Magnetar SGR 1935 + 2154, which emitted the first known rapid radio burst from inside the Milky Way in April, flared again, giving astronomers another chance to unravel the cosmic mystery.
The little dead star that sent the signal earlier this year did it again.
On October 8, 2020, the CHIME / FRB collaboration discovered SGR 1935 + 2154 emitting three millisecond radio bursts in three seconds. Following the CHIME / FRB detection, the FAST radio telescope detected something else – pulsed radio emission corresponding to the rotation period of the magnetar.
It is very interesting to see SGR 1935 + 2154 again, and I am optimistic that if we study these bursts more closely, it will help us better understand the potential relationship between magnetars and fast radio bursts, “says astronomer Deborah Goode of the University of Britain Colombia in Canada and a member of CHIME / FRB.
Until April of this year, fast radio bursts (FRBs) were ever recorded only from outside the galaxy, usually from sources millions of light years away. The first was discovered in 2007, and since then astronomers have been trying to figure out what causes them.
As the name suggests, FRBs are bursts of extremely powerful radio waves found in the sky, some of which release more energy than hundreds of millions of suns. They only last a millisecond.
Since most sources of fast radio bursts seem to flare up once and no repetition is detected, they are highly unpredictable. In addition, the ones we detect usually come so far that our telescopes cannot distinguish individual stars. Both of these characteristics make it difficult to track the FRB to either the exact source galaxy or a known cause.
But SGR 1935 + 2154 is only 30,000 light-years away. On April 28, 2020, it spat out a massive millisecond pulse that has since been dubbed FRB 200428 under the fast radio transmission naming convention.
Once the signal strength was adjusted for distance, FRB 200428 was not as powerful as the extragalactic fast radio bursts, but everything else was in line with the profile.
“If the same signal came from a nearby galaxy, such as one of the closest typical FRB galaxies, it would look like an FRB to us,” said astronomer Srinivas Kulkarni of the California Institute of Technology. “We’ve never seen anything like it before.”
We don’t know much about the three new bursts yet. Since scientists are still working on the data, it is possible that some of the early findings could change, Goode said. But now we can say that they are both similar and not similar to FRB 200428.
They are a little less powerful again, but they are all still incredibly strong, and they all lasted only milliseconds.
“Although less bright than those detected earlier this year, they are still very bright flares that we would see if they were extragalactic,” Goode added.
“One of the more interesting aspects of this discovery is that our three bursts appear to have occurred during the same rotation period. The magnetar is known to rotate every ~ 3.24 seconds, but our first and second bursts were separated by 0.954 seconds, and the second and third were separated by 1.949 seconds. This is a bit unusual, and I think we will look at it later. “
This could reveal something new and useful about the behavior of magnetars, because – let’s face it – they’re pretty weird.
Magnetars, of which only 24 have been confirmed to date, are neutron stars; it is the collapsed core of a dead star, not massive enough to turn into a black hole. Neutron stars are small and dense, about 20 kilometers in diameter, with a maximum mass of about two Suns. But magnetars add something else to this: a stunningly powerful magnetic field.
These stunning fields are about a quadrillion times more powerful than Earth’s magnetic field and a thousand times more powerful than a normal neutron star. And we still do not fully understand how they came to this.
But we know that magnetars have periods of activity. As gravity tries to hold the star together – an internal force – the magnetic field pulling outward is so powerful that it distorts the star’s shape. This results in a constant voltage that sometimes causes giant starquakes and giant magnetic flares. SGR 1935 + 2154 is undergoing such activity, which suggests a link between magnetar attacks and at least some FRBs.
Obviously, astronomers have found that the source of the first intragalactic FRBs is of great interest. When CHIME / FRB reported their discovery, other astronomers decided to look at the star, including a team led by Zhu Weiwei of the National Astronomical Observatory of China, which had access to FAST, the largest single-aperture radio telescope in the world.
And they discovered something interesting, which was also reported on the astronomer’s Telegram – pulsed radio emission. These radio pulses were nowhere near as strong as the bursts, but they are extremely rare: if confirmed, SGR 1935 + 2154 will only be the sixth pulsed radio frequency magnetar. And the pulse period turned out to be equal to 3.24781 seconds – almost exactly the rotation period of the star.
This is curious, because until now astronomers have not been able to find a connection between magnetars and radio pulsars. Pulsars are another type of neutron star; they have a more normal magnetic field, but they pulsate with radio waves as they spin, and astronomers have long tried to figure out how the two types of stars are related.
Earlier this year, Australian astronomers identified a magnetar that behaved like a radio pulsar – a possible “missing link” between the two and evidence that at least some magnetars could evolve into pulsars. SGR 1935 + 2154 might be another piece of the puzzle.
“Based on these results and the increasing burst activity, we hypothesize that the magnetar may be in the process of transforming into an active radio pulsar,” Weiwei’s team wrote.
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