are available – X-ray instrument is hard to point
promptly

25. Extension of the energy range: keV
The additional component extends also below the peak
It modifies measurements of the low-energy slopes during the prompt phase
In one case instead an inexplicable emission down measured
Will measure prompt suppression is to 1 KeV
(instead of 10 keV)!
Very few quality measurements are available – X-ray instrument is hard to point
promptly

26. Extension of the energy range
GeV-to-X-ray emission long after the prompt phase: INTEGRAL/ISGRI is very useful,
but only if lucky
2012, in preparation

27. Extension of the energy range
GeV-to-X-ray emission long after the prompt phase: INTEGRAL/ISGRI is very useful,
but only if lucky
Missing instrument
2012, in preparation

28. Extension of the energy range: optical
Optical emission during the prompt phase of the GRB has been detected in few
cases.
The challenge is to start observation of a narrow-field optical instrument in time.
It can be in fact extremely bright: reaching magnitude 5.3: stellar size object visible to a
naked eye from redshift of 0.97!

29. Extension of the energy range: optical
Optical emission during the prompt phase of the GRB has been detected in few
cases.
The challenge is to start observation of a narrow-field optical instrument in time.
Although no burst was seen simultaneously in GeV and optical
energetics and MeV spectrum comparative evolution suggest that
They might be of common origin
A single powerlaw from 1 eV to 10 GeV probably carrying
bulk of the GRB energy
It can be in fact extremely bright: reaching magnitude 5.3: stellar size object visible to a
naked eye from redshift of 0.97!

30. Extension of the energy range: optical
To study the prompt GRB optical emission the telescope has to be
extremely fast: react at <1 second.
Currently available cases are due to extreme GRB duration, presence
of a precursor or to pure luck
UFFO
The slewing mirror telescope(SMT), can slew to Field of view of SVOM will be constantly
target within 10 msec using MEMS (Micro-Electro-
monitored by a group of optical telescopes
Mechanical Systems)

31. Classification
Two components?
Large sample is required –
large instrument
Kouveliotou 1999

32. Classification
Using more then only the duration
Three components?..

33. Classification
Different divisions?..
Large instrument: INTEGRAL/SPI-ACS
Savchenko et al 2012

34. Progenitors
Two major classes: two kind of progenitors
Collapsar: hypernova – massive supernova Merging compact objects
Supported by localization in the host galaxies

35. Collapsar: direct confirmation
In some cases supernova was directly observed after a GRB – always long
But in two cases upper limit excluded supernova...

36. Neutron star merger: direct
Close compact binary must emit gravitational waves, especially before merging
LIGO: interferometer
Bulk of the short GRBs, if related to NS mergers, will be soon detectable
For local events the limit already is reached. No detection indicates that the origin
was most likely not a merger.

37. Tidal disruptions
Tidal disruptions of small objects by stellar BH or stars by
supermassive black holes lead to similar phenomena.
The difference can be seen in the afterglow.
In several cases were directly identified, but contribution to the bulk of
the events is not really known
It may be seen as a reminder that a single event should may not represent
a population (although it is often tempting in the case of GRBs)

38. Magnetar flares
Reorganization of magnetic field in extremely magnetized neutron stars also
leads to short strong bursts, sometimes confused with the GRBs.

39. Unexplained bursts
Some of the bursts lack any explanation

40. GRBs to probe history of the universe
GRBs are distant: have a potential
The most distant single event is at redshift of 9.4
Population studies suggest that there may be till ~20

41. GRBs as standard candles
The idea is to deduce luminosity from the spectral parameters
Most notably correlation between energy of the peak of the spectrum and
the luminosity is observed
The correlation is probably driven by the Lorentz
factor of the outflow

42. GRBs as standard candles

43. GRBs as probes star formation
GRBs carry unique direct information about high-redshift stars
Counting number of GRBs with
redshift one can deduce the star
formation history to
unprecedented redshift
Unbiased large sample is
required

44. GRBs line of sight
Absorption lines by different structures along the line of sight are observed
and can be used to study the structure, similarly to Lyman-alpha forest
Different elements can be probed, to higher redshift
Absorption in X-ray probes ionized medium

45. GRB as probes for vacuum dispersion
Dependency of speed of light on photon energy and polarization can be tested.
Strong upper limits are set, especially if the polarization is measured.

46. Conclusions
● Mechanism of the MeV prompt emission is still not clear but
major advances were made recently
● Classification is gradually shaping out
● Connections of the GRBs with cosmology are strengthening
● New results are expected in the coming years