The little dead star that blinded us earlier this year continues to signal on radio frequencies.
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.
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 discovered something else – pulsed radio emission corresponding to the rotation period of the magnetar.
“It’s very interesting to see SGR 1935 + 2154 again, and I’m optimistic that taking a closer look at these bursts will help us better understand the potential link between magnetars and fast radio bursts,” says astronomer Deborah Goode of the University of Britain Colombia in Canada and a member of CHIME / FRB.
The detections reported on Astronomer's Telegram are currently being analyzed.
Until April of this year, fast radio bursts were recorded only from outside the galaxy, usually from sources at a distance of millions of light years from us. The first was discovered in 2007, and since then, astronomers have been trying to figure out what causes them.
As the name suggests, fast radio bursts 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 appear to flare up once and do not repeat themselves, they are highly unpredictable. In addition, the ones we detect usually come so far away that our telescopes cannot distinguish individual stars. Both of these characteristics make it difficult to track the bursts, either to the exact source galaxy or to a known cause.
But SGR 1935 + 2154 is only 30,000 light years away. On April 28, 2020, it emitted a powerful millisecond pulse, which has since been called FRB 200428.
After 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.
We don't know much about the three new bursts so far. As scientists are still working on the data, it is possible that some of the initial findings may change. But now we can say that they are similar and not similar to FRB 200428.
They've gotten a little weaker, but they are all still incredibly strong, and they all lasted only milliseconds.
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.
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 one more thing to this: an amazingly 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 that of a normal neutron star. And we still don't fully understand how they came about.
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 produces a constant voltage that sometimes causes giant starquakes and magnetic flares.
SGR 1935 + 2154 undergoes such activity, which indicates a connection between magnetar flares and some radio bursts.
Sources: Photo: Sophia Dagnello, NRAO / AUI / NSF
