The ANtarctic Impulsive Transient Antenna (ANITA) NASA long-duration balloon payload completed its fourth flight in December 2016, after 28 days of flight time. ANITA is sensitive to impulsive broadband radio emission from interactions of ultrahigh-energy neutrinos in polar ice (Askaryan emission). We present the results of two separate blind analyses searching for signals from Askaryan emission in the data from the fourth flight of ANITA. The more sensitive analysis, with a better expected limit, has a background estimate of ${0.64}_{-0.45}^{+0.69}$ and an analysis efficiency of $82\pm 2\%$. The second analysis has a background estimate of ${0.34}_{-0.16}^{+0.66}$ and an analysis efficiency of $71\pm 6\%$. Each analysis found one event in the signal region, consistent with the background estimate for each analysis. The resulting limit further tightens the constraints on the diffuse flux of ultrahigh-energy neutrinos at energies above $10^{19.5}\mathrm{eV}$.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

The sky distribution of gamma-ray bursts (GRBs) has been intensively studied by various groups for more than two decades. Most of these studies test the isotropy of GRBs based on their sky number density distribution. In this work, we propose an approach to test the isotropy of the universe through inspecting the isotropy of the properties of GRBs such as their duration, fluences, and peak fluxes at various energy bands and different timescales. We apply this method on the Fermi/Gamma-ray Burst Monitor (GBM) data sample containing 1591 GRBs. The most noticeable feature we found is near the Galactic coordinates , , and radius . The inferred probability for the occurrence of such an anisotropic signal (in a random isotropic sample) is derived to be less than a percent in some of the tests while the other tests give results consistent with isotropy. These are based on the comparison of the results from the real data with the randomly shuffled data samples. Considering the large number of statistics we used in this work (some of which are correlated with each other), we can anticipate that the detected feature could be a result of statistical fluctuations. Moreover, we noticed a considerably low number of GRBs in this particular patch, which might be due to some instrumentation or observational effects that can consequently affect our statistics through some systematics. Further investigation is highly desirable in order to clarify this result, e.g., utilizing a larger future Fermi/GBM data sample as well as data samples of other GRB missions and also looking for possible systematics.

Context. On 2022 October 9 the brightest gamma-ray burst (GRB) ever observed lit up the high-energy sky. It was detected by a multitude of instruments, attracting the close attention of the GRB community, and saturated many detectors. Aims. GRBAlpha, a nano-satellite with a form factor of a 1U CubeSat, detected this extraordinarily bright long-duration GRB, GRB 221009A, without saturation but affected by pile-up. We present light curves of the prompt emission in 13 energy bands, from 80 keV to 950 keV, and performed a spectral analysis to calculate the peak flux and peak isotropic-equivalent luminosity. Methods. Since the satellite’s attitude information is not available for the time of this GRB, more than 200 incident directions were probed in order to find the median luminosity and its systematic uncertainty. Results. We find that the peak flux in the 80 − 800 keV range (observer frame) was F_ph^p = 1300₋₂₀₀⁺¹²⁰⁰ ph cm⁻² s⁻¹, or F_erg^p = 5.7_−0.7^+3.7 × 10⁻⁴ erg cm⁻² s⁻¹, and the fluence in the same energy range of the first GRB episode, which lasted 300 s and was observable by GRBAlpha, was S = 2.2_−0.3^+1.4 × 10⁻² erg cm⁻², or S_bol = 4.9_−0.5^+0.8 × 10⁻² erg cm⁻² for the extrapolated range of 0.9 − 8690 keV. We infer the isotropic-equivalent released energy of the first GRB episode to be E_iso^bol = 2.8_−0.5^+0.8 × 10⁵⁴ erg in the 1 − 10 000 keV band (rest frame at z = 0.15). The peak isotropic-equivalent luminosity in the 92 − 920 keV range (rest frame) was L_iso^p = 3.7_−0.5^+2.5 × 10⁵² erg s⁻¹, and the bolometric peak isotropic-equivalent luminosity was L_iso^p,bol = 8.4_−1.5^+2.5 × 10⁵² erg s⁻¹ (4 s scale) in the 1 − 10 000 keV range (rest frame). The peak emitted energy is E_p^∗ = E_p^(1+z) = 1120 ± 470 keV. Our measurement of L_iso^p,bol is consistent with the Yonetoku relation. It is possible that, due to the spectral evolution of this GRB and the orientation of GRBAlpha at the peak time, the true values of peak flux, fluence, L_iso, and E_iso are even higher.