Swift J1913.1+1946/GRB 221009A: detection of a 250-TeV photon-like air shower by Carpet-2

ATel #15669; D. D. Dzhappuev, Yu. Z. Afashokov, I. M. Dzaparova, T. A. Dzhatdoev, E. A. Gorbacheva, I. S. Karpikov, M. M. Khadzhiev, N. F. Klimenko, A. U. Kudzhaev, A. N. Kurenya, A. S. Lidvansky, O. I. Mikhailova, V. B. Petkov, E. I. Podlesnyi, N. A. Pozdnukhov, V. S. Romanenko, G. I. Rubtsov, S. V. Troitsky, I. B. Unatlokov, I. A. Vaiman, A. F. Yanin, K. V. Zhuravleva (Carpet-2 group, INR RAS)
on 12 Oct 2022; 13:56 UT
Credential Certification: Sergey Troitsky (st@ms2.inr.ac.ru)

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The X-ray and optical transient Swift J1913.1+1946 (ATel #15650; GCN #32632) is possibly associated with a gamma-ray burst GRB 221009A (Fermi GBM alert, GCN #32635, #32636). This bright transient has been observed by numerous instruments in optical, X-ray and gamma-ray bands (ATel #15651, #15653, #15655, #15656, #15660, #15661, #15662, #15663, #15664, #15665; GCN #32634 - #32671, #32676 - #32679, #32683 - #32686, #32688, #32690 - #32695 and counting). Tentative redshift from the observation of the afterglow emission is z=0.151 (GCN #32648, #32686). In case the GRB association is true, this event produced the most energetic GRB photon ever seen by Fermi LAT (ATel #15656), that of 99 GeV. Moreover, the same transient was detected by LHAASO during 2000 sec after the GRB trigger with photons up to 18 TeV, highest energies ever detected from a GRB (GCN #32677).

In a follow-up analysis of Carpet-2 data, we found an air shower consistent with being caused by a photon of 251 TeV energy, giving zero hits in the 175 m² muon detector. This event was detected at 14:32:35 UT, that is 1338 sec after the SWIFT trigger and 4536 sec after the GBM trigger. The reconstructed arrival direction is RA=289.51 deg, DEC=18.44 deg, that is 1.78 deg from the transient direction, well within the Carpet-2 angular resolution of 4.7 deg (90% CL).

Such a high-energy muonless, that is photon-like, shower is a rare event. For the declination of the transient, from a given direction within the 90% CL angular resolution, one expects one background photon-like shower of this energy to arrive in 428 days, which corresponds to the Poisson probability of a random coincidence in 4536 seconds of 1.2*10^-4, that is to the 3.8 sigma pre-trial significance. We note that it is not trivial to estimate the significance correctly in an a posteriori search. More details will be published after further elaborated analysis.

High-energy photons attenuate through production of electron-positron pairs on cosmic background radiation, and 250-TeV photons (as well as 18-TeV photons detected by LHAASO) cannot reach us from the assumed GRB redshift z=0.151 unless unconventional particle physics is involved. Examples are axion-like particles (e.g. Csaki et al. 2003, de Angelis et al. 2007, review in Troitsky 2017) or Lorentz-invariance violation (e.g. Kifune 1999, review in Martinez-Huerta et al. 2020). However, given the low Galactic latitude, b=4 deg, of the event, an association of high-energy photons with a Galactic transient remains a possible explanation.

Carpet-2 is an air-shower array at Baksan Neutrino Observatory of the Institute for Nuclear Research of the Russian Academy of Sciences, capable of detecting cosmic gamma rays above 100 TeV (see e.g. Dzhappuev et al. (2020) for a recent description and further references).