[Paper Review] Is the Gamma Ray Bursts emission suppressed at high energy?

D. Guetta, E. Pian|ArXiv.org|Oct 12, 2009

Gamma-ray bursts and supernovae3 citations

TL;DR

This study investigates whether high-energy gamma-ray burst (GRB) emission is suppressed beyond 100 MeV by comparing peak flux distributions from Fermi-GBM, BATSE, and Swift-BAT. It finds that the LAT's low detection rate of high-energy GRBs cannot be explained by simple synchrotron extrapolation, indicating intrinsic suppression of >100 MeV emission in several bursts, including GRB090217 and GRB090202b, which have soft GBM spectra.

ABSTRACT

We compare the luminosity function and rate inferred from the GBM long bursts peak flux distribution with those inferred from the Swift and BATSE peak flux distribution. We find that the GBM, BATSE and the Swift peak fluxes can be fitted by the same luminosity function implying the consistency of these three samples. Using the trigger algorithm of the LAT instrument we derive important information on the flux at 100 MeV compared to lower energy detected by the GBM. We find that the simple extension of the synchrotron emission to high energy cannot justify the low rate of GRBs detected by LAT and for several GRBs detected by the GBM, the flux at >100 MeV should be suppressed. Two bursts, GRB090217 and GRB 090202B, detected by LAT have very soft spectra in the GBM and therefore their high energy emission cannot be due to an extension of the synchrotron.

Motivation & Objective

• To assess whether the high-energy emission of GRBs is suppressed above 100 MeV, challenging the assumption of simple synchrotron extrapolation.
• To test the consistency of the luminosity function and rate evolution across Fermi-GBM, BATSE, and Swift-BAT GRB samples.
• To evaluate the LAT instrument's sensitivity and expected detection rate of high-energy GRB counterparts based on GBM-detected bursts.
• To determine whether the low number of LAT-detected GRBs (only 10 above 30 MeV) reflects instrumental limitations or intrinsic spectral suppression.
• To investigate the spectral properties of high-energy GRBs, particularly those detected by LAT, to understand the physical mechanisms behind high-energy emission.

Proposed method

• Fits the peak flux distributions of long GRBs from GBM, BATSE, and Swift-BAT in the 50–300 keV band to infer the luminosity function and rate evolution.
• Applies a luminosity function model (LFb) with parameters $ L^* $, $ \alpha $, $ \beta $, and redshift-dependent rate to all three datasets, using $ \chi^2 $ minimization for goodness-of-fit.
• Uses the LAT trigger algorithm to estimate the expected high-energy (100 MeV–10 GeV) flux for GBM-detected GRBs under the assumption of simple spectral extrapolation.
• Compares the expected LAT detection rate (based on field of view and sensitivity) with the actual observed rate to infer intrinsic suppression.
• Analyzes spectral parameters (e.g., $ E_{\rm peak} $, $ \alpha $, $ \beta $) of LAT-detected GRBs and compares them with GBM data to assess spectral softness at high energies.
• Applies the Band function and power-law models to fit spectral energy distributions and assess deviations from standard synchrotron emission.

Experimental results

Research questions

• RQ1 Is the high-energy emission of GRBs suppressed above 100 MeV, as suggested by the low detection rate of LAT compared to GBM?
• RQ2 Do the luminosity functions and rate evolution inferred from GBM, BATSE, and Swift-BAT GRB samples show consistent properties?
• RQ3 Can the low number of LAT-detected GRBs (10 above 30 MeV) be explained by instrumental sensitivity alone, or is intrinsic spectral suppression required?
• RQ4 Do GRBs detected by LAT exhibit soft spectra in the GBM band, indicating a physical suppression of high-energy emission?
• RQ5 To what extent does the observed spectral shape of high-energy GRBs deviate from simple synchrotron extrapolation?

Key findings

• The luminosity function and rate evolution inferred from GBM, BATSE, and Swift-BAT peak flux distributions are consistent, with a best-fit LFb model yielding $ \chi^2 \sim 1.3 $ for all samples.
• The GBM sample's luminosity function parameters are $ L^* = 5.5^{+1.5}_{-2} \times 10^{51} $ erg/s, $ \alpha = 0.3^{+0.1}_{-0.5} $, and $ \beta = 2.3^{+0.6}_{-0.3} $, with a rate at z=0 of $ 0.5^{+0.3}_{-0.2} $ Gpc⁻³yr⁻¹.
• The expected LAT detection rate, assuming simple spectral extrapolation, would be ~1/3 of GBM bursts above 30 MeV, but only ~5% are actually detected, indicating a significant discrepancy.
• For GRB090217 and GRB090202b, the GBM spectra are very soft, and their high-energy emission cannot be explained by simple synchrotron extrapolation, implying intrinsic suppression.
• The LAT detection rate is consistent with the earlier EGRET vs. BATSE statistics, suggesting an intrinsic paucity of high-energy emission rather than instrumental bias.
• The spectral parameters of LAT-detected GRBs (e.g., $ E_{\rm peak} $ up to 798 keV, $ \beta \approx -3.87 $) indicate hard spectra, but their low flux at >100 MeV contradicts simple extrapolation, supporting suppression.