AstroSat, India’s first multi-wavelength space-based observatory, has detected bright sub-second X-ray bursts from a new and unique neutron star with ultrahigh magnetic field (magnetar), which can help understand the intriguing extreme astrophysical conditions of magnetars.

Magnetars are neutron stars having an ultrahigh magnetic field that are much stronger than the terrestrial magnetic field. Simply put, the magnetic field of a magnetar is over one quadrillion time stronger than the magnetic field of Earth. What powers the emission of high-energy electromagnetic radiation in them is the decay of magnetic fields in these objects. Besides, magnetars display strong temporal variability, typically including a slow rotation, a rapid spin-down, bright but short bursts going on upto months-long outbursts.

One such magnetar was called SGR J1830-0645, was discovered in October 2020 by NASA's Swift spacecraft. It is relatively young (about 24,000 years) and isolated neutron star.

Motivated to study the magnetar and explore its characteristics in broad-band X-ray energies with AstroSat, scientists from the Raman Research Institute (RRI) and University of Delhi performed the timing and spectral analysis of this magnetar using two instruments onboard AstroSat --- the Large Area X-Ray Proportional Counter (LAXPC) and Soft X-Ray telescope (SXT).

“One of the key findings was the detection of 67 short sub-second X-ray bursts, with an average duration of 33 milliseconds. Of these bursts, the brightest one lasted for about 90 milliseconds”, said Dr. Rahul Sharma, the paper’s lead author and a post-doctoral fellow at RRI, an autonomous institute funded by the Department of Science and Technology.

The study, published in the Monthly Notices of the Royal Astronomical Society, concluded that SGR J1830–0645 is a unique magnetar that showcased emission line in its spectra.

The presence of emission lines and its potential origin -- either due to fluorescence of iron, proton cyclotron line feature or an instrumental effect -- remains a subject of consideration, the study noted.

“The energy-dependence in SGR J1830-0645 was different from what was observed in several other magnetars. Here, there were two thermal blackbody emission components originating from surface of neutron star (radius of 0.65 and 2.45 km). This research, thus, contributes to our understanding of magnetars and their extreme astrophysical conditions,” said Dr. Sharma.

"We noticed that the pulsed component of the overall X-ray emission showed significant variation with energy. It increased for energies up to about 5 kiloelectron Volt (keV) and showed a steep drop thereafter. This trend is different from that observed in several other magnetars," said co-author Prof. Chetana Jain from Hansraj College, University of Delhi.

The research team now plans to expand their study to understand the origin of these highly energetic emissions and decipher whether they are astrophysical or instrumental in nature.

Publication link - 

https://academic.oup.com/mnras/article/526/4/4877/7325939

AstroSat observation of the magnetar SGR J1830−0645 during its first detected X-ray outburst

Rahul Sharma, Chetana Jain, Biswajit Paul, T R Seshadri

Monthly Notices of the Royal Astronomical Society, Volume 526, Issue 4, December 2023, Pages 4877–4884

SNC/PK