## Newly Discovered Fast Radio Bursts May be Colliding Neutron Stars

by Shannon HallThe Universe is sizzling with undiscovered phenomena. Only last month astronomers heard four unexpected bumps in the night. These *Fast Radio Bursts* released torrents of energy, each occurred only once, and lasted a few thousandths of a second. Their origin has since mystified astronomers.

Dismissing my first guess, which includes a feverish Jodie Foster verifying the existence of extraterrestrial life, astronomers have found a more likely answer. Two neutron stars collide, but before doing so produce a quick burst of radio emission, which we later observe as a Fast Radio Burst.

Our first hint? These Fast Radio Bursts are extra-galactic in origin. The exact distance is quantifiable from a “dispersion measure – the frequency dependent time delay of the radio signal,” Dr. Tomonori Totani, lead author on the paper, told Universe Today. “This is proportional to the number of electrons along the line of sight.”

For all bursts, the short-wavelength component arrived at the telescope a fraction of a second before the longer wavelengths. This is due to an effect known as interstellar dispersion: through any medium, longer-wavelength light moves slightly slower than short-wavelength light.

Light from extra-galactic objects will have to travel through intergalactic space, which is teeming with electrons in clouds of cold plasma. The farther the light travels, the more electrons it will have to travel through, and the greater the time delay between arriving wavelength components. By the time light reaches the Earth, it has been dispersed, and the amount of dispersion is directly correlated with distance.

These Fast Radio Bursts are likely to have originated anywhere from 5 to 10 billion light years away.

While the exact source of these Fast Radio Bursts has been highly debated, a recent hypothesis concludes that they are the result of merging neutron stars in the distant Universe.

In the final milliseconds before merging, the rotation periods of the two neutron stars synchronize – they become tidally locked to one another as the Moon is tidally locked to the Earth. At this point their magnetic fields also synchronize. Energetic charged particles spiral along the strong magnetic field lines and emit a beam of radio synchrotron emission.

Known neutron star magnetic field strengths are consistent with the radio flux observed in these Fast Radio Bursts. The emission then ceases in a few milliseconds when the two neutron stars have collided, which explains the short duration of these Fast Radio Bursts.

Not only does this mechanism describe both the high energy and the time duration of these bursts, but they’re inferred occurrence rate as well. It’s likely that 100,000 Fast Radio Bursts occur each day. This matches the likely neutron star merger rate.

Merging neutrons stars will also create gravitational waves – ripples in the curvature of spacetime that propagate away from the event. Dr. Totani emphasized that the next step will be to perform a correlated search of gravitational waves and Fast Radio Bursts. Such a fast rate estimate is certainly good news for scientists hoping to detect gravitational waves in the nearby future.

The Universe is bursting with energy – literally – every 10 seconds, and until recently we simply had no idea. This recently discovered phenomenon is likely to be the center of a new active area of research. And I have no doubt that it will lead to exciting discoveries that just may break trends and burst into new territories.

The discovery paper may be found here, while the paper analyzing neutron stars as a likely source may be found here.

Neutron Stars, Radio Astronomy

Getting a gravity wave signature will be rather difficult. The collision of two neutron stars in effect converts their mutual potential energy into radiation and gravity waves. We can compare this to the case for two merging black holes. In that case the production of gravitational radiation will be around 10 times or less that for an equivalent black hole merger.

If you have two colliding black holes of mass M their horizon lengths by the Schwarzschild formula is R = 2GM/c^2, which we write as R = 2m. The area of the event horizons is A = 4?R^2 = 8?m^2, and for two black holes the area of the two horizons is 8?m^2. If the two black holes merge the new black hole has mass M’ or m’ = GM/c^2 and horizon area A’ = 8?m’^2. The resulting black hole is such that A’ = 2A and we compute the mass of the merged black hole m’ = sqrt{2})m, and so there is energy E = (2 – sqrt{2})m “missing” which is in the form of gravitational radiation. The minimal mass for a black hole formed by the implosion of a star is about 3.3 times the mass of the sun, so about 2 times the mass of the sun is emitted as gravitational radiation.

An estimate for the amount of gravitational radiation emitted is that the difference in potential energy for neutron stars from their surface to “infinity” is ?? = GM/rc^2, and the radius of a neutron star is about 10 times that of the equivalent Schwarzschild radius, and so the potential energy released is about 1/10 that of a black hole merger. Given this and that neutron stars are around 2 solar masses we can estimate the gravitational energy released is E < .2 solar masses.

If we assume this merger happens 1 billion light years away then we can estimate how much of that energy reaches us. At one billion light years this gravitational radiation is emitted and spread out over an area of 4? billion light years squared. The area of a gravitation wave antenna is around 1km^2 or 10^{-12}light years squared. As a result about 8×10^{-23} of that radiation passes through a gravity wave detector. If we assume .01 solar masses are emitted as gravity wave in the merger, or about 2×10^{29} kg the around 1.6×10^6kg equivalent of energy or about 1.44×10^{23}j of energy passes through the detector.

That is a lot of energy! However, the Einstein field equation tells us that curvature information G_{ab} is equal to the energy stuff T_{ab} multiplied by a factor 8?G/c^4 = 2.07×10^{-43}(j-m^2)^{-1}. This means there is a curvature change of about 2.9×10^{-20}m^{-2}, or that the interferometer arm will shift by around 10^{-9}cm. This is less than the diameter of an atom. That small coupling constant means that very little of that energy is detectable.

LC

My arithmetic says there are 60 x 60 x 24 = 86,400 seconds in a day so 100,000 fast radio bursts per day is more like one a second. Just saying.

There are 3.15×10^7 seconds in a year.

LC