1. Transient sky and LSST
Nicolas Leroy

2. Transient sky will be well covered in 2020
Large sets of instruments in 2020
SVOM Scientific prospects arXiv v1

3. Multi-messenger astronomy
Timeline KM3NET - ORCA
NB : ARCA have a similar timeline

4. Lot of physics behind
SVOM Scientific prospects arXiv v1

5. Courtesy of G. Ghirlanda
GRB orphan afterglows
γ rays
Courtesy of G. Ghirlanda
N_orph ~ N_grb(2/theta2)
~300 orphan afterglows for each on-axis GRB!

6. GRB orphan afterglows
The beaming angle of GRB jets remains a crucial issue, which impacts the energy budget of GRBs.
“isotropic equivalent” energy Eiso is computed under the assumption that the source emits isotropically and spans 6 orders of magnitude from to 1054 erg. Due to beaming, the true energy radiated by GRB jets could be smaller by up to a factor 103, but the beaming angle has been measured for only a few GRBs.
Determining the GRB progenitor nature. The observations of an orphan afterglow probe circumburst environment material less affected by the strong ionizing flux of the jet. Optical spectroscopy of GRB orphan afterglows is possibly a unique way to probe the pristine environment of the massive star GRB progenitor.

7. GRB orphan afterglows
Ghirlanda+15
LSST is the best facility to detect orphan afterglows
LSST offers a completely new method of detecting GRBs afterglows
(for the first time independently of their detection in gamma-rays).
LSST will detect the optical afterglows of on-axis and off-axis GRBs.
Synergy LLST-SVOM to study the afterglows/orphan afterglow population

8. You are needed !
From current expectations LSST will provide up to 106 transients / night
Nobody can absorb a so large sample
What parameters can we determine on the nature of the source with your measurements ?
Will help a lot to make decision in terms of ToO
Plan of observation will allow only on exceptional astrophysics event a depointing
Can we anticipate in advance the plan based on closed by galaxies ie up to maximum of binary neutron stars range with GW detectors for example ?

9. Let discuss together First point here
We also organize a PNHE workshop June in Orsay :
One session will be dedicated to LSST !