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GRB prompt emission: spectral energy distribution and light curve - - PowerPoint PPT Presentation

Oct 09, 2022 369 likes •775 views

GRB prompt emission: spectral energy distribution and light curve profile Francesco Massaro Harvard (SAO CfA) Thanks to: Barcellona J. Grindlay, R. Preece, N. Omodei June 2011 Modeling GRBs Since the discovery of GRBs Two major

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GRB prompt emission: spectral energy distribution and light curve profile

Francesco Massaro Harvard (SAO – CfA) Thanks to:

  • J. Grindlay, R. Preece, N. Omodei

Barcellona June 2011

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Modeling GRBs

Since the discovery of GRBs… Two major descriptions of their SEDs have been used: Band fucntion and Blackbody+powerlaw while the lightcurves have been described with the Norris profile We propose a new view for both the

  • 1. Spectral Energy Distributions (SEDs)

and

  • 2. Light Curves (LCs)
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FACTS…and NOT ARTIFACTS

Observational evidences (random order):

  • 1. SEDs curved and broadly peaked
  • 2. Single pulse or Spiky lightcurves
  • 3. Cosmological distances
  • 4. Variability classes (long and short GRBs)
  • 5. Presence of afterglows
  • 6. Relativistic effects
  • 7. Supernova connection
  • 8. HIC during the LC decay phase
  • 9. GeV emission (not all of them)
  • 10. Presence of X-ray flares (not all of them)
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Spectral Energy Distribution of the GRB prompt emission

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AIMS

We aim to find a model for the SED that:

  • 1. Could describe at least 90% of GRBs
  • 2. Must describe the TIME RESOLVED SPECTRA

(not only the time integrated)

  • 3. Must interpret the majority of the observed FACTS
  • 4. Must work directly on the observed spectra

(not on the “deconvolved” SED)

  • 5. Must have a strong physical background
  • 6. MUST NOT IMPROVE THE NUMBER OF PARAMETERS

Ockham Razor (Frustra fit per plura quod fieri potest per paucior)

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Since 1992…The Band Function

SWIFT, Fermi GBM, Multifrequency ToO obs. new planned missions (we hope) … but still …

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Since 1992…The Band Function

Band et al. 1993 Tavani et al. 1996 The SED shape is well described by this phenomenological model

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The thermal emission

(Ryde 2004)

  • 1. No signatures exponential cutoff
  • 2. Power-law always necessary
  • 3. Extrapolation of the power-law
  • 4. No BB photon index at low energies
  • 5. Connection low-high energies
  • 6. It cannot describe all GRBs
  • 7. No high values of curvature

Physical background !!! My view…it still has some problems

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The log parabolic spectral shape

Massaro, Grindlay & Paggi 2010

Log-parabola Energy dependent photon index A new physical parameter: the spectral curvature b

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The log parabolic spectral shape

Log-parabolic means log-normal

Parabola is the natural way to approximate functions around a minimum or a maximum --> e.g. Taylor series

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The physics!!!

The general solution of the kinetic equation is well approximated by a log-parabolic function when:

  • 1. Not only Systematic but also stochastic acceleration
  • 2. Radiative cooling + adiabatic expansion ……etc. etc.

Since Kardashev 1962

Similar ideas: Ellison et al. 2001, Pelletier et al. 2003, Stawarz & Petrosian 2006.

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The log-parabolic synchrotron spectra

F(ν)=F0(ν/ν0)- a - b Log ν/ν

Curvatures b ~ r/5

N(γ)=N0(γ/γ0)- s - r Log γ/γ b (BL Lacs): 0.05 – 0.5 b (GRBs): 0.2 – 1.2 (time resolved spectra)

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GRB 090902B

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GRB 090902B

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GRB 090902B

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GRB 090902B

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GRB 090902B

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Adiabatic expansion

Massaro & Grindlay 2011

  • Hp. Self similar scenario:

Possible interpretation of the hardness intensity correlation (HIC): The HIC has a peak index of ~ 1.6 so

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Using the log-parabola…. We can test this idea

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Spectral curvature behavior

Observed spectral behavior Simulated spectral evolution We do not see drastic variations of the curvature during GRB single pulses

Adiabatic losses do not change the shape of the SED

Massaro & Grindlay 2011

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No high values of spectral curvature (Blackbody expected b~10)

A new feature of GRBs

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A note on the synchrotron scenario

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Photon index

Synchrotron: line of death or small pitch angle?

Llody & Petrosian 2002 A clear signature of Synchrotron emission Then LCs….

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Light curves of the GRB prompt emission

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Since 1996 and 2005… The Norris profile

The LC profile can be described by this phenomenological model Single long pulse GRBs It is always asymmetric Norris et al. 2005

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Rise and decay time ratios is ~ 1/2 An example of an artifact….. 1,000,000 of Montecarlo simulations with uniform distribution

Since 1996 and 2005… The Norris profile

Massaro & Grindlay 2011 in prep.

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Modified Beta Function

3C 273 Abdo et al. 2010

Massaro & Grindlay 2011 in prep.

Vetere et al. 2006

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FACTS

Observational evidences:

  • 1. SEDs curved and broadly peaked
  • 2. Spiky lightcurves
  • 3. Cosmological distances
  • 4. Fast variability (long and short GRBs)
  • 5. Presence of afterglows
  • 6. Relativistic effects
  • 7. Supernova connection
  • 8. HIC during the LC decay phase
  • 9. No large variations of the spectral curvature
  • 10. GeV emission (not all of them)
  • 11. Presence of X-ray flares (not all of them)
  • 12. Modified Beta profile to describe LCs

(symmetric and asymmetric profiles)

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CONCLUSIONS

  • 1. Log-parabola vs Band

From the statistical point of view 4 or 5 vs 3 parameters (in agreement with Fermi LAT GRBs detections) From the physical point of view a priori physical background

  • 2. Time resolved spectra are very well described in terms of

log-parabolic model (up to now no exceptions)

  • 3. No drastic variation of the spectral curvature

during GRB single pulses (CAREFUL must be tested) (signatures of adiabatic expansion)

  • 4. A new idea to describe the LCs: modifed Beta function

More versatile than the exponential profile and without degeneracies or biases

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AIMS for the SED model

And we came out with a model

  • 1. Could describe at least 90% of GRBs
  • 2. Must describe the TIME RESOLVED SPECTRA

(not only the time integrated)

  • 3. Must interpret the majority of the observed FACTS
  • 4. Must work directly on the observed spectra

(not on the “deconvolved” SED)

  • 5. Must have a strong physical background
  • 6. MUST NOT IMPROVE THE NUMBER OF PARAMETERS

Ockham Razor (Frustra fit per plura quod fieri potest per paucior)

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And thanks for your attention

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Backup slides

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GRB conference in Rome 2004

  • R. Blandford concluding remarks
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Curved spectra in jets

Crab Pulsar (Campana et al. 2008) Cygnus A (FR II) (Carilli et al. 1991) Vela plerion (Mangano et al. 2005)

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Curved spectra in jets

BL Lacs (Massaro et al. 2006, 2008)

Log-parabolic model Mrk 501 Mrk 501 Mrk 421

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Curved spectra in jets

GPS radio sources

(Ostorero et al. 2009)

High Frequency Peakers (Maselli et al. 2009)

Radio galaxies (Katz-Stone et al. 1993)

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The synchrotron line of death

Preece et al. 1998

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Asymmetric log-parabola

The synchrotron line of death

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X-ray flares in GRB afterglows

  • 1. No drastic variations of b
  • 2. Inconsistent with thermal

(i.e. Blackbody) emission

  • 3. Same model adopted for GRB

prompt emission SWIFT time resolved spectral analysis