The Detection of Radio Wavelength Transients from Astrophysical Sources Can Provide External Triggers for Gravitational Wave (GW) Searches Within LIGO/Virgo Data

Home , Joseph Polchinski

The detection of radio wavelength transients from astrophysical sources can provide external triggers for gravitational wave (GW) searches within LIGO/Virgo data. There are a variety of sources of GWs that should also produce a radio transient, such as compact object inspirals and mergers [1], core-collapse supernovae [2], and the cusps or kinks of superconducting cosmic strings [3]. Indeed, it has been argued that a recently identified radio transient [4] was produced by the cusp of a superconducting cosmic string [5]. Radio polarization and spectral information can help distinguish among candidate sources. The inspiral of a NS-NS binary, for example, will produce a GW chirp until the binary merger occurs at which point a coalescence waveform is emitted. A radio transient is also produced during the merger as increased amounts of plasma flow along the magnetic field lines of the stellar magnetospheres [1]. A core-collapse supernova (SN), if not spherically symmetric, would produce a GW signal. In addition, the SN would produce an expanding shell of charged particles which interact with the surrounding magnetic field of the progenitor star producing a transient radio pulse [2]. The cusp or kink of a superconducting cosmic string is expected to produce GWs and electromagnetic radiation. These emissions would be observed as transient events. Such bursts could be detected by LIGO/Virgo [3]. Both the GW and the electromagnetic emissions are beamed along the direction of motion of the cusp, leading to an enhancement in the observed signal if the Earth lies within the beam. The electromagnetic luminosity may be substantially larger than the gravitational luminosity [6]. It has been argued that searches in the radio spectrum would be optimal for detections of these bursts [5]. Such cosmic strings would be part of networks created during phase transitions in the early universe. These networks could even be composed of cosmic superstrings [7]. Any results provided by LIGO/Virgo and radio observations would be valuable in constraining or confirming cosmological models. A number of factors make it effective to search for electromagnetic transients in the long-wavelength regime of the radio spectrum. The emission processes discussed above involve relativistic electrons in magnetic fields, producing a power-law spectrum with greater intensity for longer wavelengths. Transient events are by nature unpredictable, and thus all-sky searches are important, but large single-dish radio telescopes, while having large collecting area, can only sample a small solid angle of sky. However, the collecting area of simple dipole antennas is proportional to the square of the wavelength, and with proper antenna design the entire sky above the horizon is effectively observed. Thus long-wavelength radio searches can be sensitive and encompass the entire sky. Additional collecting area is gained by utilizing an array of antennas, which also provides for some directional source information. The Eight-meter-wavelength Transient Array (ETA) is one instrument that meets these criteria [8][9]; an ongoing upgrade will provide two widely separated arrays of ten crossed-dipole antennas, to allow for removal of signals due to localized radio interference (in much the same way that LIGO utilizes two widely separated interferometers). As an external trigger for LIGO/Virgo, the observation of a radio transient should provide a temporal search window for any GW signal. For long-

1 wavelength radio transients, the radio pulse will be delayed in arrival compared to the GW signal, for essentially any source model. The GW signal travels at the speed of light, but the radio pulse is dispersed and delayed by the interstellar plasma. Spectral observations, as obtained by ETA for example, yield the precise dispersion measure (DM) for a pulse, where the dispersion measure R is the line of sight integral of the free electron density, DM = nedl. So the radio pulse arrival delay is determined. For a source within the plane of the Galaxy at a typical distance of 1 kpc, DM ≈ 50 pc cm−3, thus the radio pulse is delayed by ≈150 seconds at a wavelength of 8m. The delay is proportional to DM. A transient of extragalactic origin could be delayed further (the radio transient suggested to be produced by a cosmic string cusp, as discussed above, is thought to be of extragalactic origin, and had DM ≈ 375 pc cm−3). An instantaneous pulse emitted at the source will also be temporally broadened by dispersion, but with narrow-bandwidth spectral channels the broadening in the ﬁnal analyzed pulse is determined by the channel width, and is 0.1 second at 8m using ETA, for a DM ≈ 50 pc cm−3. Additional pulse broadening due to interstellar scattering (with no associated delay), and not removable in the data processing, is of order a few seconds at 8m, for DM ≈ 50 pc cm−3. This scatter-broadening is dependent on the detailed structure and geometry of the line-of-sight irregularities in the plasma.

References

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