Gamma-ray bursts with MAXI and CALET

Satoshi Nakahira1 for the CALET collaboration

1 Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505E-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 thusGRB 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 observationwith 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 moni-tor with its sensitivity in the soft X-ray band. It has beenobserving all the sky since August 2009 to the present onthe 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-raysand the Gas Slit Camera (GSC) sensitive to 2–30 keVX-rays, which have rectangular fields of view of 90◦ and180◦, respectively. MAXI has detected about 90 GRBsin 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 instru-ments due to their dimness and soft spectra (Serino etal. 2014).

In October 2015 CALET (Torii et al. 2015) began ob-servation on the same platform with MAXI. CALETGamma-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 50keV–20 MeV. CGBM consists of two kinds of inorganicscintillator crystals LaBr3(Ce) (HXM1, 2) and BGO(SGM) with a PMT. Additionally CAL (CALorime-ter; 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 in-struments. Thus, GRB observations over an ultra-wideenergy band can be achieved.

Table 1. Properties of MAXI and CALET instruments.

MAXI CALETenergy bands 0.7–7 keV (SSC) 7–1000 keV (HXM)

2–30 keV (GSC) 100keV–20 MeV (SGM)

1 GeV–10 TeV (CAL)

instantaneous 1% (SSC) 20% (HXM, CAL)

fields of view 2% (GSC) 60% (SGM)

2. Status of CGBM

In this section, we describe the performance of theCGBM detectors, the status of calibration, data anal-ysis and the operations on orbit.

2.1. performance

CGBM unit test was conducted in January 2014. Weperformed a two-dimensional scan measurement of trans-mission efficiency and gain of the HXM entrance windowusing an X-ray generator and a radioisotope. Energy lin-earity of HXM and SGM direction dependence was alsomeasured. There was an unexpected asymmetric reduc-tion 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 effi-ciency curve can be represented by absorption feature ofLaBr3(Ce) itself. Possibly it is caused by a degradation

Fig. 1. HXM1 count rate map of 14.4 keV X-ray pencil bean taken

in ground test at January 2014.

of 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 ac-curacy and 8192 channels

• capture of an ASC (Advanced Stellar Compass) im-age 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 by

a 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 inthe high latitude region and the HV value is set to 0 Vsubsequently. 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 setto 700.7 V, 793.8 V and 867.3 V for HXM 1, HXM 2 andSGM, respectively (gains of the ground measurementscan be approximated with these values).Also, the trigger conditions are set as follows.

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Fig. 2. Total efficiency of HXM1, 2 and SGM of CGBM (on-axis).

• background integration time of 16s

• 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 Fitsformat to meet the specification of HEASoft1 providedby HEASARC2 in order to simplify the future data pub-lication.The energy response function used for energy spec-

tral analysis is created by a response generator us-ing the Geant4 (Allison 2006) simulator. We modeledCGBM detectors in detail and roughly reproduced wholeCALET geometry to reproduce back-scattered photonsfrom those structures. In order to take into account theeffects of degradation shown in section 2.1. in the cal-culation, we modeled the surface and the depth map ofthe degraded part, which are used as calibration data.Then this model is implemented in the response gener-ator. The efficiency curve obtained from CGBM’s re-sponse matrix is shown in Figure 2.We note that the response function still needs more

improvement because 1) optimization of HXM efficiencyby on-orbit data is not done yet and 2) effects of ISSstructure 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 Research

Center

3. MAXI and CGBM detected GRBs

In this section, we summarize the statistics of gamma-raybursts detected by MAXI and/or CGBM. Figure 3 showsthe angle distribution at the trigger time. In CGBM, 49GRBs 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 and

dim, 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.

Table 2. Summary of CGBM detection of MAXI bursts.

MAXI GRBs 15detected inside of the MAXI’s FoV 4

by CGBM outside of the MAXI’s FoV 1not detected CGBM HV off 2

by CGBM blocked by ISS structures 2soft and dim 5bright and soft 1

3.1. Simultaneously detected events

We briefly describe the characteristics of simultaneouslydetected GRBs.In GRB160101A and GRB160814A, the same peaks

were triggered simultaneously with MAXI and CGBM.For GRB160101A, as shown in Figure 4(a), the pulsewidth differs depending on energy bands (The width isgreater at lower energies). As shown in Figure 4(d), thejoint spectrum is well reproduced with Band functionwith α=−0.98±0.05, Ep=260.7+90

−64 keV and a 2–1000keV energy flux of 1.69×10−6 erg cm−2 s−2 (β cannotbe constrained).In the case of GRB160107A and GRB160509A, sep-

arated peaks were triggered. MAXI triggered the pre-burst 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 of

view through the GSC lead shield. The spectra mea-sured in CGBM are shown in Figure 4(e), corrected forthe HXM1, HXM2, and SGM effective areas. However,obscuration by the ISS solar panels is evident; the count

rate below 100 keV in HXM2 is lower compared to that ofHXM1, despite the greater effective area of HXM2. Anincrease in the plastic scintillator count rate in the CALCharge Detector (CHD) was also detected and reportedby Nakahira et al. (2016).

4. summary

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

References

Matsuoka, M. et al. 2009, PASJ, 61, 999Serino, M. et al. 2014, PASJ, 66, 5, 87Torii, S. for the CALET collaboration, Proc. 34th ICRC,

581 (2015)Yamaoka, M. et al. 2013, proceeding of 7th Huntsville

Gamma-Ray Burst Symposium, C1304143Allison, J. et al., 2006, IEEE Transactions on Nuclear

Science, 53,1, 270Nakahira, S. et al. 2016, GCN, 19617

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Fig. 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.

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MAXI/GSC panels show the effective areas. (d) νFν spectrum of GRB160101A fitted by the Band function and (e) energy spectra

of GRB160625A corrected for the effective areas.