F3: The transient catastrophic Universe
The capability to study transient phenomena on time scales from milliseconds to years has recently increased and shown large discovery potential. Yet, the time domain of the sky has been sparsely explored. Fermi and Swift are now showing the way at high energies, the former by mapping the whole sky every 3 hours, the latter by its rapid follow-up across many wavelengths. With the construction of wide field imagers dedicated to sky surveys on a daily timescale in the radio (Lofar, ASKAP, MeerKAT), optical (Pan-STARRS, LSST), infrared (Akari), and X-ray (eRosita/SRG), and with the ongoing surveillance in gamma rays from Fermi, there are new motivations to explore transients. Discovering new classes of fast transients is one, as it could represent new exotic or explosive events out to cosmological distances, such as merging neutron stars or evaporating black holes.
Over the next decade, a combination of increased sensitivity, larger field of view, and algorithmic developments will open up the time domain to a wide range of astronomical fields, from stellar flares and supernovae to neutron-star and black-hole births, quakes and instabilities. Near-coincidence searches with high-energy neutrino and gravitational wave signals will be actively pursued across the world. The dynamic and bursting events represent the new terra incognita.
Radio astronomy is leading this effort. Transient science is identified as a key goal for LOFAR (Europe), ASKAP (Autralia) and MeerKAT (South Africa) that are the precursor instruments of the major international SKA facility to be developed for the 2020+. The synchrotron radio emission probes with unequalled angular resolution the cooling in the ambient magnetic field of the relativistic particles ejected by compact accretors or explosions. It also constrains the kinetic feedback of such events on their environment.
Algorithmic improvements for transient detection would yield improved use of the different combinations of fields-of-view and unprecedented sensitivities offered by these instruments. The vast storage and computational requirements of transient searches, in particular for imaging interferometers, also requires the development of near real-time detection pipelines. The LOFAR project is engaged in the development of first-generation pipelines. The characterization of transients and their identification at other wavelengths represents another challenge.
In this context, the frontier project FP3 targets innovative algorithmic improvement of radio software (Compressed Sensing framework and sparse representations, Direction-Dependent Effects,…) along the scientific exploitation of the forthcoming large flow of data originating from this new generation of radio telescopes.
POSITION NAME SURNAME LABORATORY NAME GRADE, EMPLOYER WP leader Corbel Stéphane AIM Professor, University Paris Diderot WP co-leader Grenier Isabelle AIM Professor, University Paris Diderot WP co-leader Starck Jean-Luc AIM Researcher/CEA WP member Girard Julien AIM Ass. Prof., University Paris Diderot WP member Migliori Giulia AIM Post-doc, LabEx UnivEarthS (Dec 14-Dec 17) WP member Loh Alan LESIA Post-doc at LESIA/OP (former AIM PhD) WP member Jiang Ming AIM PhD Student (Start Oct 14) WP member Evangelia Tremou AIM New post-doc (Start Nov 17) WP member Floriane Cangemi AIM New PhD Student (Start Oct 17) WP member Pires Sandrine AIM Researcher/CEA WP member Bobin Jérôme AIM Researcher/CEA WP member Rodriguez Jérôme AIM Researcher/CEA
In collaboration with Cyril Tasse (GEPI, Observatoire de Paris) and the LOFAR (TKP), MeerKAT (ThunderKAT) and ASKAP(VAST) collobarations
New and large radio telescopes, that are currently developped and built, will observe the whole radio spectrum with an improved spectral, angular and especially an unprecedented time resolution. They represent a huge technical improvement over the past technologies and a data processing challenge for users that would require comparable performances, similar to what is developped for “big data” problems. At an early stage, the Frontier Project FP3 carried out the development of tools to produce robust 2D radio maps and extended the scope of application to produce two supplementary imagers working on 3D data: one dedicated to transient detection and imaging, the other on hyperspectral radio imaging. On its final stage, FP3’s main aim is to apply these new tools to a wide scope of applications in radio interferometry to generate scientific products and study the associated physical processes (either in time or in frequency).
In the current 2010-2020 era, the scientific teams that exploit the large datasets generated by continental-scaled instrument (such as LOFAR, MeerKAT, the South African precursors or SKA1-MID) have to face a huge radio data calibration and imaging challenge. On the imaging/deconvolution side, we have demonstrated with the past products generated by the FP3, that the compressed sensing framework could bring better image reconstruction with low residuals [Garsden et al., 2015; Girard et al., 2015], while taking into account some of the Direction Dependent Effects through W-Projection and A-Projections.
After this large effort on the algorithmic side of the project, we also continued pursuing the scientific exploitation of various radio and high energy facilities in order to probe the transient emission from a wide range of astrophysical objects (galactic or extragalactic). The physics of relativistic jets represents a main axis of our research (being frequently associated with transient radio emission). Adopting an observational approach, we aim at understanding how in stellar and supermassive black holes jets are formed, evolve and interact with their environment.
The past and current theoretical and numerical groundwork carried on 2D, 2D-1D and hyperspectral deconvolution, benefited from the support of the UnivEarthS Labex funding allocated to Human Ressource and to participation to specialized workshops (Jiang, Girard), conferences (Girard, Jiang, Starck, Corbel) on the topics of Compressed Sensing, Radio interferometry Imaging & image processing. The LabEx allowed two very different teams to bring their own expertise in this innovative project. During 2015/2016, JG was offered a post-doctoral fellowship and has worked at SKA South Africa, on the integration of our developped CS code in one the SKA imager candidate (DDFacet, C. Tasse, in rev.) and benefited from regular interactions with the project team (supervision of Jiang PhD Thesis, production of results for the 2D-1D study). He was hired as Ass. Prof. at Univ. Paris Diderot.
Current effort is being put on applying all the described methods on real dataset for scientific production. We especially aim at the following science case:
- Imaging of Fast-transient:
- Code acceleration:
- From sparse imaging to sparse calibration
- Scientific exploitation of various radio facilities (with specific focus on MeerKAT and NenuFAR).
Therefore, in the final funding period, we will carry out the studies in the direction of the scientific exploitation of MeerKAT and NenuFAR and the application of the 2D-1D, hyperspectral method which should open up new prospects (like the electromagnetic counterpart of gravitational wave events or other type of transients to be discovered with MeerKAT) to follow-up after UnivEarthS.
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