Talk given in the National Astronomy Meeting 2019, Lancaster, UK. A large number of neutron star mergers will be detected by LIGO/Virgo in the coming years. GW170817 confirmed that neutron star mergers would actually produce relativistic outflows (e.g. relativistic jets and cocoon). The polarimetry by LT and NRT would enable us to study the properties of the outflow in details (i.e. magnetic field structure/strength, and its angular dependence). We discuss polarization signals in the outflow emission, and we show the expected signal distributions for an upcoming neutron star merger sample.

1. NRT polarimetry and neutron
star mergers
Joseph John Fernandez & Shiho Kobayashi
Astrophysics Research Institute, LJMU

2. Credit: NASA
- High energy events
- Caused by a collimated jet of
relativistic material colliding with
ISM
- Short vs long GRBs
- Prompt emission + long lasting
afterglow

3. Credit: NASA
- Radiation due to synchrotron
emission.
- Launch mechanism: uncertain
- Origin: a little less uncertain

4. Abbott et al. 2017, PRL 119, 161101
Troja et al. 2017, Nature volume 551, 71–74
Neutron star merger GW170817:
Gravitational wave signal and EM counterpart

5. Lamb et al. 2019, ApJL 870, L15
Apparent superluminal motion of centroid → anisotropic ejecta
Light curve decay → (angular) structured jet
Mooley et al., Nature 561, 355-359

6. Credit: NASA
Polarization of the afterglow
- Forward shock → magnetic field
of surrounding ISM
- Reverse shock → magnetic field
of the original ejecta. Probe jet
launch mechanism?
· Relative magnetization
· Geometry, spatial distribution

7. Polarization of the afterglow
· Expect linear polarization
· Relativistic jet → beaming of radiation along jet axis: limited visible region
· Isotropic vs anisotropic B-field; moment of observation; location of visible region
Credit: Kobayashi, 2019

8. Polarization of the afterglow: preliminary predictions
· Simple model for the dynamics (Sari, Piran and Narayan, 1998):
· Synchrotron spectrum following standard methods (Sari, Piran and Narayan, 1998)
· Reverse shock calculated from analytic light curves (Kobayashi, 2000)
· Method for calculating light curves following Lamb & Kobayashi (2017). Gaussian jet, best fit
parameters for GW170817 afterglow (Lamb et al. 2019)

9. Observing strong reverse-shock afterglows requires ~ early observations:
1-10 day range
Strong, but fast and early
(see Lamb & Kobayashi 2018, arXiv:1903.03320v2 )

10. Observing strong reverse-shock afterglows requires ~ early observations:

11. For any polarization producing mechanism, we can estimate (assuming unpolarized FS
emission)
- Visible region B-field made up of small,
coherent patches :
- Anisotropic B-field, with component parallel
to jet front dominating:
- Large scale magnetic fields in ejecta (Steele
et. al, 2017) :
(Kobayashi (2019) and references therein)

12. The role of the NRT in the multi-messenger
astronomy era
Credit: Liverpool Telescope, LJMU
- aLIGO/VIRGO should provide many (at least a
few) triggers in coming years.
- Identification and classification of potential EM
counterparts
- Rapid response photometry and polarimetry to
the triggers (contemplated in science case)

13. Conclusions
- GW170817 follow-up revealed that (some) sGRBS are associated with NS
mergers.
- While observation angle-dependent, the RS emission can dominate for a period
of time and provide information about the original ejecta and jet launch
mechanism.
- With many GW-triggers expected, automated facilities like the NRT are
essential for follow-up of EM counterparts