A delayed 400 GeV photon from GRB 221009A and implication on the intergalactic magnetic field Zi-Qing Xia1Yun Wang1Qiang Yuan1 2and Yi-Zhong Fan1 2 1Key Laboratory of Dark Matter and Space Astronomy

2025-04-30 0 0 370.75KB 9 页 10玖币

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A delayed 400 GeV photon from GRB 221009A and implication on the intergalactic magnetic ﬁeld

Zi-Qing Xia,1Yun Wang,1Qiang Yuan,1, 2 and Yi-Zhong Fan1, 2, ∗

1Key Laboratory of Dark Matter and Space Astronomy,

Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China

2School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, China

Large High Altitude Air Shower Observatory has detected 0.2−13 TeV emission of GRB 221009A within

2000 s since the trigger. Here we report the detection of a 400 GeV photon, without accompanying prominent

low-energy emission, by Fermi Large Area Telescope in this direction with a 0.4 days’ delay. Given an in-

tergalactic magnetic ﬁeld strength of about 4 ×10−17 G, which is comparable to limits from TeV blazars, the

delayed 400 GeV photon can be explained as the cascade emission of about 10 TeV gamma rays. We estimate

the probabilities of the cascade emission that can result in one detectable photon beyond 100 GeV by Fermi

Large Area Telescope within 0.3−1 days is about 2% whereas it is about 20.5% within 0.3−250 days. Our

results show that Synchrotron Self-Compton explanation is less favored with probabilities lower by a factor of

about 3 −30 than the cascade scenario.

Introduction

The measurement of the intergalactic magnetic ﬁeld strength (BIGMF), one of the fundamental parameters of the astrophysics

that may be related to how the Universe starts/evolves and carries the information of the primordial magnetic ﬁelds [1,2], is

rather challenging. So far, it has not been reliably measured, yet [3–5]. One promising method, initially proposed by Plaga in

Ref. [6], is to explore the arrival times of gamma rays from extra-galactic TeV transients such as Gamma-ray Bursts (GRBs,

[7–10]) and blazars [11,12]. The basic idea is that, before reaching the observer, the primary TeV gamma rays will be absorbed

by the diﬀuse infrared background and then generate ultra-relativistic electron-positron (e±) pairs with Lorentz factors of γe≈

9.8×105(1 +z)(ϵγ/1 TeV), where zis the redshift of the source and ϵγis the observed energy of the primary gamma rays.

These pairs will subsequently scatter oﬀthe ambient cosmic microwave background (CMB) photons, and boost them to an

average energy of ϵγ,2nd ≈0.8 (1 +z)2(ϵγ/1 TeV)2GeV. Unless BIGMF is very low (say, ≤10−20 G), the presence of intergalactic

magnetic ﬁeld will play the dominant role in delaying the arrival of the secondary gamma rays. Hence the observation of delayed

gamma-ray emission can in turn impose a tight constraint or give a direct measurement of BIGMF [6,13,14].

Such an idea has been applied to the long-lasting MeV-GeV afterglow emission of GRB 940217 [6,15,16] and then to other

GRBs, including for instance GRB 130427A and GRB 190114C [17,18], two bursts with powerful very high energy gamma-

ray radiation [19,20]. In these studies, the limits of BIGMF ≥10−21 −10−17 G have been set. Among all bursts detected so

far, GRB 221009A is distinguished by its huge power (the isotropic equivalent gamma-ray radiation energy is about 1055 erg),

low redshift (z=0.151), and very strong TeV gamma-ray emission [21–27]. Therefore, it is an ideal target to search for

the cascade emission and hence probe the intergalactic magnetic ﬁeld. GRB 221009A triggered the Fermi Gamma-Ray Burst

Monitor (GBM) on 2022-10-09 13:16:59 UT (T0, MET 687014224), about 1 hour earlier than the Swift [21,22]. After 200 s of

the Fermi-GBM trigger, the Fermi Large Area Telescope (LAT) [28] detected strong high-energy emission from GRB 221009A

and the photon ﬂux averaged in the time interval of T0+200 −T0+800 s is about 10−2ph cm−2s−1[25]. The Large High

Altitude Air Shower Observatory (LHAASO) has detected above 10 TeV emission of the extraordinary powerful GRB 221009A

within about 2000 s after the trigger [27,29,30]. Though GRB 221009A is about 90 deg from the boresight at T0, the Dark

Matter Particle Explorer (DAMPE) also observed the signiﬁcantly increasing unbiased-Trigger counts within 227 −233 s after

the trigger [31].

In this work we analyze the long-term Fermi-LAT data and report the detection of a 400 GeV photon, without associated

prominent low-energy emission, at approximately 0.4 days after the trigger. We show such a delayed 400 GeV photon can be

explained as the cascade emission of 10 TeV photons with an intergalactic magnetic ﬁeld strength of about 4 ×10−17 G, which

is more favored than Synchrotron Self-Compton (SSC) emission explanation.

Results

The signiﬁcant detection of a delayed 400 GeV photon. In the long-term Fermi-LAT observations for GRB 221009A,

we ﬁnd there is a 397.7 GeV photon arriving at T0+33554 s (approximately 0.4 days) without accompanying GeV photons

(see methods subsection The Fermi-LAT data). As shown in Fig. 1, the location of this amazing event is RA =288.252◦and

Dec =19.763◦given by the red ﬁlled dot, which is nicely in agreement with the Swift/Ultra-violet Optical Telescope (UVOT)

localization (the blue triangle, RA =288.265◦and DEC =19.774◦, [21]) as well as that of LHAASO’s Water Cherenkov

∗Corresponding author:Yi-Zhong Fan (yzfan@pmo.ac.cn)

arXiv:2210.13052v3 [astro-ph.HE] 10 May 2024

Detector Array (LHAASO-WCDA, the gold star, RA =288.295◦and Dec =19.772◦, [29]) and Very Long Baseline Array

(VLBA, the grey square, RA =288.264◦and Dec =19.773◦, [32]). Pre-GRB 221009A, just two photon events larger than

100 GeV had been found in 14 years of Fermi-LAT observations within 0.5 degree of GRB 221009A, suggesting a rather low

background level at energies above 100 GeV (One was observed with the energy of 268.1 GeV at the location of RA =288.51◦

and Dec =20.08◦. The other 107.1 GeV photon was located at RA =288.47◦and Dec =19.54◦). Giving its spatial and temporal

coincidence with GRB 221009A, we conclude that this 400 GeV photon is indeed physically associated with this monster. We

calculate the probability that this LAT event belongs to GRB 221009A within the (T0+0.3−T0+1) days interval using the

gtsrcprob tool (in the Fermitools package). It turns out to be 0.9999937, corresponding to a signiﬁcance level of 4.4σ. Note that

this 400 GeV photon is among the ULTRACLEAN class events and the possibility for being a mis-identiﬁcation of a cosmic ray

is very low. Therefore, we have identiﬁed the most energetic GRB photon detected by Fermi-LAT so far. The previous records

are a 95 GeV photon from GRB 130427A [19] and then a 99.3 GeV photon from GRB 221009A at an early time [23].

An intergalactic magnetic ﬁeld strength of about 4×10−17 G needed in the cascade scenario. The spectral energy

distributions (SEDs) measured in the time intervals after the burst of 0.05 −0.3 days (blue), 0.3−1 days (yellow) and 0.3−250

days (red) are reported in the panel (a) of Fig. 2(see methods subsection The Fermi-LAT energy spectral analysis). We ﬁnd that

the emissions in these time intervals show diﬀerent behaviors: for the 0.05 −0.3 days interval, it is dominated by low-energy

radiation; but for the later time intervals of 0.3−1 days and 0.3−250 days, we only detect the single 400 GeV photon without

accompanying low energy emission. In principle, the delayed GeV-TeV emission could be either from the SSC radiation of the

forward shock electrons or the cascade emission of about 10 TeV prompt gamma-rays. The LHAASO collaboration has reported

that the SSC afterglow from a very narrow jet, as proposed in Ref. [33], can explain the TeV emission within ﬁrst 2000 s [29].

With multi-band afterglow light curves of this event (see Ref. [34] for more details, and an updated paper with the structured jet

[35]), we obtain the expected SSC afterglow emission for the corresponding time intervals plotted as the dashed lines in panel

(a) of Fig. 2. As for the 0.05 −0.3 days interval, we ﬁnd the SEDs measured by the Fermi-LAT can be well described by this

SSC model. While in the later time intervals of 0.3−1 days and 0.3−250 days, the ﬂuxes measured by Fermi-LAT around

400 GeV are larger by about 3 orders of magnitude than that expected by the SSC model, which implies that it is challenging to

produce such a 400 GeV photon via the SSC process. Hence we concentrate on exploring the cascade scenario.

Within the cascade scenario, the yielding e±pairs have a Lorentz factor of approximately 107, and the delay of the arrival

time of the secondary GeV−TeV photons is governed by the deﬂection of the e±pairs by the intergalactic magnetic ﬁeld. To

account for a delay time of 0.4 days, we need an intergalactic magnetic ﬁeld strength of approximately 4 ×10−17 G (assuming a

coherence scale of 1 Mpc), which is comparable with limits set by Fermi-LAT observations of TeV blazars [36,37] (see methods

subsection Analytical estimate of the intergalactic magnetic ﬁeld strength).

Then we conduct Monte Carlo simulation of the cascade scenario with the BIGMF =4×10−17 G to obtain the expected cascade

ﬂux (see methods subsection Numerical simulation of the cascade scenario and the estimate of BIGMF.). As shown in panel (a)

of Fig. 2, the expected emission ﬂux at around 400 GeV in the cascade scenario is higher than that of the SSC model, implying

that the former is more likely the origin of the photon.

Discussion

GRB 221009A is the most powerful gamma-ray bursts detected so far. Thanks to its rather low redshift z=0.151, the emission

has been detected up to the energy of about 13 TeV. Because of the high optical depth of the universe to such energetic gamma

rays, the intrinsic spectrum likely extends to an even higher energy range and most of these primary photons have been absorbed

by the far-infrared background before reaching us. The resulting ultra-relativistic e±pairs will up-scatter and then boost the CMB

photons to sub-TeV energy. Motivated by such a prospect, we analyze the long term of the Fermi-LAT gamma-ray observations

in the direction of GRB 221009A and successfully identiﬁed a 400 GeV photon, without accompanying any low-energy gamma

rays, at 0.4 days after GRB 221009A.

Motivated by the facts that the SSC afterglow model is hard to account for the data and a simple analytical estimate suggests

that the cascade scenario can account for the data, we conduct the simulation of cascade with the Elmag 3.03 package and adopted

the intrinsic energy spectrum of the GRB as a power-law (PL) with an index of 2.4 reported by the LHAASO collaboration [29].

In the time intervals of 0.3 −1 days and 0.3 −250 days, the cascade spectra with BIGMF =4×10−17 G peak at several hundred

GeV, and the corresponding probabilities that the Fermi-LAT observed one cascade photon with energy larger than 100 GeV are

about 2.0% and 20.5%, respectively (see methods subsection Numerical simulation of the cascade scenario and the estimate of

BIGMF). Hence, we suggest that the detection of the 400 GeV photon is by chance (i.e., it is a small probability event). Anyhow,

the cascade scenario for the 400 GeV photon is preferred over the SSC model with probabilities higher by a factor of 3 −30.

Although the diﬀerence is not very signiﬁcant, the cascade scenario seems to be a better explanation for the delayed 400 GeV

photon. Note that due to the limited observation, it is diﬃcult to draw a strong conclusion. Considering the uncertainty of

intrinsic spectrum above 13 TeV, we also take the power-law with an exponential cutoﬀ(PLEcut) at 20 TeV model into account

(see methods subsection Uncertainty in the intrinsic energy spectrum). The cascade ﬂuxes above 10 GeV for the PLEcut cases

are also signiﬁcantly higher than that expected by the SSC model, as shown in Fig. 3. For both two intrinsic spectral models (PL

and PLEcut), the possibility of a weaker IGMF BIGMF =1×10−18 G can be ruled out by the Fermi-LAT upper limits at about

10 GeV in the 0.3 −250 days interval. A higher BIGMF, say ≥10−16 G, is also disfavored because of the resulting lower cascade

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Adelayed400GeVphotonfromGRB221009AandimplicationontheintergalacticmagneticfieldZi-QingXia,1YunWang,1QiangYuan,1,2andYi-ZhongFan1,2,∗1KeyLaboratoryofDarkMatterandSpaceAstronomy,PurpleMountainObservatory,ChineseAcademyofSciences,Nanjing210023,China2SchoolofAstronomyandSpaceScience,UniversityofSciencea...

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A delayed 400 GeV photon from GRB 221009A and implication on the intergalactic magnetic field Zi-Qing Xia1Yun Wang1Qiang Yuan1 2and Yi-Zhong Fan1 2 1Key Laboratory of Dark Matter and Space Astronomy.pdf

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A delayed 400 GeV photon from GRB 221009A and implication on the intergalactic magnetic field Zi-Qing Xia1Yun Wang1Qiang Yuan1 2and Yi-Zhong Fan1 2 1Key Laboratory of Dark Matter and Space Astronomy

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