It’s a difficult and winding journey to get to the Sardinia Radio Telescope (SRT). An hour or so from the tough Italian coastal city of Cagliari, its rugged mountainous location lies within easy reach of a state-of-the-art 64-meter radio telescope that has been constantly in the news during its decade of operation.
A stone’s throw from pastures for sheep and cows, its location was chosen largely because it was somewhat sheltered from the wind, fairly quiet, and situated on a plateau rising about 700 meters above the Mediterranean.
The fully steerable SRT’s biggest claim to fame to date has been the first detection of the lowest frequency fast radio burst (FRB) yet found. Fast radio bursts are intense, millisecond pulses of radio waves located at cosmic distances. In the case of the SRT discovery, FRB 180916 was found at 328 MHz in a star-forming region within a massive spiral galaxy located about 500 million light-years away in the northern constellation Cassiopeia.
However, the €60 million SRT does not undergo a technical update until at least September. This is because in addition to general radio astronomy observations, the SRT operates as part of the European Space Agency’s (ESA) Deep Space Network.
But when turned on, the SRT routinely monitors between 300MHz to 116GHz, which is roughly equivalent to mainstream TV at the lower end of today’s car radar in advanced cars. More than a thousand separate aluminum plates keep the telescope surface in its parabolic alignment as each is supported by electromechanical actuators.
Sergio Poppi, head of SRT operations in the telescope’s control room. Credit: Bruce Dorminy
As for the origin of these FRBs?
They are initially characterized by those that appear to be one-time bursts and those like FRB 180916 that recur, sometimes at specific intervals.
A one-time explosion can be caused by a one-time explosion, such as the merger of two neutron stars.
Repeaters cannot be caused by a one-time explosion, Maura Bellia, a radio astronomer at the Cagliari Astronomical Observatory in Sardinia, told me via email. In the case of recurring explosions, the currently favored explanation is that magnetars produce them, Belyea says.
Magnetars are the “strongest” neutron stars: with extremely intense magnetic fields that appear to be powered by enormous losses of magnetic energy. Our Milky Way Galaxy is full of magnetars, however, all fast radio bursts detected so far appear to be extragalactic.
One thing is certain. The source of these fast radio bursts must be an astrophysically compact object. This narrows the list of sources down to black holes, neutron stars, or white dwarfs.
White dwarfs are the least active of the three, so we tend to rule them out, and black holes don’t typically have this kind of short-range emission, Belyea says.
This leaves some subclass of neutron stars as a possible explanation.
But a typical burst from FRB 180916 is emitted in milliseconds.
This means that a lot of energy is being released in a very short time and that the motor behind it is very powerful, because it keeps losing that kind of energy and providing more, Belyea says.
Image of FRB 180916’s host galaxy (center) obtained using NOIRLab’s 8-meter Gemini-North telescope on Maunakea, Hawaii. Source: Gemini Observatory/NOIRLab/NSF/AURA
In contrast, our Sun’s brightest solar flares are emitted over a period of minutes to hours. So, fast radio bursts are inherently the product of some kind of extreme astrophysics.
As for the source of FRB 180916?
SRT observations showed that FRB 180916 could not be surrounded by a thick nebula; That would block low-frequency radiation, Belyea says. So, this fast radio burst is thought to be not among the youngest, but perhaps middle-aged, although still very active, she says.
Other transient phenomena
The SRT team is also tracking other transient phenomena that typically glow at other wavelengths (mainly X-rays) such as magnetars, microquasars, and gamma-ray bursts.
There could be a connection between how all these systems work, Belyea says. “We also regularly monitor pulsars and try to study their individual and giant pulsations, and the activity of the magnetosphere of pulsars may be a miniature version of what happens in fast radio bursts,” she says.
SRT monitoring feature
Using the SRT, Belya says she and her colleagues can make simultaneous observations at 300 MHz and 1.5 GHz, thanks to the telescope’s dual-beam receiver. It says that fast radio bursts have been observed within the radio range from 110 MHz to 8 GHz.
What’s most puzzling about these FRBS?
“We still don’t know whether it represents one phenomenon or two, or even more than that,” says Belya. She wonders whether those that are not repeated come from explosions that leave nothing behind?
Could they be a type of binary neutron star?
This is the hardest question of all; Suppose the surfaces of magnetars can be somewhat turbulent due to extremely high magnetic fields, says Belya. So, we might expect moments when something “breaks” and leads to explosive phenomena, she says.
Think “starquakes”
When enormous forces are trapped and then released, an explosion will occur, says Belia. But as with earthquakes, this is unpredictable, she says.
SRT and its control and operations headquarters. Credit: Bruce Dorminy
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