Gamma-ray bursts with MAXI and CALET
Satoshi Nakahira1 for the CALET collaboration
1Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505
E-mail(SN): nakahira@crab.riken.jp
Abstract

MAXI and CALET are wide-field observatories that have been operating on the same platform(JEM/EF of the ISS). They have overlapping fields of view and continuous energy ranges, and thus
GRB observations in a wide energy band can be achieved. From the start of the CALET observation inOctober 2015 to December 2016, gamma ray burst monitor for CALET (CGBM) detected 49 GRBs, fiveof which were also detected with MAXI. In this paper, we will describe the gamma-ray burst observation
with CGBM and MAXI including simultaneously detected Gamma-ray bursts.
Key words: CALET, MAXI, GRB

1.

MAXI and CGBM observations of gamma-ray bursts

MAXI (Matsuoka et al. 2009) is a unique wide-field monitor with its sensitivity in the soft X-ray band. It has been
observing all the sky since August 2009 to the present on
the exposed facility of Japan Experiment Module “Kibo”
on the international space station (ISS). MAXI has theSolid State Camera (SSC) sensitive to 0.7–7 keV X-rays
and the Gas Slit Camera (GSC) sensitive to 2–30 keV
X-rays, which have rectangular fields of view of 90◦ and180◦ , respectively. MAXI has detected about 90 GRBs
in the past seven and a half years of observations. Half ofthe GRBs detected by MAXI were found to be “MAXIonly bursts”, which were not detected by other instruments due to their dimness and soft spectra (Serino etal. 2014).
In October 2015 CALET (Torii et al. 2015) began observation on the same platform with MAXI. CALET
Gamma-ray burst monitor (CGBM; Yamaoka et al.
2013) has two types of scientific instruments: HXM(Hard X-ray Monitor) with a sensitivity in 7–1000 keVand SGM (Soft Gamma Monitor) with a sensitivity in 50
keV–20 MeV. CGBM consists of two kinds of inorganic
scintillator crystals LaBr3 (Ce) (HXM1, 2) and BGO(SGM) with a PMT. Additionally CAL (CALorimeter; the main detector of CALET) has a sensitivity togamma-rays from 1 GeV to 10 TeV.
As summarized in Table 1, MAXI and CALET havecomplementary energy bands, and one-third of theMAXI/GSC’s field of view overlaps with all other instruments. Thus, GRB observations over an ultra-wide
energy band can be achieved.

Table 1. Properties of MAXI and CALET instruments.

MAXICALET

energy bands

0.7–7 keV (SSC)
2–30 keV (GSC)

7–1000 keV (HXM)
100keV–20 MeV (SGM)

instantaneousfields of view

1% (SSC)

20% (HXM, CAL)

2% (GSC)

60% (SGM)

1 GeV–10 TeV (CAL)

2. Status of CGBM
In this section, we describe the performance of theCGBM detectors, the status of calibration, data analysis and the operations on orbit.
2.1. performance
CGBM unit test was conducted in January 2014. We
performed a two-dimensional scan measurement of transmission eﬃciency and gain of the HXM entrance windowusing an X-ray generator and a radioisotope. Energy linearity of HXM and SGM direction dependence was alsomeasured. There was an unexpected asymmetric reduction of the transmission eﬃciency in the low energy bandof HXM 1 (Fig. 1).
Detection eﬃciency and gain at the higher energies
were almost uniform. Therefore, it is considered thatthe degradation of the eﬃciency occurs at the surfaceof the crystal. Energy dependency of the measured eﬃciency curve can be represented by absorption feature ofLaBr3 (Ce) itself. Possibly it is caused by a degradation

60

SGM
40

HXM1
HXM2

020

Effective Area (cm2)80

Total Efficiency

10100
Energy (keV)

1000

104Fig. 2. Total eﬃciency of HXM1, 2 and SGM of CGBM (on-axis).
Fig. 1. HXM1 count rate map of 14.4 keV X-ray pencil bean taken
in ground test at January 2014.

• background integration time of 16sof scintillating light emission eﬃciency due to hydrationof LaBr3 (Ce) in several µm of the surface by small airleaks.
As for SGM, no major problem was found. Its lower
energy threshold on the orbit was ∼30 keV.
2.2. operation
CGBM regularly acquires two kinds of periodic data:• PH data: 1/8-second intervals with 512 channels• TH data: 4-second intervals with 8 channels
Also, when a GRB is triggered on the orbit,• capture of photon event data with 62.5µs time accuracy and 8192 channels• capture of an ASC (Advanced Stellar Compass) image with an exposure time of 0.5 second• change of CAL observation mode from HE mode(>10 GeV) to LE mode (>1 GeV)
will be executed.
The high voltage operation on orbit is controlled bya schedule command uploaded almost daily. Since theinclination angle of the ISS is as high as 51.6 degrees, toavoid false triggers and damage due to increase/variationof the charged particle rate, the trigger is turned oﬀ in
the high latitude region and the HV value is set to 0 V
subsequently. At the beginning of the CGBM operation,
we started with ∼45% of observation time referring tothe count rate map of the RBM (Radiation Belt Monitor)
equipped on MAXI. Now it is operating at observationeﬃciency of about 60%. The operating HV value is set
to 700.7 V, 793.8 V and 867.3 V for HXM 1, HXM 2 andSGM, respectively (gains of the ground measurements
can be approximated with these values).
Also, the trigger conditions are set as follows.

• foreground integration time of 1/4s, 1/2s, 1s, 4s• trigger energy range and significance:– HXM 10–50 keV with 8.5σ
– SGM 40–230 keV with 7σ
2.3. data analysis
We are making eﬀorts to develop CGBM data in the Fits
format to meet the specification of HEASoft1 provided
by HEASARC2 in order to simplify the future data publication.
The energy response function used for energy spectral analysis is created by a response generator using the Geant4 (Allison 2006) simulator. We modeled
CGBM detectors in detail and roughly reproduced whole
CALET geometry to reproduce back-scattered photons
from those structures. In order to take into account theeﬀects of degradation shown in section 2.1. in the calculation, we modeled the surface and the depth map of
the degraded part, which are used as calibration data.
Then this model is implemented in the response generator. The eﬃciency curve obtained from CGBM’s response matrix is shown in Figure 2.
We note that the response function still needs more
improvement because 1) optimization of HXM eﬃciency
by on-orbit data is not done yet and 2) eﬀects of ISS
structure including solar panels or robotic arms are notfully taken into account in the response calculation.

*1 http://heasarc.nasa.gov/lheasoft/*2 NASA’s High Energy Astrophysics Science Archive ResearchCenter3. MAXI and CGBM detected GRBs
In this section, we summarize the statistics of gamma-ray
bursts detected by MAXI and/or CGBM. Figure 3 shows
the angle distribution at the trigger time. In CGBM, 49
GRBs have been detected since the operation started, 40of which are long, 9 are short, and 37 are localized byother satellites.
MAXI detected 15 GRBs during the same period. As
summarized in Table 2, five of them were also detectedwith CGBM, including one that came through the leadshield of the MAXI GSC.
Five of the CGBM-undetected GRBs were soft anddim, and the remaining one (GRB160705A) was a brightand soft burst, which emitted mainly below the sensitiveenergy bands of HXM. GRB160705A was found close tothe Galactic plane (b∼5◦ ), and it may be another kindof phenomenon such as an X-ray burst.

rate below 100 keV in HXM2 is lower compared to that of
HXM1, despite the greater eﬀective area of HXM2. An
increase in the plastic scintillator count rate in the CAL
Charge Detector (CHD) was also detected and reported
by Nakahira et al. (2016).

Table 2. Summary of CGBM detection of MAXI bursts.

References
Matsuoka, M. et al. 2009, PASJ, 61, 999
Serino, M. et al. 2014, PASJ, 66, 5, 87
Torii, S. for the CALET collaboration, Proc. 34th ICRC,
581 (2015)
Yamaoka, M. et al. 2013, proceeding of 7th Huntsville
Gamma-Ray Burst Symposium, C1304143
Allison, J. et al., 2006, IEEE Transactions on Nuclear
Science, 53,1, 270
Nakahira, S. et al. 2016, GCN, 19617

MAXI GRBs
detectedby CGBM
not detectedby CGBM

inside of the MAXI’s FoV
outside of the MAXI’s FoVCGBM HV oﬀ
blocked by ISS structures
soft and dimbright and soft

15
41
2
2
5
1

3.1. Simultaneously detected events
We briefly describe the characteristics of simultaneously
detected GRBs.
In GRB160101A and GRB160814A, the same peaks
were triggered simultaneously with MAXI and CGBM.
For GRB160101A, as shown in Figure 4(a), the pulse
width diﬀers depending on energy bands (The width isgreater at lower energies). As shown in Figure 4(d), thejoint spectrum is well reproduced with Band function
with α=−0.98±0.05, Ep =260.7+90−64 keV and a 2–1000
keV energy flux of 1.69×10−6 erg cm−2 s−2 (β cannot
be constrained).
In the case of GRB160107A and GRB160509A, separated peaks were triggered. MAXI triggered the preburst emission of GRB160107A 45 seconds before themain peak detected by CGBM (Figure 4(b)), and caughta bright X-ray flare of GRB160509A during an earlyphase of the afterglow, 320 seconds after the main peakdetected with CGBM (Figure 4(c)).
GRB 160625A came from outside the MAXI field ofview through the GSC lead shield. The spectra measured in CGBM are shown in Figure 4(e), corrected for
the HXM1, HXM2, and SGM eﬀective areas. However,obscuration by the ISS solar panels is evident; the count

4. summary
Since October 2015, CALET/CGBM has detected 49
gamma-ray bursts, and five of them were also detectedby MAXI. Simultaneously observed bursts indicate different properties depending on the energy bands. The
MAXI and CGBM broadband observation is thought tobe useful for understanding gamma-ray bursts from different aspects.

180°160927A

160821A

151006A
160623A
SGR1935+2154

90°160227A
160607A
160102A

SGR1935+2154
160118A
160104B

160223B

160509A

151205C 160509A

161004B

151112A

151216A

0°90151107B
160625B160910AA
160720A

160408A
A

60

160824B
160726A

160422A

160316A

30

160815A
160530B

161109A

160709A
161106A

160802A151212B

151231A
160106A
A160107
160107A
151227B
160814A
151212A
151225A 151210
151210B
160705A
160419A
160101A

160206A
161123A

161010A

270°

120

frontFig. 3. Incident angle distribution of GRBs detected by MAXI and CGBM. The red circles indicate the MAXI GRBs, the green crosses show
those of CGBM. The hatched regions in cyan and magenta are the fields of view of the GSC and SSC, respectively. The gray region shows
ISS structure viewing from the CGBM position.
(b) GRB160107A0counts /0.125sec

0.050−100102030SGM100-1000 keV

250−1001020
TIME since 2016/01/01T00:43:53−80080160240 keV
HXM1+2 7-100

1000counts s−1 cm−2Eff.Area500−80120counts /0.125sec

counts /0.125sec

30
HXM1+27-100 keV080

160

240

SGM100-1000 keV

60
0−80

30080
160
TIME since 2016/01/07T22:20:42

240

SSC 0.7-7 keV

0.3
0.0
40

0100

200

300

400

0100

200

300

400

0100200300

400

0100500

GSC2-20 keV 5.020040

2.5500

Eff.Area

204

100.6

counts /2.0 sec

10(c) GRB160509A8counts /0.125sec

0GSC2-20 keV

20

counts /0.125sec−10100−205.02.5

150−205030
counts /1.000 sec

300−20counts /0.125sec

GSC2-20 keV

effective Area

counts /0.250 sec(a) GRB160101A

0.0

HXM1+2 7-100 keV

200200500
SGM100-1000
keV

100
0200
300
400
TIME since 2016/05/09T08:58:50

500(e) energy spectrum of GRB160101A
1(d) νFν spectrum of GRB160101A

0.01
10 −3
10 −4
10 −610 −5Counts cm −2 s −1 keV −10.1

HXM1
HXM2SGM

10100

1000

10 4Energy (keV)

Fig. 4. MAXI/GSC, CGBM/HXM and SGM light curves of (a)GRB160101A, (b)GRB160107A and (c)GRB160509A. Red lines in theMAXI/GSC panels show the eﬀective areas. (d) νF ν spectrum of GRB160101A fitted by the Band function and (e) energy spectraof GRB160625A corrected for the eﬀective areas.