Fast Radio Bursts: First Distance Measurement Options

[Mongo]

Fast Radio Bursts: First Distance Measurement

Have we finally traced a Fast Radio Burst to its place of origin? News from the CSIRO (Commonwealth Scientific and Industrial Research Organisation) radio telescopes in eastern Australia, along with confirming data from the Japanese Subaru instrument in Hawaii, suggests the answer is yes. Fast Radio Bursts (FRBs) are transient radio pulses that last scant milliseconds. In that amount of time, they have been known to emit as much energy as the Sun emits in 10,000 years. And exactly what causes FRBs is still a mystery.

Take the so-called ‘Lorimer Burst’ ( FRB 010724) which was discovered in archival data from 2001 at the Parkes radio telescope in New South Wales. Here we’re dealing with a 30-jansky dispersed burst that was less than 5 milliseconds in duration. Although the burst appeared roughly in the direction of the Small Magellanic Cloud, the FRB is not thought to be associated with our galaxy at all. A 2015 event, FRB 110523, was discovered in data from the Green Bank dish in West Virginia, with an origin thought to be as much as six billion light years away.

Sixteen FRBs have been detected, but according to this news release from the Max Planck Institute for Radio Astronomy, researchers believe they are a much more common phenomenon. A new paper in Nature now tells us that the Parkes telescope detected a new Fast Radio Burst (FRB 150418)) on April 18, 2015. But in this case, we have a twist. Evan Keane (Square Kilometre Array Organisation, Jodrell Bank UK) and an international team have been developing a system to detect FRBs within seconds, which makes it possible to alert other telescopes to pinpoint the source.

The system, part of the Survey for Pulsars and Extragalactic Radio Bursts (SUPERB), allows quick detection and transmission of the relevant information to other telescopes. Using it, within two hours of the Parkes detection the Australia Telescope Compact Array (ATCA) 400 kilometers to the north was able to trace the FRB to a radio source that lasted six days before finally fading out. What the Subaru telescope adds to the mix was that it found a galaxy at optical wavelengths that matches up with the radio source detected by the Compact Array. Unlike the previous sixteen detected bursts, we now have a far more precise fix on a location.

The culprit turns out to be an elliptical galaxy with a redshift consistent with a distance of six billion light years. This marks the first time a host galaxy has been determined for an FRB, and the first time an FRB’s distance has been measured. Simon Johnston (Head of Astrophysics at CSIRO) notes that the galaxy in question is well past its prime period for star formation, an unexpected result. Johnston speculates that the cause of the FRB may be two neutron stars colliding rather than any process associated with the birth of young stars.

I wonder if LIGO would have been able to detect this event, if it were operating at full sensitivity back then?

[Gerald]

The LIGO observation was estimated to be about 1.3e9 light years away. This one is estimated at 6e9 light years.
By an equally strong source, the signal would be about 1/20 by an inverse square rule. Assuming the observed LIGO signal been 3-fold as intense as currently detectable, and an improvement of another factor of 3, this presumed merger of two neutron stars would probably be below the detection limit of LIGO at full sensitivity.
With 4 active instances LIGO might just achieve detectability, under the above assumptions. But neutron stars are considerably lighter than the presumably observed black hole merger.

[fredk]

The sensitivity of a detector like LIGO falls only with the inverse of the distance, not the inverse squared. This is because they measure the strain amplitude, not the power in the wave. This means they could reach farther much better than you'd expect via the inverse square law.

However, the source LIGO detected was a pair of ~30 solar mass black holes merging. Neutron stars are always less than a couple of solar masses. Also, neutron stars are less dense than black holes. So the expected amplitude for the same source distance would be considerably smaller for a neutron star merger than for a 30 solar mass BH merger. Also the frequency would be higher for NS merger, whereas the LIGO BH event frequency was right near the detector noise minimum.

So I think corrent LIGO would not be able to see such an event. But in the future hopefully...