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Register a new userRapid Spectral Variability of a Giant Flare from a Magnetar in NGC 253
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Magnetars are slowly-rotating neutron stars with extremely strong magnetic fields ($10^{13-15}$ G), episodically emitting $sim100$ ms long X-ray bursts with energies of $sim10^{40-41}$ erg. Rarely, they produce extremely bright, energetic giant flares that begin with a short ($sim0.2$ s), intense flash, followed by fainter, longer lasting emission modulated by the magnetar spin period (typically 2-12 s), thus confirming their origin. Over the last 40 years, only three such flares have been observed in our local group; they all suffered from instrumental saturation due to their extreme intensity. It has been proposed that extra-galactic giant flares likely constitute a subset of short gamma-ray bursts, noting that the sensitivity of current instrumentation prevents us from detecting the pulsating tail, while the initial bright flash is readily observable out to distances $sim 10-20$ Mpc. Here, we report X- and gamma-ray observations of GRB 200415A, which exhibits a rapid onset, very fast time variability, flat spectra and significant sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extra-galactic magnetar, noting that GRB 200415A is directionally associated with the galaxy NGC 253 ($sim$3.5 Mpc away). The detection of $sim3$ MeV photons provides definitive evidence for relativistic motion of the emitting plasma. The observed rapid spectral evolution can naturally be generated by radiation emanating from such rapidly-moving gas in a rotating magnetar.
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Magnetars are young, highly magnetized neutron stars that produce extremely rare giant flares of gamma-rays, the most luminous astrophysical phenomena in our Galaxy. The detection of these flares from outside the Local Group of galaxies has been predicted, with just two candidates so far. Here we report on the extremely bright gamma-ray flare GRB 200415A of April 15, 2020, which we localize, using the Interplanetary Network, to a tiny (20 sq. arcmin) area on the celestial sphere, that overlaps the central region of the Sculptor galaxy at 3.5 Mpc from the Milky Way. From the Konus-Wind detections, we find a striking similarity between GRB 200415A and GRB 051103, the even more energetic flare that presumably originated from the M81/M82 group of galaxies at nearly the same distance (3.6 Mpc). Both bursts display a sharp, millisecond-scale, hard-spectrum initial pulse, followed by an approximately 0.2 s long steadily fading and softening tail. Apart from the huge initial pulses of magnetar giant flares, no astrophysical signal with this combination of temporal and spectral properties and implied energy has been reported previously. At the inferred distances, the energy released in both flares is on par with that of the December 27, 2004 superflare from the Galactic magnetar SGR 1806-20, but with a higher peak luminosity. Taken all together, this makes GRB 200415A and its twin GRB 051103 the most significant candidates for extragalactic magnetar giant flares, both a factor of five more luminous than the brightest Galactic magnetar flare observed previously, thus providing an important step towards a better understanding of this fascinating phenomenon.
The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the compact star merger GRB model is found to have difficulty reproducing such an event in a nearby distance. Future detections and follow-up observations of similar events are essential to firmly establish the connection between SGR giant flares and a subsample of nearby short GRBs.
NGC 253 is a starburst galaxy of SAB(s)c type with increasing interest because of its high activity at unrivaled closeness. Its energetic event is manifested as the vertical gas features in its central molecular zone, for which stellar feedback was proposed as the driving engine. In order to pursue details of the activity, we have undertaken a kinematic analysis of the ALMA archive data of CO($J$=3--2) emission at the highest resolution $sim$3 pc. We revealed that one of the non-rotating gas components in the central molecular zone shows a loop-like distribution of $sim$200 pc radius. The loop is associated with a star cluster, whereas the cluster is not inside the loop and is not likely as the driver of the loop formation. Further, we find that the bar potential of NGC 253 seems to be too weak to drive the gas motion by the eccentric orbit. As an alternative we frame a scenario that magnetic acceleration by the Parker instability is responsible for the creation of the loop. We show that the observed loop properties are similar to those in the Milky Way, and argue that recent magnetro-hydrodynamics simulations lend support for the picture having the magnetic field strength of $gtrsim$100 $mu$G. We suggest that cluster formation was triggered by the falling gas to the footpoint of the loop, which is consistent with a typical dynamical timescale of the loop $sim$1 Myr.
Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100 GeV) data from gamma-ray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analysed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE gamma-ray data can be described by a power law in energy with differential photon index Gamma=2.14 pm 0.18_stat pm 0.30_sys and differential flux normalisation at 1 TeV of F_0 = (9.6 pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14} TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE gamma-ray spectrum reveals a photon index of Gamma=2.24 pm 0.14_stat pm 0.03_sys and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 pm 1.0_stat pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE gamma-ray data results in a differential photon index Gamma=2.34 pm 0.03 with a p-value of 30%. The gamma-ray observations indicate that at least about 20% of the energy of the cosmic rays capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in the HE and VHE gamma-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the gamma-ray production, the single overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of cosmic-ray sources in the starburst region.
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