An international group of researchers has succeeded in measuring for the first time the characteristics of a flare on a distant magnetar. A magnetar is a rare compact type of neutron star teeming with energy and magnetism. The magnetar they have studied is about 13 million light years away, in the direction of the NGC 253, a prominent galaxy in the Sculptor group of galaxies.
The flare, which spewed within a few tenths of a second as much energy as the Sun would shed in 100,000 years, was captured accidentally on April 15, 2020, by the Atmosphere-Space Interactions Monitor instrument (ASIM) of the International Space Station.
Rare and exotic
This data was then analysed by the researchers over the period of a year to throw light into the structure of the flare, and thereby, into the nature of such magnetars. This is the first study to characterise such a flare from so distant a magnetar. The research was published in the journal, Nature.
Magnetars are relatively rare objects, with only about thirty having been spotted within the Milky Way so far. The present magnetar is only the second one to be studied which is located outside the galaxy and is also the furthest, at 13 million light years distance.
How magnetars form
During the course of their evolution, massive stars – with masses around 10-25 times the mass of the Sun – eventually collapse and shrink to form very compact objects called neutron stars. A subset of these neutron stars are the so-called magnetars which possess intense magnetic fields. These are highly dense and have breathtakingly high rotation speeds – they have rotational periods that can be just 0.3 to 12.0 seconds. “We believe that size of the object was around 20 km in diameter with mass around 1.4 times the mass of the Sun,” says Shashi Bhushan Pandey of the Aryabhata Research Institute of Observational Sciences, Nainital, who is one of the authors of the paper.
Magnetars have high magnetic fields in the range of 1015 gauss and they emit energy in the range given by luminosities of 1037 – 1040 joules per second. Compare this to the luminosity of the sun which is in the order of 1026 joules per second – a factor of at least 1011 lower.
Further, these magnetars emit violent flares. “The observations revealed multiple pulses, with a first pulse appearing only for about tens of microseconds, much faster than other extreme astrophysical transients,” said Alberto J. Castro-Tirado, from the Andalusian Institute for Astrophysics (IAA-CSIC), Spain, and lead author of the paper, in a release circulated by the Department of Science and Technology, Government of India.
The observed giant flare lasted approximately 160 milliseconds and during this time 1039 joules of energy was released. The flare spewed as much energy in a tenth of a second that our Sun will radiate in 100,000 years, according to the paper.
Eruptions in magnetars are believed to be due to instabilities in their magnetosphere, or “starquakes” produced in their crust - a rigid, elastic layer about one kilometre thick. This causes waves in the magnetosphere, and interaction between these waves causes dissipation of energy.
Magnetars are very difficult to observe when they are silent. It is only during a flare that they can be observed, and these flares are so short-lived that it presents a formidable problem. “They are mostly observed or seen in active transient phases which are very short in duration and are very faint in general for any available instruments or telescopes,” says Dr Pandey.
It was also a serendipitous find because, as Dr Pandey explains: “ASIM is mainly designed with its large effective area to observe terrestrial gamma ray flashes. It was a great coincidence that this bright transient flash was observed by ASIM instrument.”
According to Dr Pandey, studying these flares will not only help us understand the physics of magnetars, it will also help in understanding fast radio bursts, which are among the most enigmatic phenomena in astronomy.