1. GRM brief introduction
Institute of High Energy Physics, CAS

2. 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).

3. 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 T 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.

4. 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.

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

6. 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

7. GRM working principle

8. 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

9. 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

10. 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

11. 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