Colloquium: Multimessenger astronomy with gravitational waves and high-energy neutrinos

Many of the astrophysical sources and violent phenomena observed in our Universe are potential emitters of gravitational waves and high-energy cosmic radiation, including photons, hadrons, and presumably also neutrinos. Both gravitational waves (GW) and high-energy neutrinos (HEN) are cosmic messengers that may escape much denser media than photons. They travel unaffected over cosmological distances, carrying information from the inner regions of the astrophysical engines from which they are emitted (and from which photons and charged cosmic rays cannot reach us). For the same reasons, such messengers could also reveal new, hidden sources that have not been observed by conventional photon-based astronomy. Coincident observation of GWs and HENs may thus play a critical role in multimessenger astronomy. This is particularly true at the present time owing to the advent of a new generation of dedicated detectors: the neutrino telescopes IceCube at the South Pole and ANTARES in the Mediterranean Sea, as well as the GW interferometers Virgo in Italy and LIGO in the United States. Starting from 2007, several periods of concomitant data taking involving these detectors have been conducted. More joint data sets are expected with the next generation of advanced detectors that are to be operational by 2015, with other detectors, such as KAGRA in Japan, joining in the future. Combining information from these independent detectors can provide original ways of constraining the physical processes driving the sources and also help confirm the astrophysical origin of a GW or HEN signal in case of coincident observation. Given the complexity of the instruments, a successful joint analysis of this combined GW and HEN observational data set will be possible only if the expertise and knowledge of the data is shared between the two communities. This Colloquium aims at providing an overview of both theoretical and experimental state of the art and perspectives for GW and HEN multimessenger astronomy.

Figure 1

Left: Schematic depiction of the fireball mechanism, with the characteristic time and distance scales associated with the different phases. The prompt (burst) phase is due to internal shocks in the relativistically expanding fireball, producing strong gamma- and x-ray emission. The afterglow arises from the cooling fireball and its interaction with the surrounding medium; it is associated with x-ray, optical, and radio emission. From www.swift.ac.uk. Right: Current limits set by IceCube in its 40- and 59-string configurations (IC40 and $\mathrm{IC}40+59$) on the quasidiffuse GRB neutrino flux, together with predictions based on recent numerical calculations taking into account uncertainties on the astrophysical parameters and those due to the limited statistics of bursts. Also shown is the extrapolated limit that one can expect from 10 years of operation with the full IceCube detector (IC86). From 115.

Figure 2

Summary of the upper bounds on the duration of GRB emission processes taken into account in the total GW and HEN coincidence time window. (a) Active central engine before the relativistic jet has broken out of the stellar envelope [note that recent estimates of jet propagation within a stellar envelope suggest that this phase lasts typically 10–20 s; see 61, 62]; (b) active central engine with the relativistic jet broken out of the envelope; (c) delay between the onset of the precursor and the main burst; (d) duration corresponding to 90% of GeV photon emission; (e) time span of central engine activity. The top of the figure shows a schematic drawing of a plausible emission scenario. From 50.

Figure 3

Optical scheme of a gravitational-wave interferometric detector, consisting of two twin laser beams propagating in km-long arms oriented at 90° to each other. The Fabry-Pérot cavities enable the storage of the beams, thereby increasing by a significant factor their effective path length; the suspended, highly reflective mirrors play the role of test masses. A gravitational wave would cause a difference in the optical path lengths in each arm, which can be inferred by measuring the interference pattern at the photodiode. Adapted from Y. Aso (GECo, Columbia University).

Figure 4

Detector noise spectra from LIGO and Virgo associated with the two joint science runs, showing the increase of sensitivity of the detectors. Left panel: The best sensitivity associated with S5/VSR1 data set (2007), along with that of GEO (2008). From 6. Right panel: Typical sensitivities for LIGO and Virgo associated with S6/VSR2-3 (2009–2010) data sets. From 12.

Figure 5

Time chart of the data-taking periods for the ANTARES (and KM3NeT), IceCube, LIGO, and Virgo experiments, indicating the respective (achieved or planned) upgrades of the detectors. The IceCube detector is now complete and will be operating for at least another 5 years, with possible upgrades in the meantime. The deployment of the KM3NeT neutrino telescope, which will take place in parallel with the operation of ANTARES, is expected to last 3–4 years, possibly starting in 2014. The detector will be taking data with an increasing number of photodetectors before reaching its final configuration. A larger-scale upgrade to the next generation of GW interferometers (Advanced LIGO and Virgo) is ongoing and data taking should start again around 2015.

Figure 6

Left: Schematic view of the ANTARES detector, with an illustration of its detection principle: an upgoing ${\nu }_{\mu }$ interacting with matter in the seabed will produce a muon that can cross the detector, emitting a cone of Cherenkov light that will be detected by the array of photosensors. ANTARES consists of 12 instrumented lines anchored to the seabed at a depth of 2475 m, about 40 km off the coast of Toulon (France). The lines are 450 m long and kept up by a buoy; each of them supports 25 detection storeys separated by 14.5 m and supporting triplets of optical modules looking 45° downward. The interline separation is about 70 m. Adapted from http://antares.in2p3.fr. Right: Schematic view of the IceCube detector at the South Pole. The strings are deployed from the surface and instrumented between 1450 and 2450 m; every string supports 60 optical modules enclosing one downward-looking PMT each. The interline separation is about 125 m. The sketch also shows the position of the prototype detector AMANDA, of the low-energy infill DeepCore, and of the surface array IceTop. From http://icecube.wisc.edu.

Figure 7

Current experimental sensitivities to differential cosmic neutrino flux $E^{-2}dN/dE$ (dotted lines) and limits (points) as a function of declination for time-integrated searches of pointlike sources. The results of AMANDA-II (14) and its successor IceCube, in its 40-string configuration (18), as well as those of ANTARES; also displayed are the limits from the MACRO (41) and Super-Kamiokande (211) experiments, which are not principally devoted to neutrino astronomy. From 31.

Figure 8

Schematic flow diagram of a HEN-triggered search for GWs. Each neutrino candidate (with its time and directional information) provided by a HEN telescope acts as an external trigger for the X-pipeline, which searches the combined GW data flow from all active interferometers (ITFs) for a possible concomitant signal. The size of the spatial search window is related to the angular accuracy associated with each HEN candidate. The background estimation and the optimization of the selection strategy are performed using time-shifted data from the off-source region in order to avoid contamination by a potential GW signal. Once the search parameters are tuned, the box is opened and the analysis is applied to the on-source data set.

Figure 9

Schematic flow diagram of a joint GW and HEN search pipeline. The inputs of the pipeline are, besides data from HEN and GW detectors, the astrophysical source distribution from a galaxy catalog, as well as the coincidence time window used for the search. Spatially and temporally coincident neutrinos can be clustered, potentially increasing the significance and decreasing the false alarm rate of a coincident GW and HEN signal. Combining this information in a joint test statistic, one can evaluate the results to look for individual or statistical detection of signal candidates. Upon nondetection, the results can be used to determine an upper limit on the source population.

Figure 10

Expected upper limits for a GW and HEN source population as inferred from a joint GW and HEN search with 1 yr of coincident observation time, respectively, with IceCube IC22 and initial Virgo and LIGO detectors (left panel), and with full IceCube IC86 and Advanced Virgo and LIGO detectors (right panel). The results take into account the blue-luminosity-weighted galaxy distribution. The benchmark source parameters are $E_{\mathrm{iso}}^{\mathrm{GW}}$ (total energy emitted in GW) and $n_{\mathrm{HEN}}$ (number of detected neutrinos from a source at 10 Mpc). The population rate upper limit (color scale) is expressed in logarithmic units of the number of sources per Milky Way-equivalent galaxy per year. The horizontal lines show predictions for $n_{\mathrm{HEN}}$ from the emission models of 220 and 44. From 51.