Astrophysicists from UNLV and Nanjing University revise theoretical framework of Gamma Ray Bursts

Tony Allen from UNLV News Center summarized the work.

The mysteries of the cosmos continue to amaze astronomers, and with each new observation comes a chance to deepen – or upend – our understanding of the universe.

In the Dec. 7 issue of the journal Nature, an international team of astrophysicists report the discovery of a unique cosmological gamma-ray burst (GRB) that defies prevailing theories of how the violent cosmic explosions form. This “oddball” burst led the team to propose a new model, or source, for certain types of GRBs.

Gamma-ray bursts are the most luminous and violent explosions in the universe. They signify the deaths of stars or collisions of stellar remnants. Observed GRBs are typically placed into two categories: short- or long-duration GRBs. Long GRBs originate from the deaths of massive stars, and are typically associated with bright optical transients named supernovae. Short GRBs have a duration of less than two seconds and originate from the collisions of two neutron stars or a neutron star and a black hole, and are typically associated with more faint optical transients known as kilonovae.

For decades, GRBs nestled nicely into these cozy categories. Until now.

On Dec.11, 2021, a GRB triggered several gamma-ray detectors in space, including NASA’s Fermi Gamma-ray Telescope and the Neil Gehrels Swift Observatory. This burst, with a duration of nearly 70 seconds, would typically be regarded as a normal long GRB. That is, until multiple teams from the U.S. and Europe performed follow-up observations and discovered a surprising signature.

“This GRB includes two parts: a 13-second long hard spike and a 55-second softer extended emission,” said UNLV alumnus and study corresponding author Bin-Bin Zhang, who’s currently with China’s Nanjing University. “The duration of the 13-second hard spike should have completely excluded this burst from the short GRB category.”

In other words, instead of showing a much brighter supernova, as expected, the observation was consistent with a kilonova that is more typically associated with a short GRB.

“Such a peculiar GRB was the first of its kind ever detected,” said UNLV astrophysics professor Bing Zhang, co-corresponding-author of the Nature paper. “This discovery not only challenged our understanding of GRB origins, it also requires us to consider a new model for how some GRBs form.”

The research team believes that this unique GRB, known as GRB 211211A, likely formed through collision between a neutron star and a white dwarf, what’s known as a WD-NS merger.

White dwarfs are earth-sized objects that form from the death of low-mass stars – those with a mass smaller than that of about eight of our Suns. Neutron stars form when more massive stars, those with a mass of between about eight and 20 Suns, die off. When even larger stars die, they form black holes directly.

Massive, low-density stars make long-duration GRBs whereas high-density stars, including neutron stars, make short duration GRBs. According to UNLV’s Zhang, white dwarfs have intermediate densities, which make them ideal origins for the type of GRB discovered in 2021 as it displays an intermediately long duration without involving a massive star.

“Despite the relatively large number of GRBs observed each year, the unique signature of GRB 211211A pushed the envelope of our current categorial systems and required a new way of thinking,” said Zhang. “After careful review, the only merger scenario that made sense was that of a white dwarf and neutron star.”

UNLV doctoral student Shunke Ai and a student from Nanjing University collaborated to develop a detailed model to interpret the peculiar kilonova signature observed by GRB 211211A. Ai found that if a WD-NS merger leaves behind a rapidly spinning neutron star, known as a magnetar, the additional energy injection from the magnetar combined with the nuclear reaction energy from the material thrown during the burst can account for the kilonova emission observed for GRB 211211A. About the Study

The study, “A long-duration gamma-ray burst with a peculiar origin”, appeared Dec. 7 in the journal Nature. The paper includes 10 co-authors from 4 institutions, with UNLV and Nanjing University being the lead institutions. Published in the same issue are three parallel papers that report the detection of the kilonova. This paper focuses on the peculiar gamma-ray emission itself and proposes the WD-NS merger model to interpret the data.

 

(Image credit: Anyu Lei and Jing Chen, Nanjing University School of Arts)

International Team of Astronomers closer to an understanding of Fast Radio Bursts

Author: Tony Allen from UNLV News Center.

Nearly 15 years after the discovery of fast radio bursts (FRBs), the origin of the millisecond-long, deep-space cosmic explosions remains a mystery.

That may soon change, thanks to the work of an international team of scientists – including UNLV astrophysicist Bing Zhang – which tracked hundreds of the bursts from five different sources and found clues in FRB polarization patterns that may reveal their origin. The team’s findings were reported in the March 17 issue of the journal Science.

FRBs produce electromagnetic radio waves, which are essentially oscillations of electric and magnetic fields in space and time. The direction of the oscillating electric field is described as the direction of polarization. By analyzing the frequency of polarization in FRBs observed from various sources, scientists revealed similarities in repeating FRBs that point to a complex environment near the source of the bursts.

“This is a major step towards understanding the physical origin of FRBs,” said Zhang, a UNLV distinguished professor of astrophysics who coauthored the paper and contributed to the theoretical interpretation of the phenomena.

To make the connection between the bursts, an international research team, led by Yi Feng and Di Li of the National Astronomical Observatories of the Chinese Academy of Sciences, analyzed the polarization properties of five repeating FRB sources using the massive Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Robert C. Byrd Green Bank Telescope (GBT). Since FRBs were first discovered in 2007, astronomers worldwide have turned to powerful radio telescopes like FAST and GBT to trace the bursts and to look for clues on where they come from and how they’re produced.

Though still considered mysterious, the source of most FRBs is widely believed to be magnetars, incredibly dense, city-sized neutron stars that possess the strongest magnetic fields in the universe. They typically have nearly 100% polarization. Conversely, in many astrophysical sources that involve hot randomized plasmas, such as the Sun and other stars, the observed emission is unpolarized because the oscillating electric fields have random orientations.

That’s where the cosmic detective work kicks in.

In a study the team originally published last year in Nature, FAST detected 1,652 pulses from the active repeater FRB 121102. Even though the bursts from the source were discovered to be highly polarized with other telescopes using higher frequencies – consistent with magnetars – none of the bursts detected with FAST in its frequency band were polarized, despite FAST being the largest single-dish radio telescope in the world.

“We were very puzzled by the lack of polarization,” said Feng, first author on the newly released Science paper. “Later, when we systematically looked into other repeating FRBs with other telescopes in different frequency bands – particularly those higher than that of FAST, a unified picture emerged.”

According to Zhang, the unified picture is that every repeating FRB source is surrounded by a highly magnetized dense plasma. This plasma produces different rotation of the polarization angle as a function of frequency, and the received radio waves come from multiple paths due to scattering of the waves by the plasma.

When the team accounted for just a single adjustable parameter, Zhang says, the multiple observations revealed a systematic frequency evolution, namely depolarization toward lower frequencies.

“Such a simple explanation, with only one free parameter, could represent a major step toward a physical understanding of the origin of repeating FRBs,” he says.

Di Li, a corresponding author of the study, agrees that the analysis could represent a corner piece in completing the cosmic puzzle of FRBs. “For example, the extremely active FRBs could be a distinct population,” he says. “Alternatively, we’re starting to see the evolutionary trend in FRBs, with more active sources in more complex environments being younger explosions.”

The study, “Frequency-dependent polarization of repeating fast radio bursts—implications for their origin,” appeared March 17 in the journal Science. It includes 25 co-authors from 11 institutions and is part of long-running collaboration among institutions. In addition to UNLV and NAOC, collaborating institutions also include Yunnan University, Princeton University, Western Sidney University, Peking University and Green Bank Observatory, USA. 

(Image credit: Jingchuan Yu, Beijing Planetarium)