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Multi-messenger astrophysics, from gravitational waves to the high-energy universe

by Yannis Karyotakis - 21 March 2012

Just like radio-astronomy has done in the 20th century and gamma-ray astronomy is currently achieving, the detection of gravitational waves (GW) will open a new way to observe the Universe. GW will shed light on poorly understood, powerful astrophysical events, directly probing the masses and mass distributions involved in mergers and explosions: in fact they would provide spectacular demonstrations of conversion of rest mass into propagating energy at macroscopic scales. Virgo to which LAPP members have made important contributions is one of the large scale interferometric GW detectors, operating within the worldwide GW detector network together with the partner LIGO instruments. Over the recent years, these detectors have taken science data at the sensitivities provided by their initial and enhanced configurations. They are currently being upgraded to their advanced configurations and are expected to come back online in 2015. The targeted sensitivity should allow not only to make the first detections, but to truly enter the era of GW astronomy with routine detections. One of the accomplishments of the initial Virgo and LIGO was to start exploring multi-messenger approaches, through searches of GW associated with electromagnetic events such as gamma ray bursts, and partnerships with X-ray, optical and radio telescopes to follow-up on interesting GW candidates detected by low-latency searches. LAPP has been actively involved in both of these aspects.

The goal of the project is to boost the achievement of the science programme associated with Advanced Virgo (AdV), through dedicated commissioning work to speed up the progress toward AdV’s nominal sensitivity, and by exploring in depth the path of multi-messenger astronomy (jointly with instruments like LSST, HESS/CTA, AMS, or neutrino observatories). Additionally, the similarities between the optical benches involved in Virgo and LSST also make it worth considering joint technical developments. More generally, the ENIGMASS Labex provides the opportunity to further study how to best combine the information from GW and other messengers. For example, nearby, powerful galactic accelerators, e.g. pulsar and pulsar wind nebulae, are sources of non-thermal emission and represent the majority of the TeV gamma-ray galactic sources discovered in the last years. They may prove suitable to a multi-messenger campaign, including cosmic-ray leptons and GW. Such an approach will require a population study of galactic sources with present (e.g. Fermi) and next-generation observatory (e.g. CTA). The detection/discovery of tens of quiet radio pulsars by means of VHE gamma-ray observations will provide potential unknown and close-by galactic sources of continuous gravitational waves. Developing and applying new methods dedicated to a stack analysis of these pulsars and by means of these two messengers (gammas and GW) is of paramount importance.

The multi-messenger approach is not limited to local, astrophysical sources, but can be extended to cosmologically interesting objects. One example is provided by first stars (so called Population III) whose birth put an end to the so-called dark ages. They are expected to be massive stars, ending their life in violent explosions, acting as powerful engines of re-ionisation. At LAPTh, a new model for GRBs has been proposed. These GRBs are the result of the death of very massive stars disrupted by pair instability processes. One prediction of the model is the increase in the number of GRBs with redshift, ultimately related to the presence of Population III stars. Again, a multimessenger strategy is crucial to shed new light on this early epoch, for example attacking the Population III problem via direct observations (LSST), via infrared background fluctuations inferred from CMB surveys (PLANCK), or constraints on the magnitude and z-dependence of the extra-galactic background light via the absorption spectra features manifested by astrophysical sources (namely Active Galactic Nuclei and GRBs) probed by TeV gamma-ray observatories (HESS/CTA). Additionally, refining the understanding of these phenomena may improve our sensitivity to effects of new physics: one example is the sensitivity to DM annihilation cross section via its impact on re-ionisation probed in CMB. Yet another example of very promising information which is suitable to a combined analysis within the Labex perimeter is the reconstruction of the CMB lensing map from the PLANCK satellite data: this cosmological signal sees a strong implication of LPSC, via the development of suitable analysis methods.