J/A+A/703/A101 GRB X-ray plateaus (Guidorzi+, 2025)
Gamma-ray burst X-ray plateaus as evidence of pre-prompt afterglow.
Guidorzi C., Maccary R., Maistrello M., Kobayashi S., Bulla M., Frontera F.
<Astron. Astrophys. 703, A101 (2025)>
=2025A&A...703A.101G 2025A&A...703A.101G (SIMBAD/NED BibCode)
ADC_Keywords: GRB ; Gamma rays ; X-ray sources
Keywords: methods: data analysis - methods: statistical -
gamma-ray burst: general
Abstract:
Most gamma-ray burst (GRB) X-ray afterglow light curves are
characterised by a plateau, followed by a normal power-law decay,
which is interpreted as afterglow emission, that is radiation emitted
by the shocked interstellar medium that is swept up by the blast wave.
Despite the numerous alternative interpretations, the origin of the
plateau remains unclear. In the early years of the Neil Gehrels Swift
Observatory, it was suggested that the plateau might be afterglow
radiation, that started before the prompt gamma-ray emission, and its
time profile would be an artefact of assuming the start time of the
prompt gamma-ray emission as zero time (the so-called "prior
activity model"). We aim to test the plausibility of the prior
activity model by leveraging the current Swift sample of early X-ray
afterglows of GRBs with measured redshifts, which is more than eight
times larger than the one originally used (463 vs. 56). We modelled
the GRB rest-frame X-ray afterglow luminosities assuming a simple
power-law with the true reference time preceding the prompt gamma-ray
emission trigger time by T0 and the X-ray luminosity L0 at the trigger
time as free parameters. We tested each case applying both chi2 and
runs tests. For 90% GRBs of our sample, the model provided a
successful description. In ten cases the afterglow peak is identified
and modelled appropriately. Using the 300 GRBs with accurate
parameters' estimates, we confirm the anti-correlation between L0
and T 0 with 0.7dex scatter. In addition, selecting the subsample of
180 from the literature with reliable estimates of
isotropic-equivalent released energy Egamma,iso, peak luminosity
Lgamma,iso, and intrinsic peak energy Ep,i of the nuFnu spectrum of
the prompt gamma-ray emission, we find a correlation between L0, T0,
and Egamma,iso (0.4dex scatter) over nine decades in L0 and common to
all kinds of GRBs. The afterglow likely begins in most cases before
the start of the detected prompt gamma-ray emission by a
lognormally-distributed rest-frame delay with a mean of 103s and
0.8dex dispersion. As also suggested by the recent discoveries of
Einstein Probe of X-ray emission starting long before the prompt
gamma-rays, our results suggest that the occurrence of prior activity
could be much more frequent than what has tacitly been assumed so far.
Description:
Table 3 reports, for a selection of GRBs, the time intervals that
contained internal activity and that, as such, were excluded from the
modelling. Tables 4 and 5 report the results of the modelling,
including the best fit parameters, for the two classes of GRBs: long,
short, short with extended emission (Table 4) and the AG-rise group
(Table 5).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 23 22 Time intervals that were excluded from the fit
because of the presence of internal activity,
for a restricted list of GRBs
table4.dat 172 453 Best-fit parameters for the main group of GRBs
table5.dat 178 10 Best-fit parameters for the AG-rise set
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- GRB GRB name
9- 15 I7 s tstart Start time of internal activity interval
17- 23 I7 s tstop Stop time of internal activity interval
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- GRB GRB name
9- 15 F7.5 --- z Redshift
18- 25 E8.2 s tstart Start time of the interval used for fit
27- 34 E8.2 s tstop Stop time of the interval used for fit
38- 42 F5.2 [10-7W] logL0 Best-fit of afterglow luminosity
(L0 in erg/s)
45- 48 F4.2 [10-7W] e_logL0 Error on logL0
53- 56 F4.2 [s] logT0 Best-fit of time measured since the true
beginning of the event
59- 62 F4.2 [s] e_logT0 Error on logT0
67- 70 F4.2 --- alpha Best-fit power-law index
73- 76 F4.2 --- e_alpha Error on alpha
82- 89 E8.2 --- Pvalchi2 p-value of chi2 test
96-100 F5.2 [10-7W] b_logL0 Lower boundary of 90% confidence interval
for logL0
102-106 F5.2 [10-7W] B_logL0 Upper boundary of 90% confidence interval
for logL0
110-113 F4.2 [s] b_logT0 Lboundary of 90% confidence interval
for logT0
116-119 F4.2 [s] B_logT0 Uboundary of 90% confidence interval
for logT0
123-126 F4.2 --- b_alpha Lboundary of 90% confidence interval
for alpha
128-131 F4.2 --- B_alpha Uboundary of 90% confidence interval
for alpha
134-141 E8.2 --- PvalRuns p-value of runs test
144-147 I4 --- Np Number of points
150-152 A3 --- Cl [L S SEE] GRB Class (G1)
155-162 F8.3 s T90 T90 duration
165-172 F8.3 s e_T90 ?=-1 Error on T90
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- GRB GRB name
9- 14 F6.4 --- z Redshift
17- 23 E7.1 s tstart Start time of the interval used for fit
25- 31 E7.1 s tstop Stop time of the interval used for fit
34 I1 --- FSflag [0/1] Forward Shock (1), Reverse Shock (0)
37- 41 F5.2 [10-7W] logL0 Best-fit of afterglow luminosity
(L0 in erg/s)
43- 46 F4.2 [10-7W] e_logL0 Error on logL0
51- 54 F4.2 [s] logT0 Best-fit of time measured since the true
beginning of the event
56- 59 F4.2 [s] e_logT0 Error on logT0
64- 67 F4.2 [s] logTp Best-fit of time measured since the true
zero time
69- 72 F4.2 [s] elogTp Error on logTp
77- 80 F4.2 --- p Best-fit p (e- energy distribution index)
82- 85 F4.2 --- e_p Error on p
90- 97 E8.2 --- PvalChi2 p-value of chi2 test
102-106 F5.2 [10-7W] b_logL0 Lower boundary of 90% confidence interval
for logL0
108-112 F5.2 [10-7W] B_logL0 Upper boundary of 90% confidence interval
for logL0
116-119 F4.2 [s] b_logT0 Lower boundary of 90% confidence interval
for logT0
122-125 F4.2 [s] B_logT0 Upper boundary of 90% confidence interval
for logT0
129-132 F4.2 [s] b_logTp Lower boundary of 90% confidence interval
for logTp
134-137 F4.2 [s] B_logTp Upper boundary of 90% confidence interval
for logTp
141-144 F4.2 --- b_p Lower boundary of 90% confidence interval
for p
146-149 F4.2 --- B_p Upper boundary of 90% confidence interval
for p
152-159 E8.2 --- PvalRuns p-value of runs test
161-163 I3 --- Np Number of points
165 A1 --- Cl [L S] GRB Class (G1)
167-172 F6.2 s T90 T90 duration
174-178 F5.2 s e_T90 Error on T90
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Global notes:
Note (G1): GRB Class as follows:
L = long burst
S = short burst
SEE = short burst with extended emission
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Acknowledgements:
Cristiano Guidorzi, guidorzi(at)fe.infn.it
(End) Cristiano Guidorzi [Univ. Ferrara], Patricia Vannier [CDS] 29-Sep-2025