Fundación Galileo Galilei - INAF Telescopio Nazionale Galileo 28°45'14.4N 17°53'20.6W 2387.2m A.S.L.

Telescopio Nazionale Galileo helps to detect a magnetar giant flare in M 82

On 2023, November 15, at 15:36:22 UT the European Space Agency (ESA) INTEGRAL satellite detected a short (about 0.1 second duration) flash of gamma-ray radiation from the nearby galaxy M 82 (12 million light years).

The possibility of having a short duration GRB in such a nearby galaxy immediately triggered the interest of the astronomical community. A short GRB occurring at the distance of the M 82 galaxy would have displayed a bright afterglow emission at the X-rays and optical frequency.

A series of follow-up observations were soon activated by a team led by S. Mereghetti. Multi-filter follow-up optical observations of this event were carried out with different telescopes between about 5 and 12 h from the event. In particular the November 16th deep photometry in the z_sdss, i_sdss and r_sdss band has been taken with DOLORES (Device Optimized for the LOw RESolution) instaled at the Telescopio Nazionale Galileo.

The short, bright flash was initially classified as a Gamma-ray burst (GRB), i.e. an extra-galactic explosion that release collimated outflows of matter with speeds close to the speed of light. GRBs are divided into long and short classes on the basis of their duration. In particular, short GRBs last from a few tens of milliseconds up to about 2 seconds. This events are usually associated with the merger of two compact objects and they are also emitters of gravitational waves. Thanks to the deep observations of this event obtained with DOLORES it was posible to exclude a GRB-like optical afterglow. At the same time no detection of gravitational waves coincident with the gamma-ray flash has been reported by the LIGO-Virgo-KAGRA collaboration.

The non-detection of a bright afterglow nor of a gravitational wave signal led the authors to conclude that the short and intense flash of gamma-ray radiation was not due to a short GRB but to another astrophysical phenomenon, namely a magnetar giant flare. Magnetars are a peculiar class of neutron stars (the extremely dense remnants of supernovae explosions). They are characterised by extreme magnetic fields and can undergo very rapid and intense burst of gamma-ray radiations, known as "giant flares" that suddenly reach luminosities billions of times higher than that of the Sun.

These giant flares are an extremely rare phenomenon, whose origin is still unknown. Just three of these events have been observed in the past 50 years: two occurred in our Milky Way and the third in the Large Magellanic Cloud.

The detection of a magnetar giant flare in the M82 galaxy, beyond the horizon of previous detections of similar events, opens therefore exciting perspectives for future detections of such phenomena in external galaxies with the next generation of high sensitivity instruments.

The results of these observations have been published on Nature:

Optical image of M82 created from data obtained with the TNG

Figure: Optical image of M82 created from data obtained with the TNG (RGB scale: z band, red; i band, green; r band, blue). The error circle obtained from the INTEGRAL satellite for the bright gamma-ray flash detect on 2023 Nov. 15 has a radius of 2 arcmin. See paper for more details.