GRM brief introduction Institute of High Energy Physics, CAS

Sciences GRM will contribute to GRB-related studies in the following aspects: a) GRB physics including progenitor, jet mechanism and components, energy dissipation mechanism and radiation mechanism. b) Multi-messenger studies including gravitational wave, neutrinos and high-energy cosmic rays. c) Cosmology and fundamental physics. GRM will significantly contribute to Terrestrial Gamma-ray Flashes (TGFs). TGFs are brief (~ 1ms) and energetic (up to several MeV) radiation events originated from thunderstorm and lightning. Almost all of TGFs are discovered by Gamma-ray detectors in the low Earth orbit (e.g. Fermi/GBM, RHESSI).

Scientific requirements (SR) [SR1] Discover a variety of GRBs, including short GRBs lasting 5 ms to 2 s, long GRBs up to 1000 s, X-ray rich GRBs. [SR2] Provide fast trigger to short GRBs. [SR3] Observe GRBs in wide energy band (4 keV – 5 MeV) from T0-5 min to T0 + 10 min. [SR4] Provide synergy observation to Gravitational Wave objects. [SR5] Discover and observe TGFs. . Discover GRBs out of ECLAIRs’ FOV but inside of GRM’s FOV. Help ECLAIRs improve GRB discovery capability. According to the fact that Fermi/GBM find more short GRBs than Swift/BAT, GRM could find more short GRBs that couldn’t trigger ECLAIRs. This will enable GRM to study GRB temporal and spectral features including precursor, extended emission, soft excess, thermal component, etc. The overlap in energy band with ECLAIRs will provide cross-calibration and crosscheck between GRM and ECLAIRs. Although the direct evidence is still missing, NS-NS or BH-NS merger is widely believed to be progenitor of short GRBs. Meanwhile, such compact mergers are good candidate for gravitational wave detectors, such as LIGO. According to in-flight observation made by Fermi/GBM and RHESSI, TGFs last from 50 μs to several ms, in an energy range of 10 keV to several MeV. Electron and positrons are also occasionally observed from TGFs.

Functional requirements (FR) [FR1] GRM shall provide the spectral observation on GRBs from 15 keV to 5000 keV; [FR2] GRM shall provide on-board trigger and measure duration of GRBs (especially short and hard GRBs); [FR3] GRM shall measure the GRBs’ peak energy in hard X and soft gamma ray band in near real-time; GRM shall be combined with ECLAIRs to estimate GRB peak energy more accurately; [FR4] GRM shall provide the alert of entering high particle flux places (e.g. SAA). [FR5] GRM shall provide rough localization for GRBs.

GRM specifications Number of units 3 (different orientation) SR4, FR5 Energy range 15-5000 keV SR3, FR1 Field of view +/-60° Sensitive area >200 cm2 (each unit) Dead time <8 µs SR5 Temporal resolution <20 µs Energy resolution ≤16%@60 keV FR3 Burst observation rate >90 / year GRB localization error ~5 degree (for high flux GRBs)

GRM devices & functions 241Am+PS+SiPM GRD calibration SAA alert Data process, Data management Power supply, etc. PS: to monitor particle flux to reject Particle Precipitation Events NaI(Tl): X-ray detector (15~5000 keV) 3 GRDs with different orientations

GRM working principle

GRM working mode PLM mode GRM mode Comments OFF mode GRM off LV off; HV off; LLC low S/H mode Orbital maneuver Configuration Wait/configuration Wait: LV on; HV off; LLC low Configuration: LV on; HV on; LLC high SAA LV on; HV off; switched by CMD or GPM alert GRB observation GRB LV on; HV on; switched by CMD or trigger rate Non-GRB observation Background Detection “off” mode Background mode when no GRB or other transient sources are detected. GRM collects and download rebinned spectra. Event data of about 5 min is temporarily stored. Calibration of the GRD detector can be performed. Burst mode Once GEB find count rate of GRDs increases significantly at T0, the GRM automatically enters burst mode. commands from ground or broadcasted by ECLAIRs. GEB will transfer event data [T0-5min,T0+10min]to ground After T0+10min GRM automatically enters bkg mode SAA mode GRM uses GPM data to determine “SAA” region Uploaded “SAA” region

GRM data – through X band In GRB mode (from T0-5min to T0+10min) Event-by-event data. 6~7 bytes per event which includes energy, time, pulse width, status, etc. Rebinned spectra (same as spectra in background mode) Rebinned spectra (additional) In background mode Rebinned spectra Data type Channels Bytes/chann Num. of spectra Time slice(s) GRD 32 1 6 0.04 8 0.01 Data type Channels Bytes/chann Num. of spectra Time slice (s) GRD 128 2 6 8 0.256 GPM 4 1

GRM data – through S band Count rates of detectors every second Calibrated spectra of 3 GRDs every minute Pulse width spectra of 3 GRDs every minute Temperature, HV/LV, currents, etc. every 10 seconds

GRM data – through VHF GRM – PDPU – ECLAIRs GRM – VHF – GWAC Rebinned spectra All involved GRDs in one spectrum; no bkg spectrum; Epeak can’t be derived Level 1b trigger package through VHF Data Bits Content D1 32 Ltime (D31～D0) (time in second from PPS.) D2 Stime (D31～D0) (time in second from GEB. TBD) D3 16 Ptime (D15～D0) (sub-second time from GEB.) D4 12 Start channel (or energy) of ΔE D5 End channel (or energy) of ΔE D6 4 Δt (4 Δts) D7 8 Significance level D8 16+32 Coordinates of GRB (θ, φ) (precision: 1degree) + Platform attitude D9 Count of GPM in Δt D10 256 Rebinned spectrum in Δt (16 channels, 2 Byte/channel) D11 Light curves in ΔE (16Δt, 2 Byte/bin) D12 GRDs getting involved (***0) Total 656+32 (656 bits can be used in one VHF package) GRM trigger package to ECLAIRs Data Bits Content D1 32 Ltime (D31～D0) D2 Stime (D31～D0) (TBD, may be removed) D3 16 Ptime (D15～D0) D4 Start channel (or energy) of ΔE D5 End channel (or energy) of ΔE D6 8 Δt D7 Significance level D8 Coordinates of GRB (θ, φ) (precision: 1degree) D9 Count of GPM in Δt D10 4 CRC code Total 156 752bits -48bits header – 32bits time – 16 bits CRC =656 bits for one VHF package