A Comprehensive Study of Bright Fermi-GBM Short Gamma-Ray Bursts II. Very Short Burst and Its Implications_2

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Citation: Hu, Y.-Y.; Huang, Y.-L.;

Huang, J.-W; Zhu, Z.; Tang, Q.-W. A

Comprehensive Study of Bright

Fermi-GBM Short Gamma-Ray

Bursts: II. Very Short Burst and Its

Implications. Universe 2022,8, 152.

https://doi.org/10.3390/

universe8100512

Academic Editors: Gang Zhao,

Zi-Gao Dai and Da-Ming Wei

Received: 8 August 2022

Accepted: 27 September 2022

Published: 1 October 2022

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universe

Article

A Comprehensive Study of Bright Fermi-GBM Short Gamma-Ray

Bursts: II. Very Short Burst and Its Implications

Ying-Yong Hu, Yao-Lin Huang, Jia-Wei Huang, Zan Zhu and Qing-Wen Tang *

Department of Physics, Nanchang University, Nanchang 330031, China

*Correspondence: qwtang@ncu.edu.cn

Abstract:

A thermal component is suggested to be the physical composition of the ejecta of several bright

gamma-ray bursts (GRBs). Such a thermal component is discovered in the time-integrated spectra of

several short GRBs as well as long GRBs. In this work, we present a comprehensive analysis of ten very

short GRBs detected by Fermi Gamma-Ray Burst Monitor to search for the thermal component. We found

that both the resultant low-energy spectral index and the peak energy in each GRB imply a common hard

spectral feature, which is in favor of the main classiﬁcation of the short/hard versus long/soft dichotomy

in the GRB duration. We also found moderate evidence for the detection of thermal component in eight

GRBs. Although such a thermal component contributes a small proportion of the global prompt gamma-

ray emission, the modiﬁed thermal-radiation mechanism could enhance the proportion signiﬁcantly, such

as in subphotospheric dissipation.

Keywords: gamma-ray bursts; thermal component; prompt phase; non-thermal component

1. Introduction

Gamma-ray Burst (GRB) is the most intense transient astrophysical phenomena in the

universe. Long GRB (LGRB, duration longer than 2 s) is believed to origin from the core-

collapse of a massive star [

–

]. However, short GRB (SGRB, duration less than 2 s) shares a

distinct origin, such as a merger process of two compact objects, e.g., two neutron stars (NS-NS),

which is ﬁrst proved by a gravitational-wave GRB, GRB 170817A [

]. For both types of GRBs,

the dominate component of the gamma-ray emission at the photospheric radius, where energy

dissipation takes place in the optically thick regime, is the so-called quisa-thermal component,

which gradually fades across the burst duration. This thermal component represented by a

standard blackbody function is discovered in several bright GRBs, such as GRB 090902B, GRB

120323A and GRB 170206A [

–

]. Some modiﬁed blackbody modesl to reproduce the thermal

photospheric emission are proposed and employed to ﬁt the spectra of several GRBs [

–

Subphotospheric emission is also a popular mechanism for the thermal component in the Fermi

era [

–

]. Photospheric emission from a structured jet or a hybrid relativistic outﬂow is

invoked in some GRBs [

–

]. Photospheric emission models via the Compton scattering are

also proposed to broaden the thermal peak [23–26].

In the ﬁreball model, GRB photosphere often occurs in the early phase, after which there

is the emission region dominated by the internal shock, thus the thermal component and

the non-thermal component would dominate the different emission phase [

]. The spectral

evolution that includes the thermal and non-thermal component is also conﬁrmed in several

GRBs [

–

]. In this work, we select a GRB sample with a very short duration detected by

Fermi Gamma-Ray Burst Monitor (Fermi/GBM) to judge these features, such with a burst

duration shorter than 0.05 s, which often cannot be performed the time-resolved spectral

Universe 2022,8, 152. https://doi.org/10.3390/universe8100152 https://www.mdpi.com/journal/universe

arXiv:2210.00410v1 [astro-ph.HE] 2 Oct 2022

Universe 2022,8, 152 2 of 14

analysis. In these GRBs, the thermal component would dominate prompt gamma rays if the

ﬂux of the thermal emission exceeds that of the non-thermal emission. In Section 2, we present

the sample selection and describe the method of spectral ﬁtting. The results for four spectral

models are presented in Section 3. A short discussion is given in Section 4. We present the

summary and conclusions in Section 5.

2. Data Reduction

2.1. Sample Selection

Fermi/GBM has two types of scintillation detectors, 12 Sodium Iodide units (NaIs, ‘n0’ to

‘n9’, ‘na’ and ‘nb’) and 2 Bismuth Germanate units (BGOs, ‘b0’ and ‘b1’). NaIs cover the photon

energy between about

8 keV

and

1 MeV

while BGOs between about

200 keV

and

40 MeV

[

Among 3339 GRBs detected by Fermi-GBM as of August 6, 2022, 14 GRBs are selected with

T90

(50–300 keV) less than 0.05 s, four of which are excluded with a low signal-to-noise ratio (SNR

4, [

]), e.g., GRBs with the GBM trigger of bn141102112, bn161115745, bn200423579 and

bn210119121. Please note that

T90

is a temporal duration between GBM

T05

and

T95

, which are

the moments when 5% and 95% of the total GRB energy ﬂuence is accumulated, respectively.

For each GRB, we selected all the detectors as in the Fermi/GBM catalog, as shown in Table

1. Please note that we abandoned one or more NaI detectors in some GRBs for a low SNR,

e.g., ‘n1’ in GRB 081229, ‘n8’ + ’n9’ in GRB 091126, and ‘n6’ in GRB 120616. GBM data can be

downloaded from the public data site of Fermi/GBM (20 August 2022)

. NaI lightcurves of

two GRB are plotted with the binned time of 0.008 s, GRB 090802 and GRB 160822, as shown in

Figure 1.

Table 1. Information of our GRB sample.

GRB Name GBM Trigger T90 T05 T95 Detetor bkg. Selection

(s) (s) (s) (s)

081229 bn081229675 0.032 −0.016 0.016 ’n2’,’n5’,’b0’ [−25, −10], [15, 30]

090802 bn090802235 0.048 −0.016 0.032 ’n2’,’n5’,’b0’ [−25, −10], [15, 30]

091126 bn091126389 0.024 −0.008 0.016 ’n6’,’n7’,’nb’,’b1’ [−25, −10], [15, 30]

120616 bn120616630 0.048 −0.048 0 ’n3’,’n7’,’b0’ [−25, −10], [15, 30]

160822 bn160822672 0.040 −0.016 0.024 ’n9’,’na’,’b1’ [−25, −10], [15, 30]

171108 bn171108656 0.032 −0.016 0.016 ’n9’,’nb’,’b1’ [−25, −10], [15, 30]

180103 bn180103090 0.016 −0.016 0 ’n4’,’b0’,’b1’ [−25, −10], [15, 30]

180602 bn180602938 0.008 −0.016 −0.008 ’n0’,’n1’,’n3’,’b0’ [−25, −10], [15, 30]

190505 bn190505051 0.032 −0.016 0.016 ’n0’,’n1’,’n3’,’n5’,’b0’ [−25, −10], [15, 30]

201221 bn201221591 0.032 −0.016 0.016 ’n6’,’n7’,’n9’,’b1’ [−25, −10], [15, 30]

Universe 2022,8, 152 3 of 14

(a)090802

(b)160822

Figure 1.

NaI lightcurves of GRB 090802 and GRB 160822. The blue line is the observed lightcurve, the

green line is the ﬁtted background line and the orange shadow region is the GBM

T90

duration. The

binned time is 0.008 s.

2.2. Spectral Fitting

GBM time-tagged event (TTE) data are used in the spectral analysis. Instrument response

ﬁles are selected with

rsp

2 ﬁles. We ﬁt the background rates with auto-selected orders polyno-

mials using the Nelder-Mead method for ten GRBs, two of which are plotted in Figure 1. For

NaI detectors, we select photons in channels between about 8 keV and

1000 keV

and exclude

photons in channels between about 33 and 36 keV (Iodine K-edge, [

]). For BGO detectors,

photons in channels between about 200 keV to 40 MeV are included.

We ﬁrst select two non-thermal spectral models to ﬁt the gamma-ray spectra, e.g., the Band

function model (BAND), the cutoff power-law function model (CPL). Two standard blackbody

function (BB)-joined models are employed, the BAND + BB model and the CPL + BB model.

For the above four models, BAND model is written as the so-called Band function [

], such as

NE(BAND) = ABAND











Epiv )αe[−E/E0],E≤(α−β)E0

((α−β)E0

Epiv )(α−β)e(β−α)(E

Epiv )β,E≥(α−β)E0

(1)

where

is the photon index before and after the typical energy of

(α−β)E0

, and

is the

break energy in the

FE(= ENE)

spectrum, and

Epeak = (

+α)E0

is the peak energy in the

EFE(= E2NE)

spectrum [

]. CPL could be regarded as the lower energy segment of the BAND

model but with an exponential cutoff-power-law decay in the high-energy band, such as

NE(CPL) = ACPL(E

Epiv

)Γe−E/Ec, (2)

where

is the photon index and

is the cutoff energy.

Epiv

in both models is the pivot energy

and ﬁxed at 100 keV. BB component is usually modiﬁed by the standard Planck spectrum,

which is given by the photon ﬂux,

NE(BB) = ABB

exp[E/kT]−1, (3)

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Citation:Hu,Y.-Y.;Huang,Y.-L.;Huang,J.-W;Zhu,Z.;Tang,Q.-W.AComprehensiveStudyofBrightFermi-GBMShortGamma-RayBursts:II.VeryShortBurstandItsImplications.Universe2022,8,152.https://doi.org/10.3390/universe8100512AcademicEditors:GangZhao,Zi-GaoDaiandDa-MingWeiReceived:8August2022Accepted:27September2022...

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