#  Insight-HXMT dedicated 33-day observation of SGR J1935+2154 I. Burst
Catalog

Ce Caimissing Key Laboratory of Particle Astrophysics, Institute of High
Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049,
China  University of Chinese Academy of Sciences, Chinese Academy of Sciences,
Beijing 100049, China  Wang-Chen Xuemissing Key Laboratory of Particle
Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,
19B Yuquan Road, Beijing 100049, China  University of Chinese Academy of
Sciences, Chinese Academy of Sciences, Beijing 100049, China  Cheng-Kui Li*
<EMAIL_ADDRESS>Key Laboratory of Particle Astrophysics, Institute of High
Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049,
China  Shao-Lin Xiong*<EMAIL_ADDRESS>Key Laboratory of Particle
Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,
19B Yuquan Road, Beijing 100049, China  Shuang-Nan Zhang*<EMAIL_ADDRESS>Key Laboratory of Particle Astrophysics, Institute of High Energy Physics,
Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, China  Lin Lin
Department of Astronomy, Beijing Normal University, Beijing 100088, People’s
Republic of China  Xiao-Bo Li  Key Laboratory of Particle Astrophysics,
Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan
Road, Beijing 100049, China  Ming-Yu Ge  Key Laboratory of Particle
Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,
19B Yuquan Road, Beijing 100049, China  Hai-Sheng Zhao  Key Laboratory of
Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of
Sciences, 19B Yuquan Road, Beijing 100049, China  Li-Ming Song  Key Laboratory
of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of
Sciences, 19B Yuquan Road, Beijing 100049, China  University of Chinese
Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China  Fang-
Jun Lu  Key Laboratory of Particle Astrophysics, Institute of High Energy
Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, China
University of Chinese Academy of Sciences, Chinese Academy of Sciences,
Beijing 100049, China  Shu Zhang  Key Laboratory of Particle Astrophysics,
Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan
Road, Beijing 100049, China  Yan-Qiu Zhang  Key Laboratory of Particle
Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,
19B Yuquan Road, Beijing 100049, China  University of Chinese Academy of
Sciences, Chinese Academy of Sciences, Beijing 100049, China  Shuo Xiao  Key
Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese
Academy of Sciences, 19B Yuquan Road, Beijing 100049, China  University of
Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049,
China  You-Li Tuo  Key Laboratory of Particle Astrophysics, Institute of High
Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049,
China  Qi-Bin Yi  Key Laboratory of Particle Astrophysics, Institute of High
Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049,
China  School of Physics and Optoelectronics, Xiangtan University, Xiangtan
411105, Hunan, China  Zhi Wei Guo  Key Laboratory of Particle Astrophysics,
Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan
Road, Beijing 100049, China  College of physics Sciences Technology, Hebei
University, No. 180 Wusi Dong Road, Lian Chi District, Baoding City, Hebei
Province 071002, China  Sheng Lun Xie  Key Laboratory of Particle
Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,
19B Yuquan Road, Beijing 100049, China  School of Physical Science and
Technology, Central China Normal University, Wuhan 430097, China  Yi Zhao  Key
Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese
Academy of Sciences, 19B Yuquan Road, Beijing 100049, China  Department of
Astronomy, Beijing Normal University, Beijing 100088, People’s Republic of
China  Zhen Zhang  Key Laboratory of Particle Astrophysics, Institute of High
Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049,
China  Qing-Xin Li  Department of Astronomy, Beijing Normal University,
Beijing 100088, People’s Republic of China  Jia-Cong Liu  Key Laboratory of
Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of
Sciences, 19B Yuquan Road, Beijing 100049, China  University of Chinese
Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China  Chao
Zheng  Key Laboratory of Particle Astrophysics, Institute of High Energy
Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, China
University of Chinese Academy of Sciences, Chinese Academy of Sciences,
Beijing 100049, China  Ping Wang  Key Laboratory of Particle Astrophysics,
Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan
Road, Beijing 100049, China

###### Abstract

Magnetars are neutron stars with extreme magnetic field and sometimes manifest
as soft gamma-ray repeaters (SGRs). SGR J1935+2154 is one of the most prolific
bursters and the first confirmed source of fast radio burst (i.e. FRB 200428).
Encouraged by the discovery of the first X-ray counterpart of FRB, Insight-
Hard X-ray Modulation Telescope (Insight-HXMT) implemented a dedicated 33-day
long ToO observation of SGR J1935+2154 since April 28, 2020. With the HE, ME
and LE telescopes, Insight-HXMT provides a thorough monitoring of burst
activity evolution of SGR J1935+2154, in a very broad energy range (1-250 keV)
with high temporal resolution and high sensitivity, resulting in a unique
valuable data set for detailed studies of SGR J1935+2154. In this work, we
conduct a comprehensive analysis of this observation including detailed burst
search, identification and temporal analyses. After carefully removing false
triggers, we find a total of 75 bursts from SGR J1935+2154, out of which 70
are single-pulsed. The maximum burst rate is about 56 bursts/day. Both the
burst duration and the waiting time between two successive bursts follow log-
normal distributions, consistent with previous studies. We also find that
bursts with longer duration (some are multi-pulsed) tend to occur during the
period with relatively high burst rate. There is no correlation between the
waiting time and the fluence or duration of either the former or latter burst.
It also seems that there is no correlation between burst duration and hardness
ratio, in contrast to some previous reports. In addition, we do not find any
X-ray burst associated with any reported radio bursts except for FRB 200428.

magnetars: general — magnetars: individual (SGR J1935+2154) — X-rays: bursts

##  1 Introduction

Soft gamma-ray repeaters (SGRs), manifestation of celestial objects
unpredictably emitting bursts in hard X-rays and soft gamma-rays, are deemed
to be rotating neutron stars with ultra-strong surface magnetic fields up to
1014 $-$ 1015 G, known as magnetars (Thompson & Duncan, 1995; Kouveliotou et
al., 1998). These magnetar bursts are most likely powered by the release of
the magnetic field energy triggered by the starquakes of neutron stars
(Thompson & Duncan, 1995) or the reconnection of the magnetic field lines
(Lyutikov, 2003).

Magnetars are also plausible sources of fast radio bursts (FRBs), which are
another kind of mysterious cosmological flashes in the radio band. This
connection was first built on the sufficient energy preserved by the strong
magnetic field of magnetars and the compatible rate of magnetar bursts in
X-rays with that of FRBs (Popov & Postnov, 2010). Recently it has been proved
by observations of a bursting magnetar SGR J1935+2154.

SGR J1935+2154 is one of the most prolific magnetars in our galaxy, which
first triggered the Burst Alert Telescope (BAT) aboard the Neil Gehrels Swift
Observatory (hereafter Swift) through a short hard X-ray burst on July 5th,
2014 (Stamatikos et al., 2014). Since then, SGR J1935+2154 has been
sporadically active, with several outburst episodes happened in the years of
2014, 2015, 2016, 2019 and 2020 (Younes et al., 2017; Lin et al., 2020b, a).
On April 27th, 2020, SGR J1935+2154 entered an active phase with the emission
of hundreds of bursts. Later, a very bright fast radio burst (FRB 200428) was
detected by CHIME and STARE2 from the direction of the bursting magnetar
(CHIME/FRB Collaboration et al., 2020; Bochenek et al., 2020).

This FRB is found to be associated with an unusual non-thermal X-ray burst
accurately localized to SGR J1935+2154 by Insight-HXMT (Li et al., 2021) and
INTEGRAL (Mereghetti et al., 2020) and jointly detected by other gamma-ray
detectors including AGILE (Tavani et al., 2020) and Konus-Wind (Ridnaia et
al., 2021), providing an unambiguous proof that SGR J1935+2154 is the first
confirmed source that can radiate the FRB.

Insight-HXMT is China’s first X-ray astronomy satellite (Zhang et al., 2020,
2018), launched on June 15th, 2017. It is composed of three telescopes with
different energy bands: the High Energy X-ray telescope (HE, 20$–$250 keV)
(Liu et al., 2019), the Medium Energy X-ray telescope (ME, 5$–$30 keV) (Cao et
al., 2019) and the Low Energy X-ray telescope (LE, 1$–$15 keV) (Chen et al.,
2019). With wide energy band and high time resolution, Insight-HXMT plays an
important role in searching for the X-ray and gamma-ray transient, with
duration from milliseconds to seconds, as gamma-ray bursts (GRBs) (Cai et al.,
2021), high-energy emission of FRBs (Guidorzi et al., 2020a, b) and
electromagnetic counterpart of gravitational waves (Li et al., 2018; Zhang et
al., 2020; Cai et al., 2021).

Insight-HXMT discovered an X-ray burst associated with FRB 200428 (called FRB
200428-Associated Burst hereafter) from the Galactic magnetar SGR J1935+2154
(Li et al., 2021). We first suggested that there are two narrow peaks in this
X-ray burst that are the high energy counterparts of the two radio peaks in
FRB 200428 (CHIME/FRB Collaboration et al., 2020; Bochenek et al., 2020; Li et
al., 2021). This breakthrough discovery encouraged us to implement a dedicated
month long Target of Opportunity (ToO) observations to continuously monitor
SGR J1935+2154, to give a substantially deep depiction of the burst activity
evolution of this magnetar. During this long ToO, the first X-ray bursts of
SGR J1935+2154 detected by Insight-HXMT is about 13 hours after the onset of
burst activity, which is marked by the very first burst detected by Fermi/GBM
and Swift/BAT at 2020-04-27T18:26:20 UTC (Palmer & BAT Team, 2020a; Fletcher &
Fermi GBM Team, 2020; Palmer & BAT Team, 2020b).

Thanks to the wide energy band and high sensitivity of Insight-HXMT, this ToO
observation attain a unique sample of bursts in 1 keV to 250 keV with a flux
lower limit of about 1 $\times 10^{-9}$ erg cm-2 s-1 (Zhang et al., 2020),
which significantly enlarge the burst samples of SGR J1935+2154 detected by
other instruments (e.g., Swift (Gehrels et al., 2004), NICER (Gendreau et al.,
2016), Fermi/GBM (Meegan et al., 2009)) during this interesting burst episode
with the emission of a FRB.

With this Insight-HXMT burst sample, we implement a series of detailed data
analyses. As the paper I of this paper series for Insight-HXMT observation of
SGR J1935+2154, we focus on the burst search, classification and verification
of bursts, and temporal analyses. In paper II, we will give the detailed time-
integrated spectral analysis of these bursts from SGR J1935+2154.

This paper is organized as follows: In section 2, we summarize the
observations and data reduction. Section 3 describes the trigger algorithm,
classification and verification of bursts. We present the catalog analysis
method and results in section 4. Finally, we give the discussion and summary
in Section 5 and Section 6.

Table 1: Insight-HXMT observations of SGR J1935+2154 and Blank Sky (for background study). Target Name |  obsID |  Start Time1 (UTC) |  End Time1 (UTC) |  Exposure1 (ks)
---|---|---|---|---
SGR J1935+2154 2 |  P0314003001 |  2020-04-28T07:14:51.000 |  2020-04-29T12:02:36.000 |  60
SGR J1935+2154 2 |  P0314003002 |  2020-04-30T06:58:23.000 |  2020-05-06T01:20:55.000 |  290
SGR J1935+2154 2 |  P0314003003 |  2020-05-06T01:20:55.000 |  2020-05-08T01:03:20.000 |  100
SGR J1935+2154 2 |  P03140054 |  2020-05-08T01:03:20.000 |  2020-06-01T00:00:01.000 |  1200
Blank Sky 3 |  P01012935 |  2017-11-02T05:00:57.000 |  2018-03-27T05:24:47.000 |  1796

##  2 Observation and Data SAMPLES

Since 2020-04-28T07:14:51 UTC, Insight-HXMT started the dedicated 33-day (a
total span time of 2851.2 ks) ToO observations of SGR J1935+2154, which had
emitted hundreds of short X-ray bursts and triggered a series of astronomical
satellites starting from 2020 April 27 (Palmer & BAT Team, 2020b). This
Insight-HXMT dedicated long ToO ended at 2020-06-01T00:00:01 UTC with a total
effective exposure of 1650 ks, and covered the time of FRB 200428. The
detailed observation time history of Insight-HXMT is listed in Table 1.

To facilitate the joint observations of SGR J1935+2154 with multi-wavelength
telescopes, a preliminary search of X-ray bursts has been done and a
preliminary list of X-ray bursts has been released on the Insight-HXMT
website111http://hxmtweb.ihep.ac.cn/bursts/392.jhtml. In the present work, we
refine the burst search for SGR J1935+2154 X-ray bursts and implement a
comprehensive analysis with the refined bursts sample, with the data acquired
from the telescopes of HE, ME and LE of Insight-HXMT. We obtain the screened
event files with high time resolution by analysing the 1K data of HE, ME and
LE with the up-to-date version (version 2.04) of Insight-HXMT Data Analysis
Software package (hereafterHXMTDAS) 222http://www.hxmt.cn/.

First, the raw event files are processed to produce calibrated event files (Li
et al., 2020). We use the commands hepical, mepical and lepical to calibrate
the photon events of HE, ME and LE, respectively. The HE command hepical is
also used to remove the spike events, which are caused by the interactions of
high energy cosmic rays with satellite materials and usually occurred
simultaneously in a few NaI and CsI detectors (Wu et al., 2022). The ME
command megrade is used to calculate the event grade and deadtime of each
Field Programmable Gate Array (FPGA). The LE command lerecon is used to
reconstruct two spilt events and assign event grades.

The commands of hegtigen, megtigen and legtigen are used to select the good
time intervals (GTIs). The ELV and SAA flag parameters (“ELV$>$1” and “SAA
flag=0”) are set to exclude the time intervals when the satellite flies
through SAA or the targeted source is blocked by the Earth. In addition, the
megticorr and legticorr commands are used to generate new GTI files to further
eliminate some bad time intervals, including some events with higher grades,
which are not easy to calibrate.

Finally, the commands hescreen, mescreen and lescreen are used to extract the
good events using the GTIs above. The screened event files including these
good events are used to search for X-ray bursts down to ms timescales. We use
all un-blinded detectors of LE, ME and NaI detectors of HE (CsI detectors of
HE is not used since the spectra of SGR bursts are not hard enough to leave
signals in CsI crystals). The selected energy bands of HE, ME, LE are 28$-$250
keV, 10$-$30 keV and 1$-$10 keV, respectively.

Table 2: Trigger algorithms and criteria used in the burst joint search of SGR J1935+2154 and the test results of the search on the blank sky data. Timescales (s) |  Phases (s) |  Telescopes |  ${C}_{\rm HE}^{1}$ |  ${N}_{\rm HE}^{2}$ |  ${C}_{\rm ME}^{3}$ |  ${C}_{\rm LE}^{3}$ |  GTIs4 (s) |  ${N_{\rm FT}^{5}}$ |  ${P_{\rm FA}^{6}}$ |  Sig7
---|---|---|---|---|---|---|---|---|---|---
0.005 |  0/0.0025 |  HE&ME |  1 |  4 |  3 |  - |  1.33$\times 10^{6}$ |  0 |  - |  -
0.005 |  0/0.0025 |  HE&LE |  1 |  4 |  - |  3 |  1.12$\times 10^{6}$ |  1 |  4.45e-09 |  5.7$\sigma$
0.005 |  0/0.0025 |  ME&LE |  - |  - |  5 |  5 |  1.12$\times 10^{6}$ |  0 |  - |  -
0.005 |  0/0.0025 |  HE&ME&LE |  1 |  3 |  1 |  1 |  1.12$\times 10^{6}$ |  1 |  4.45e-09 |  5.7$\sigma$
0.01 |  0/0.005 |  HE&ME |  2 |  5 |  2 |  - |  1.33$\times 10^{6}$ |  0 |  - |  -
0.01 |  0/0.005 |  HE&LE |  2 |  5 |  - |  2 |  1.12$\times 10^{6}$ |  0 |  - |  -
0.01 |  0/0.005 |  ME&LE |  - |  - |  5 |  5 |  1.12$\times 10^{6}$ |  9 |  8.01e-08 |  5.2$\sigma$
0.01 |  0/0.005 |  HE&ME&LE |  2 |  1 |  3 |  3 |  1.12$\times 10^{6}$ |  1 |  8.90e-09 |  5.6$\sigma$
0.02 |  0/0.01 |  HE&ME |  3 |  3 |  3 |  - |  1.33$\times 10^{6}$ |  0 |  - |  -
0.02 |  0/0.01 |  HE&LE |  3 |  3 |  - |  3 |  1.12$\times 10^{6}$ |  1 |  1.78e-08 |  5.51$\sigma$
0.02 |  0/0.01 |  HE&ME&LE |  3 |  1 |  3 |  3 |  1.12$\times 10^{6}$ |  6 |  1.06e-07 |  5.18$\sigma$
0.04 |  0/0.02 |  HE&ME |  3 |  3 |  8 |  - |  1.33$\times 10^{6}$ |  0 |  - |  -
0.04 |  0/0.02 |  HE&LE |  3 |  3 |  - |  8 |  1.12$\times 10^{6}$ |  14 |  4.98e-07 |  4.89$\sigma$
0.04 |  0/0.02 |  HE&ME&LE |  3 |  1 |  8 |  8 |  1.12$\times 10^{6}$ |  0 |  - |  -

##  3 BURST SEARCH and IDENTIFICATION

---
Figure 1:  The count rate (counts s-1 ) distribution of the background of the
three telescopes of Insigth-HXMT during blank sky observations. The blue and
purple lines represent LE (1$-$10 keV) and ME (10$-$30 keV) respectively. The
green line is for NaI detector #04 of HE (28$-$250 keV), of which the counts
of spikes are tried to be removed by HXMTDAS. NaI detector #04 could represent
other NaI detectors of HE. The HE detectors are denoted as 00$-$17 (detector #
16 is the blinded detector which is not used in this work).
---
Figure 2:  Top: Summed light curves of all NaI detectors for a burst candidate which was included in the preliminary bursts list but identified as spikes in this work. $T_{0}$ is the trigger time. The blue line is the light curve made with raw data without removing spikes (i.e. Level 1K data), while the orange line is the light curve made with the Level 1L data for which the counts of spikes were tried to be filtered out by HXMTDAS. However, the residual spike counts are still clearly seen. Bottom: The difference between the raw light curve and the light curve after removing spikes shown in the top panel. |
---|---
Figure 3:  The light curves of 18 NaI detectors of HE. Left: $T_{0}$ is the
trigger time of a burst candidate which is identified as the SGR J1935+2154
burst. Count increases are seen in all detectors, except for the blinded
detector (det #16). Right: Light curves for a burst candidate which is
identified as a spike in this work. The excess counts are only seen in a few
adjacent detectors which is the characteristics of spikes.

Figure 4:  Examples of Insight-HXMT detected bursts of SGR J1935+2154. For
each burst, light curves are shown for LE ( 1$-$10 keV, top panels), ME
(10$-$30 keV, middle panels) and HE ( 28$-$250 keV, lower panels). The
background of each burst is shown by the blue dotted line. The events of LE
data of burst #37 are lost due to the instrumental effect (e.g., bright earth,
bad events with higher grade). The HE data of burst #74 suffered from data
saturation in the time interval marked as green shadow. Burst #23 represents
weak bursts in our sample with the trigger timescale of 0.01 s and trigger
threshold of $\sim$ 5 $\sigma$.

###  3.1 Burst Search

The blind search is based on the signal-to-noise ratio (i.e. SNR) method which
has been applied to gamma-ray burst search in the HE CsI detectors of Insight-
HXMT (see Cai et al. (2021) for more details). This method is used to search
for magnetar bursts in the HE NaI detectors of Insigth-HXMT. In order to
unveil the soft and faint bursts that might not be found with only HE
detectors (which has higher energy band than that of LE and ME), we use three
telescopes (HE, ME and LE) of Insight-HXMT to jointly search for bursts (e.g.,
(Guidorzi et al., 2020a, b)), covering a series of timescales ranging from 5
ms to 40 ms, with two phase offsets ((Cai et al., 2021)). Given the limited
counts for weak bursts, Poisson statistics is assumed (e.g., (Younes et al.,
2020)).

To help to set the search strategies and to estimate the expected counts in
LE, ME and HE for substantially weak soft bursts, we simulate the Insight-HXMT
observations of the SGR J1935+2154 bursts measured by NICER. The expected
counts are calculated by multiplying the softest spectrum (burst # 222) from
NICER (see Table 2 in Younes et al. (2020), i.e. $kT=0.5_{-0.1}^{+0.1}$ keV,
$R^{2}=970_{-560}^{+1300}$ km2) with the responses of LE, ME and HE detectors.
It results in 2 counts, 0 counts and 0 counts in LE, ME and HE respectively,
with a duration of about 0.2 s, which means that there is some signal in LE
while no signal in ME and HE for such kind of soft and weak bursts (flux of
10-9 erg cm-2 s-1 in 0.5-10 keV as measured by NICER). However, given the low
background of LE (i.e. $\sim 1$ counts in 0.1 s), LE could marginally detect
such weak and soft bursts but with a low significance (less than $\sim
3\sigma$). We note that, the ratio of model predicted counts of LE and ME is
roughly to be 1 for bursts with $kT>2.5$ keV, which constitutes about $10\%$
of the NICER samples (see Table 2 in Younes et al. (2020)). However, the
spectral parameters of NICER samples are derived from a relatively limited
energy range of 1$-$10 keV.

According to the count ratios between the three telescopes (HE, ME and LE) in
the simulations shown above, some bursts with softer spectra are likely
detected by LE only. However, the background of LE is very complicated, which
is mainly composed of two parts: particle background ( $>$7 keV) and the
diffuse X-ray background ( $<$7 keV) (Liao et al., 2020a), and the background
count rate distribution (see Figure 1) is very wide and far deviating from a
simple Poisson distribution. Therefore, it is not easy to identify the SGR
J1935+2145 bursts, especially for weak bursts, only using the light curves of
LE. We leave the detailed burst search with LE data only to the future work.

Therefore, in this paper, considering the quality of the burst sample and
available data, we require that a burst should be observed by at least two
telescopes during the burst search and verification, thus three telescopes of
Insight-HXMT could be divided into four combinations: HE&ME, HE&LE, ME&LE and
HE&ME&LE. Following the experience in previous search studies (Cai et al.,
2021; Guidorzi et al., 2020a, b), all un-blinded detector units are summed
together for ME and LE, respectively, while the 17 NaI units of HE are used
separately.

The detailed search algorithms and trigger criterion are shown in Table 2,
including different combinations of three telescopes, timescales, phases, the
threshold of detector number of HE and net counts of HE, ME and LE. A burst
candidate is found when the excess in the net counts of two or more telescopes
exceeds a preset threshold of significance. Different polynomial ﬁts are
applied to the light curves of each telescope to estimate the background count
rate within a 100-s time window centered on the searched time bin. Then the
background counts of each time bin are obtained through the interpolation of
the background model.

###  3.2 Tests with Blank Sky Observations

The backgrounds of the three telescopes of Insight-HXMT are relatively
complicated due to their relatively large field of view. The LE background is
mainly caused by the particles in orbit and the diffuse cosmic X-ray
background (Liao et al., 2020a), while that of the ME is primarily contributed
by the charged particles (Guo et al., 2020). The background of HE is more
complicated than that of LE and ME, since it is dominated by the activated
isotopes (with relatively longer decay time) which is related to the passages
of SAA, together with other components including particles and albedo
scattering (Liao et al., 2020b).

In order to set a proper search criterion and estimate the significance for
each burst candidate, it is important to know how the background of the three
telescopes vary with time and the distribution of background counts on those
short timescales used in the SGR burst search. To study the background
variation, calibrate our search algorithm, and derive the false alarm
probability, we execute the same search pipeline to a group of blank sky
background observations (i.e. observations of the selected blank sky regions
without bright sources), which spans from 2017-11-02T05:00:57.000 to
2018-03-27T05:24:47.000 (a total span time of 909 hours, or 78537.6 ks), as
listed in Table 1.

According to these blank sky observations, there are significant extra-Poisson
components in the background distributions of all three telescopes (see Figure
1), which are the imprint of the complicated background behaviors. Such
complicated distributions of background indeed require one to derive the
significance (i.e. false alarm probability) of the searched burst candidate
directly from the background data with real variations rather than from the
SNR of the searched time bin ((Cai et al., 2021)).

We calibrate the trigger criterion by searching for any count excess in the
blank sky data with the same trigger threshold as used in the SGR burst
search. Detailed results are listed in Table 2, including the common good time
intervals (GTIs) of different telescopes, total numbers of false triggers
$N_{\rm FT}$ and false alarm probability ${P_{\rm FA}}$ of the blank sky at
four timescales. The false alarm probability (${P_{\rm FA}}$) is defined as,

${P_{\rm FA}}=\frac{N_{\rm FT}}{N_{\rm S}},$ (1)

where $N_{\rm FT}$ and $N_{\rm S}$ represent the numbers of false triggers and
the number of searches of different timescales and different combinations of
telescopes. The search number is calculated using the length of common good
time intervals divide by timescales and taking into account of the phase
shifts.

As shown in Table 2, based on the detailed calibration with blank sky data,
the search threshold for SGR burst is generally equivalent to at least a 5
$\sigma$ detection (Gaussian).

###  3.3 Burst Identification

In the preliminary analysis released on the website of Insight-HXMT
333http://hxmtweb.ihep.ac.cn/bursts/392.jhtml, we reported 133 burst
candidates, which triggered the blind search with rough burst identification.
In this work, we execute a refined analysis for each burst candidate with the
careful removal of various false triggers (including instrumental effects and
real bursts from other sources rather than SGR J1935+2154) and a comprehensive
processing of all the data of the three telescopes.

Spikes are short and soft pulses in the HE light curve caused by fake events
output by the readout electronics when there is a very large energy deposition
in HE detectors. The presence of spikes in the HE data is a well-known
instrumental effect and has been studied in detail (Wu et al., 2022). Here, we
find that the spike events cannot be removed completely by the current version
(version 2.04) of the HXMTDAS, which cause some false triggers, as shown in
Figure 2.

We identify each burst candidate carefully and filter out spike events based
on the following features. The spikes usually occur on only one or a few
adjacent NaI and CsI detectors (see the right panel in Figure 3) with low
energy events (i.e. the pulse height less than about channel 35) (Wu et al.,
2022), while the count increases of all non-blinded NaI detectors should be
comparable for a SGR J1935+2154 burst (see the left panel in Figure 3) since
the energy responses are similar for all non-blinded NaI detectors for the
targeted source in the pointed observations. This is an important criterion to
identify spikes. In addition, there is no spike event on LE and ME data, which
can also be used to identify SGR J1935+2154 bursts mostly triggered by HE.

Apart from the SGR bursts, the blind search could unveil various bursts from
other sources, including GRBs and TGFs (Terrestrial-Gamma Flashes). The
typical duration of short bursts from SGRs ranges from 0.1 s to 1 s (Collazzi
et al., 2015; Lin et al., 2020a). In contrast, GRBs would appear in a much
broader duration ranging from milliseconds to more than thousands of seconds.
Indeed, Insight-HXMT/HE has been served as a wide-field gamma-ray monitor for
GRBs, mainly using the CsI detectors of HE (Zhang et al., 2020; Cai et al.,
2021). We find that some short GRBs (nominally with duration less than 2 s)
would look similar to the SGR bursts in NaI detectors when they incident the
HE telescope from the front side. Thus GRBs could be a potential contamination
source to the SGR bursts. However, the spectra of GRBs usually extend from
tens of keV up to several MeV, much harder than SGR bursts, thus GRBs are
mostly detected by CsI instead of NaI and leave much more counts in CsI than
NaI detectors. This feature is opposite to the fact that SGR bursts are soft
and mostly detected by NaI and barely leave signals in CsI detectors in this
pointed observation of SGR J1935+2154. Therefore, we use the light curves of
both NaI and CsI detectors to identify and remove GRBs from the burst
candidates of SGR J1935+2154.

TGFs are short intense gamma-ray flashes produced in the lightning process in
the atmosphere of the Earth (Fishman et al., 1994; Enoto et al., 2017). Its
duration ranges from sub-millisecond to several milliseconds with a hard
spectrum in the energy range of hundreds of keV to tens of MeV (Nemiroff et
al., 1997; Briggs et al., 2013). There are a wealth of TGF detections by
Insight-HXMT (Yi et al., In Prep.). Similar to the case of GRBs, TGFs are hard
in spectrum and leave more counts in CsI than NaI detectors. Moreover, TGFs
are much shorter than typical SGR bursts. Based on these characteristics, it
is convenient to identify TGFs out of the detected bursts.

In summary, we check each burst candidate found by the blind search and filter
out the false triggers including spikes, GRBs and TGFs. We obtained a sample
of 75 bursts which are confidently identified as from SGR J1935+2154, to our
best knowledge. The burst samples are listed in Table 4, including the FRB
200428-Associated burst (i.e. burst #15).

---
Figure 5:  Burst activity evolution of SGR J1935+2154 in 1-day time bin as
seen with Insight-HXMT and Fermi/GBM (Lin et al., 2020a). The first burst in
this activity phase was observed by Swift/BAT and Fermi/GBM around
2020-04-27T18:26:20.138. The start time of Insight-HXMT observation was about
13 hours later. There is no observation from 2020-04-29T12:02:36.000 UTC to
2020-04-30T06:58:23.000 UTC, resulting in incomplete monitoring of SGR
J1935+2154 in these three days (purple dashed box) (also see Table 1). The
daily burst rate is the number of bursts divided by the effective exposure
time when the source is monitored (i.e., the source is not blocked by the
Earth and the satellite is not in SAA). The blue and orange bars represent the
burst rates of Insight-HXMT and Fermi/GBM, respectively. The red line is the
trigger time of FRB 200428-Associated burst.

##  4 Catalog Analysis AND RESULTS

As shown above, we identify a total of 75 bursts in the dedicated 33-day ToO
observation for SGR J1935+2154. We show light curves of 9 bursts in the sample
in Figure 4. The trigger time of each burst is listed in Table 4. Most bursts
are detected by all three telescopes, while a few bursts are detected by only
one or two telescopes since other telescopes data are unavailable to use (e.g.
out of the GTI).

###  4.1 Burst Activity

We define the daily burst rate ($R$, in units of bursts per day, 24 hours) as:

${R}=\frac{N}{P},$ (2)

where $N$ is the observed burst number per day and $P$ represents the
percentage of the effective observation time in a full day, excluding the time
intervals of Earth blocking of the SGR J1935+2154 and SAA passages of Insight-
HXMT, during which the instrument (observation) would be turned off.

Note that, due to the schedules of ToO observations, the observation coverage
is incomplete in three days (i.e. 2020-04-28, 2020-04-29 and 2020-04-30), for
which the burst rate ($R$) also accounts for the non-observation time
intervals.

As shown in Figure 5, the burst rate generally decreases with time during the
33-day evolution of burst activity, with a few small re-active peaks. This
Insight-HXMT dedicated long ToO observation caught up the later part of the
most active phase and monitored the entire process from very active to
inactive in burst rate of SGR J1935+2154. Thanks to the broad energy band
(from 1 keV to 250 keV) and high sensitivity, Insight-HXMT detected much more
bursts than other instruments (e.g. Fermi/GBM), providing a unique valuable
burst data set to the community.

---
Figure 6:  Illustration of burst duration. Light curves of burst # 56 are
shown in separate panels. The yellow, orange and blue shadows represent the
duration of LE, ME and HE, respectively. The red dashed lines indicate the
common duration of the three telescopes defined in this work.

|
---|---

Figure 7:  Left: The burst distribution of the whole burst sample. The best
fit log-Gaussian functions and corresponding mean values are dotted lines. The
blue, purple, black and orange lines represent the duration of LE, ME, HE and
common, respectively. The red line is FRB 200428-Associated Burst. Right: The
burst history of SGR J1935+2154 in the top panel. Other captions of the top
panel are the same as Figure 5. The scatter of duration versus their trigger
time since 2020-04-28T00:00:00 is shown in the four bottom panels. The gray
dotted lines represent the time interval of no observation of Insight-HXMT
(also see Table 1). The red star is the FRB 200428-Associated Burst. Multi-
pulse bursts are shown in red. The bursts with longer duration (larger than
350 ms, as marked by horizontal blue lines) are mostly found in the gray
shadows.

.

---
Figure 8:  The distribution of waiting time for 21 time intervals (i.e., time difference between two successive bursts). The black dashed lines show the best fit log-normal function with $\mu=572{\pm 82}$ s and $\sigma=0.44{\pm 0.06}$ (the standard deviation in the logarithm scale). The distribution of the continuous observation time length is shown in the blue shadows (right vertical axis). The percentage of the continuous observation time length above 1000 s is 87$\%$. The red line is the FRB 200428-Associated Burst, whose trigger time is used to calculate waiting time for one time interval. |
---|---
Figure 9: Left: The scatter plot of the waiting time and common duration
($T_{\rm bb}$) for 21 bursts. Right: The scatter plot of the waiting time and
fluence (see paper II) for 21 bursts. The blue and green points represent the
two bursts used to obtain the waiting time. The red star is the FRB
200428-Associated Burst, which is the latter burst of their pair bursts. The
red square is the earlier burst of the FRB 200428-Associated Burst.

###  4.2 Burst Duration

Following previous studies (Lin et al., 2013), the bursts duration ${T}_{\rm
bb}$ is calculated with the Bayesian blocks method (Scargle et al., 2013). We
perform the Bayesian blocks to measure the duration for all bursts using the
screened event data of 10-s burst time window, including both pre-burst and
post-burst regions. The blocks with a duration longer than 6 s are treated as
background, while blocks with duration less than the spin period (i.e., 3.24
s) of SGR J1935+2154 are considered as part of the burst region(Lin et al.,
2013). The background count rate is estimated with the mean rate of the
background blocks. The burst blocks with less count rate than the background
are excluded. Some bursts with multiple pulses (e.g., burst #49 in Figure 4)
have at least two subsequent blocks, along with bursts-free (or quiescent)
intervals, so the burst duration is defined as the time length from the start
of the first burst block to the end of the last block within the burst time
window, which represents the entire burst duration. As an example of the
definition of burst duration, light curves of burst # 56 are shown in Figure
6.

The duration (${T}_{\rm bb}$) of each burst measured by the three telescopes
of Insight-HXMT are listed in Tabel 4. The burst duration distributions of LE,
ME and HE are shown in the left panel of Figure 7. These distributions are
well fit with lognormal functions with the mean value of $130{\pm 25}$ ms and
$\sigma=0.62{\pm 0.08}$ for LE, $86{\pm 7}$ ms and $\sigma=0.61{\pm 0.03}$ for
ME, $102{\pm 11}$ ms and $\sigma=0.48{\pm 0.04}$ for HE. 444All errors
presented in this paper are for 1 $\sigma$ confidence level, unless otherwise
stated. We define the common time interval (duration) of LE, ME and HE, where
the start time is the maximum start time while the end time is the minimum end
time of three telescopes. The common duration is also listed in Table 4. For
bursts detected by less than three telescopes, the common duration is obtained
with the same manner. We also plot the distribution of common duration, which
can be described with a Gaussian function of $\mu=64{\pm 8}$ ms and
$\sigma=0.59{\pm 0.05}$, as shown in the left panel of Figure 7.

We also checked the evolution of the burst duration, as shown in the right
panel of Figure 7. We find that longer bursts (with duration larger than 350
ms) mostly occurred during those time intervals with higher burst rate (i.e.,
the gray shadows).

Table 3: Burst duration measured by Insight-HXMT telescopes.

Samples | Individual Sample1 | Common Sample2
---|---|---
telescope |  number of bursts |  $\mu$ (ms) |  $\sigma^{3}$ |  number of bursts |  $\mu$ (ms) |  $\sigma^{3}$
LE |  60 |  $130{\pm 25}$ |  $0.62{\pm 0.08}$ |  60 |  $130{\pm 25}$ |  $0.62{\pm 0.08}$
ME |  74 |  $86{\pm 7}$ |  $0.61{\pm 0.03}$ |  60 |  $92{\pm 6}$ |  $0.59{\pm 0.02}$
HE |  75 |  $102{\pm 11}$ |  $0.48{\pm 0.04}$ |  60 |  $119{\pm 13}$ |  $0.48{\pm 0.04}$
Common4 |  75 |  $64{\pm 8}$ |  $0.59{\pm 0.05}$ |  60 |  $78{\pm 6}$ |  $0.61{\pm 0.03}$

|
---|---
Figure 10:  Left: The scatter plot of hardness ratio versus duration. Right:
The evolution of hardness ratio of the each burst in 28$-$250 keV and 10$-$30
keV, 10$-$30 keV and 1$-$10 keV. The gray dotted lines represent the time
interval of no observation (also see Table 1). The red star is the FRB
200428-Associated Burst.

###  4.3 Burst Waiting Time

We measure the waiting time (${\delta}_{t}$) between successive bursts which
fall within an uninterrupted GTI,

${\delta}_{t}={t}_{i+1}-{t}_{i},$ (3)

where ${t}_{i+1}$ and ${t}_{i}$ represent the trigger times of ($i+1$)-th
(latter) and $i$-th (previous) bursts, respectively.

To obtain the continuous observation time intervals (i.e. without
interruption) with Insight-HXMT, we exclude the time intervals when the
satellite passes through SAA or SGR J1935+2154 is blocked by the Earth. During
those continuous observation time intervals, there are 21 waiting times
${\delta}_{t}$. The distribution of these waiting times is shown in Figure 8,
which ranges from 5.38 s to 1935.94 s and could be fitted to a log-normal
function with a peak of $572{\pm 82}$ s and $\sigma=0.44{\pm 0.06}$. The
length distribution of the continuous observation time interval is also shown
in Figure 8, which naturally provides the maximum value of the waiting time.

The scatter plot of waiting time versus $T_{\rm bb}$ and fluence (see paper
II) is shown in Figure 9. We find that there is no significant correlation
between the waiting time and the fluence or the burst duration either for the
previous burst or the latter burst.

###  4.4 Burst Hardness Ratio

The net count of each burst is estimated as:

${C}_{i}={S}_{i}-{B}_{i},$ (4)

where ${S}_{i}$ and ${B}_{i}$ represent the total counts and background counts
in the burst duration, respectively. The net counts are computed for HE, ME,
and LE, respectively.

The hardness ratio is the ratio of the net count rates in different energy
bands. We derive the hardness ratio between 10$-$30 keV to 1$-$10 keV with ME
and LE data, and hardness ratio between 28$-$250 keV to 10$-$30 keV with ME
and HE data. There are 60 bursts detected by all three telescopes of Insight-
HXMT, which could be used for hardness ratio study. The relationship between
common burst duration versus hardness ratio of these 60 bursts is shown in the
left panel of Figure 10. Hardness ratios between 10$-$30 keV to 1$-$10 keV and
between 28$-$250 keV to 10$-$30 keV of these bursts spread over a large range
from 0.52 to 25.60 and from 0.32 to 3.86, respectively. There is no
correlation between the duration and the hardness ratio. As shown in the right
panel of Figure 10, there is also no significant trend in the evolution of the
burst hardness across the observation time.

Note that there are 7 bursts (see Table 4) for which HE or LE data suffered
from data saturation (Xiao et al., 2020). Thus, the hardness ratio calculation
is not applicable for them. We report the detailed spectral analysis to study
the hardness of the bursts in paper II, considering the data saturation and
deadtime effects.

##  5 DISCUSSIONS

In the following, we compare our results to SGR J1935+2154 bursts observed by
other instruments (e.g. GBM, NICER) and other magnetars.

Insight-HXMT started this dedicated 33-day ToO observation of SGR J1935+2154
about 13 hours after the initial Fermi/GBM and Swift/BAT triggers, and
detected a total of 75 bursts with the burst daily rate (i.e. number of bursts
divided by the effective exposure time per day) varying from $\sim$ 56 bursts
day-1 to none burst, as shown in Figure 5. We note that during this
observation of SGR J1935+2154 with Insight-HXMT, only 12 bursts were detected
by Fermi/GBM (Lin et al., 2020a). There are 7 bursts jointly observed by
Insight-HXMT and Fermi/GBM. The remaining 5 bursts are invisible to Insight-
HXMT (e.g., the satellite flies through SAA, or SGR J1935+2154 is blocked by
the earth). The much higher burst rate of Insight-HXMT than Fermi/GBM is
consistent with the fact that Insight-HXMT has a much wider energy band
coverage (1 - 250 keV), was pointed to the source, and thus has much higher
sensitivity than Fermi/GBM. Although there is no burst jointly observed by
Insight-HXMT and NICER, we find that Insight-HXMT could detect most of the
bursts found by NICER (Younes et al., 2020), based on the simulations of
Insight-HXMT observations (see Section 3.1).

Regarding the burst morphology of the light curve structure, all bursts could
be generally classified to two groups: single-pulse bursts or multi-pulse
bursts. We require that multi-pulse bursts must have distinctive pulses
separated by non-emission time intervals (i.e. quiescent period). We find that
in this sample the majority of bursts (70/75, $\sim$ 93$\%$) shows single-
pulse (e.g., burst #2, burst #4 in Figure 4), and there are only 5 multi-pulse
bursts, most of which are shown in Figure 4 (burst #21, burst #49, burst #52,
burst #74). We notice that the spectra (see paper II) of the multi-pulse
bursts are similar to those of the single-pulse bursts.

The burst duration follows a log-normal distribution, as the case for SGR
J1935+2154 bursts seen by other instruments as well as the bursts from other
magnetars (e.g., (Collazzi et al., 2015; Younes et al., 2020). There are 7
bursts which are detected by both Insight-HXMT and GBM. The burst duration of
Insight-HXMT is slightly longer than that of GBM for these 7 bursts (Lin et
al., 2020a). Note that the three telescopes of Insight-HXMT have narrow fields
of view and low background instruments working in 1 to 250 keV, while
Fermi/GBM is an all-sky monitor with relatively high background operating
above 8 keV. This is likely responsible for the difference of duration between
Insight-HXMT and Fermi/GBM. We also find that the duration of LE is slightly
longer on average than ME and HE (see Table 4.2 for details), which is also
evident in the statistical properties of the whole burst sample. Nevertheless,
some bursts show interesting behavior that the HE duration is longer than that
of LE and ME (e.g. burst #52 in Figure 4). The different duration measured by
LE, ME and HE is likely a reflection of the spectral evolution. Interestingly,
we notice that the durations are different for the bursts in the periods with
different burst rates; the burst with a longer duration and with multi-pulse
tends to occur in the more active periods (see Figure 7).

The distribution of waiting time between successive SGR J1935+2154 bursts
observed by Insight-HXMT is well described by a log-Gaussian function (Figure
8), which is also found in other observations of SGR J1935+2154 and that of
other magnetars (e.g. Göğüş et al. (1999, 2000); Cheng et al. (2020)). Younes
et al. (2020) reported the waiting time distribution of SGR J1935+2154 bursts
during burst storms. Göğüş et al. (1999) showed that the waiting time of SGR
1900+14 is a log-normal function with a peak of 49 s. Göğüş et al. (2000)
showed the waiting time of SGR 1806-20 is also a log-normal function that
peaks at 97 s. The peak waiting time of SGR J1935+2154 observed by Insight-
HXMT is 571 s, much longer than those measurements mentioned above, which
mainly reflects that the SGR J1935+2154 burst rate (burst activity) is
relatively low during this ToO observation. In principle, the waiting time
between bursts depends on the burst activity, instrumental sensitivity and the
time length of continuous observation (see Figure 8).

There are three reported radio bursts from SGR J1935+2154. One was detected by
Five-hundred-metre Aperture Spherical radio telescope (FAST) (Li et al., 2018)
at 2020-04-30T21:43:00.500 UTC (Zhang et al., 2020), and the other two were
detected by Westerbork RT1 (Wb) at 2020-05-24T22:19:19.674 UT and
2020-05-24T22:19:21.070 UT, respectively (Kirsten et al., 2020). At the time
of the first radio burst of FAST, SGR J1935+2154 was visible to Insight-HXMT,
which operated as normal, while it was invisible to Insight-HXMT for the
latter two radio bursts. We check our burst list and do not find any X-ray
bursts associated with these radio bursts. We estimate the 3 $\sigma$ upper
limit flux is 2.5 $\times 10^{-9}$ erg cm-2 s-1 in the 1$-$250 keV energy
band, assuming the same spectral parameters with that of the FRB
200428-Associated burst (Li et al., 2021) and 1 s duration timescale.

##  6 Summary

In this paper, we analyze the Insight-HXMT data of the dedicated 33-day ToO
monitoring of SGR J1935+2154, initiated by the breakthrough discovery of the
X-ray burst associated with the first FRB confirmed from a magnetar. Based on
the preliminary analysis, we refine the burst search and classification, and
verify a total of 75 bursts using the available data of all three telescopes
(HE, ME and LE) of Insight-HXMT. Thanks to the broad energy band and high
sensitivity, Insight-HXMT detects a burst sample significant larger than that
of other instruments during the same observation period.

We implemented extensive studies on the statistical characteristics of these
75 bursts, including the burst duration, waiting time and hardness ratio. The
duration follows log-normal distribution with mean values of 130.22 ms, 85.52
ms and 101.95 ms for LE, ME and HE, respectively. The waiting time
distribution is well fitted with a log-normal function with the mean value of
571.53 s. Moreover, the two hardness ratios spread extensively from 0.52 to
25.60 and from 0.32 to 3.86, respectively. We find that most bursts are
single-pulsed with only 5 with multiple pulses. Those bursts with a longer
duration tend to happen when the magnetar experiences burst active periods. So
far, we do not find any X-ray bursts associated with any reported radio bursts
except for FRB 200428.

Given that there are many multi-wavelength observations to SGR J1935+2154
during April and May of 2020, this Insight-HXMT dedicated 33-day long ToO
observation provides an unprecedented data set in 1-250 keV for more detailed
analysis to depict the full picture of the burst activity evolution of SGR
J1935+2154 for both pre- and post- the generation of FRB.

Table 4: The Insight-HXMT SGR J1935+2154 burst list of the dedicated 33-day
ToO observation from April 28 to June 1, 2020.

ID | Trigger time | ${T}^{1}_{\rm bb}$ | ${T}^{2}_{\rm{st}}$ | ${T}^{3}_{\rm bb(HE)}$ | ${T}^{4}_{\rm{st}(HE)}$ | ${T}^{3}_{\rm bb(ME)}$ | ${T}^{4}_{\rm{st}(ME)}$ | ${T}^{3}_{\rm bb(LE)}$ | ${T}^{4}_{\rm{st}(LE)}$ | $C^{5}_{\rm HE}$ | $C^{5}_{\rm ME}$ | $C^{5}_{\rm LE}$
---|---|---|---|---|---|---|---|---|---|---|---|---
| in UTC | (s) | (s) | (s) | (s) | (s) | (s) | (s) | (s) | counts | counts | counts
1S1 | 2020-04-28T08:03:34.300 | 0.180 | 0.010 | 0.180 | 0.010 | 0.180 | 0.010 | 0.180 | 0.010 | 603 | 337 | 382
2 | 2020-04-28T08:05:50.080 | 0.413 | 0.026 | 0.455 | 0.024 | 0.413 | 0.026 | 0.768 | -0.010 | 431 | 271 | 587
3 | 2020-04-28T08:14:45.985 | 0.018 | 0.020 | 0.040 | 0.011 | 0.019 | 0.019 | 0.030 | 0.020 | 36 | 10 | 11
4 | 2020-04-28T09:08:44.280 | 0.186 | 0.036 | 0.308 | 0.034 | 0.186 | 0.036 | 0.186 | 0.036 | 224 | 41 | 51
5 | 2020-04-28T09:40:10.980 | 0.064 | 0.004 | 0.064 | 0.004 | 0.127 | 0.004 | 0.531 | -0.416 | 40 | 31 | 112
6 | 2020-04-28T09:46:05.300 | 0.174 | -0.137 | 0.202 | -0.142 | 0.174 | -0.137 | 0.412 | -0.178 | 58 | 16 | 84
7S2G | 2020-04-28T09:51:04.634 | 0.958 | -0.096 | 0.993 | -0.131 | 1.014 | -0.096 | 0.958 | -0.096 | 2512 | 1731 | 1770
8∗ | 2020-04-28T09:51:39.394 | 0.091 | 0.013 | 0.117 | 0.013 | 0.104 | 0.000 |  $\cdots$ |  $\cdots$ | 149 | 34 |  $\cdots$
9 | 2020-04-28T10:54:23.850 | 0.036 | 0.036 | 0.070 | 0.033 | 0.044 | 0.033 | 0.036 | 0.036 | 117 | 33 | 12
10 | 2020-04-28T11:12:58.520 | 0.052 | 0.036 | 0.130 | 0.020 | 0.052 | 0.036 | 0.180 | 0.010 | 120 | 24 | 34
11 | 2020-04-28T11:24:28.120 | 0.011 | 0.014 | 0.011 | 0.014 | 0.011 | 0.014 | 0.011 | 0.014 | 19 | 2 | 3
12∗ | 2020-04-28T11:30:36.180 | 0.015 | 0.012 | 0.015 | 0.012 | 0.015 | 0.012 |  $\cdots$ |  $\cdots$ | 44 | 2 |  $\cdots$
13 | 2020-04-28T14:20:52.519 | 0.425 | -0.003 | 0.637 | -0.003 | 0.455 | -0.033 | 0.645 | -0.036 | 182 | 126 | 524
14 | 2020-04-28T14:20:57.900 | 0.060 | 0.047 | 0.218 | 0.042 | 0.060 | 0.047 | 0.112 | 0.044 | 285 | 47 | 35
15S2 | 2020-04-28T14:34:24.150 | 1.20 | -0.20 | 1.20 | -0.20 | 1.20 | -0.20 | 1.20 | -0.20 | 6805 | 2275 | 3762
16 | 2020-04-28T17:15:26.237 | 0.070 | 0.015 | 0.085 | 0.002 | 0.083 | 0.002 | 0.164 | 0.015 | 149 | 32 | 27
17 | 2020-04-28T19:00:29.948 | 0.039 | 0.017 | 0.039 | 0.017 | 0.109 | -0.025 | 0.055 | 0.005 | 38 | 11 | 10
18 | 2020-04-28T19:01:59.850 | 0.051 | 0.041 | 0.721 | -0.500 | 0.053 | 0.039 | 0.131 | 0.041 | 585 | 82 | 84
19∗ | 2020-04-29T00:17:40.942 | 0.017 | 0.029 | 0.017 | 0.029 | 0.017 | 0.029 |  $\cdots$ |  $\cdots$ | 50 | 7 |  $\cdots$
20 | 2020-04-29T11:12:39.397 | 0.027 | 0.016 | 0.041 | 0.008 | 0.027 | 0.016 | 0.027 | 0.016 | 58 | 3 | 3
21G | 2020-04-29T11:13:57.650 | 0.700 | -0.480 | 0.758 | -0.485 | 0.788 | -0.499 | 0.700 | -0.480 | 1001 | 181 | 125
22 | 2020-04-30T09:25:22.750 | 0.054 | 0.005 | 0.054 | 0.005 | 0.054 | 0.005 | 0.054 | 0.005 | 33 | 5 | 3
23 | 2020-04-30T15:41:53.947 | 0.050 | 0.023 | 0.067 | 0.023 | 0.051 | 0.023 | 0.050 | 0.023 | 51 | 6 | 4
24 | 2020-04-30T17:12:52.837 | 0.127 | 0.012 | 0.144 | -0.002 | 0.130 | 0.009 | 0.146 | 0.012 | 367 | 123 | 113
25 | 2020-05-01T15:05:56.635 | 0.045 | 0.016 | 0.116 | 0.016 | 0.045 | 0.016 | 0.045 | 0.016 | 170 | 22 | 28
26 | 2020-05-01T15:15:20.876 | 0.018 | 0.026 | 0.040 | 0.010 | 0.024 | 0.024 | 0.018 | 0.026 | 97 | 6 | 6
27 | 2020-05-02T05:40:53.151 | 0.021 | 0.013 | 0.037 | 0.006 | 0.042 | -0.008 | 0.037 | 0.013 | 42 | 8 | 7
28 | 2020-05-02T10:17:26.000 | 0.016 | 0.035 | 0.040 | 0.033 | 0.016 | 0.035 | 0.029 | 0.035 | 167 | 38 | 27
29 | 2020-05-02T10:25:25.777 | 0.290 | 0.019 | 0.298 | 0.011 | 0.467 | 0.017 | 0.385 | 0.019 | 269 | 25 | 63
30 | 2020-05-02T10:46:20.850 | 0.053 | -0.006 | 0.053 | -0.006 | 0.053 | -0.006 | 0.053 | -0.006 | 51 | 5 | 1
31∗ | 2020-05-03T04:30:59.050 | 0.028 | 0.008 | 0.038 | 0.008 | 0.035 | 0.001 |  $\cdots$ |  $\cdots$ | 67 | 3 |  $\cdots$
32∗ | 2020-05-03T17:12:55.600 | 0.086 | -0.041 | 0.086 | -0.041 | 0.258 | -0.041 |  $\cdots$ |  $\cdots$ | 72 | 19 |  $\cdots$
33GS1∗ | 2020-05-03T23:25:13.250 | 1.216 | 0.000 | 1.216 | 0.000 | 1.216 | 0.000 |  $\cdots$ |  $\cdots$ | 5129 | 2593 |  $\cdots$
34 | 2020-05-04T00:48:07.343 | 0.255 | 0.028 | 1.076 | 0.026 | 0.256 | 0.027 | 0.317 | 0.028 | 406 | 50 | 52
35 | 2020-05-04T13:20:00.700 | 0.081 | -0.027 | 0.081 | -0.027 | 0.081 | -0.027 | 0.081 | -0.027 | 82 | 7 | 4
36∗ | 2020-05-05T02:30:28.450 | 0.040 | 0.017 | 0.040 | 0.017 | 0.040 | 0.017 |  $\cdots$ |  $\cdots$ | 39 | 3 |  $\cdots$
37∗ | 2020-05-05T12:09:29.750 | 0.028 | -0.006 | 0.028 | -0.006 | 0.028 | -0.006 |  $\cdots$ |  $\cdots$ | 93 | 15 |  $\cdots$
38 | 2020-05-06T21:25:16.350 | 0.198 | -0.127 | 0.198 | -0.127 | 0.198 | -0.127 | 0.198 | -0.127 | 189 | 25 | 8
39 | 2020-05-06T22:48:21.550 | 0.035 | 0.016 | 0.044 | 0.007 | 0.035 | 0.016 | 0.053 | 0.016 | 55 | 4 | 5
40 | 2020-05-07T21:05:41.345 | 0.028 | 0.025 | 0.202 | 0.010 | 0.046 | 0.021 | 0.033 | 0.020 | 245 | 34 | 40
41 | 2020-05-08T06:17:16.589 | 0.186 | 0.026 | 0.339 | -0.099 | 0.186 | 0.026 | 0.192 | 0.020 | 602 | 124 | 114
42 | 2020-05-08T09:17:05.185 | 0.008 | 0.023 | 0.071 | -0.022 | 0.008 | 0.023 | 0.053 | -0.004 | 77 | 3 | 10
43 | 2020-05-08T09:49:21.134 | 0.052 | 0.012 | 0.283 | 0.006 | 0.053 | 0.011 | 0.458 | 0.012 | 158 | 7 | 23
44 | 2020-05-08T19:23:36.028 | 0.039 | 0.031 | 0.041 | 0.031 | 0.040 | 0.030 | 0.039 | 0.031 | 57 | 16 | 5
45∗ | 2020-05-08T19:37:25.270 | 0.020 | 0.011 | 0.033 | 0.011 | 0.023 | 0.008 |  $\cdots$ |  $\cdots$ | 53 | 13 |  $\cdots$
46∗ | 2020-05-09T01:56:38.750 | 0.060 | 0.000 | 0.060 | 0.000 | 0.252 | -0.164 |  $\cdots$ |  $\cdots$ | 118 | 13 |  $\cdots$
47∗ | 2020-05-10T05:00:28.195 | 0.218 | -0.040 | 0.219 | -0.040 | 0.289 | -0.111 |  $\cdots$ |  $\cdots$ | 340 | 77 |  $\cdots$
48S1∗ | 2020-05-10T06:12:01.622 | 0.873 | 0.021 | 1.113 | -0.219 | 0.936 | 0.021 |  $\cdots$ |  $\cdots$ | 12208 | 7265 |  $\cdots$
49 | 2020-05-10T06:16:41.100 | 0.389 | 0.034 | 0.452 | 0.018 | 0.401 | 0.022 | 0.404 | 0.034 | 337 | 55 | 52
50 | 2020-05-10T06:20:09.400 | 0.103 | 0.019 | 0.113 | 0.014 | 0.107 | 0.015 | 0.354 | 0.019 | 199 | 47 | 62
51 | 2020-05-10T08:55:46.300 | 0.116 | 0.031 | 0.122 | 0.027 | 0.116 | 0.031 | 0.117 | 0.031 | 317 | 34 | 44
52 | 2020-05-10T18:53:01.040 | 1.008 | 0.036 | 1.180 | 0.010 | 1.110 | 0.000 | 1.008 | 0.036 | 507 | 84 | 39
53 | 2020-05-10T20:16:22.000 | 0.383 | 0.137 | 0.390 | 0.130 | 0.537 | 0.134 | 0.408 | 0.137 | 168 | 32 | 18
54S2G | 2020-05-10T21:51:16.221 | 0.700 | 0.030 | 0.700 | 0.030 | 0.700 | 0.030 | 0.700 | 0.030 | 8550 | 4726 | 2312
55 | 2020-05-10T22:08:09.000 | 0.022 | 0.026 | 0.063 | 0.025 | 0.023 | 0.025 | 0.051 | 0.026 | 77 | 15 | 12
56 | 2020-05-11T04:22:52.560 | 0.053 | 0.040 | 0.093 | 0.000 | 0.060 | 0.040 | 0.076 | 0.026 | 132 | 67 | 67
57 | 2020-05-11T17:15:43.320 | 0.120 | 0.018 | 0.120 | 0.018 | 0.219 | 0.015 | 0.139 | 0.018 | 231 | 102 | 60
58 | 2020-05-12T08:35:19.700 | 0.057 | 0.030 | 0.195 | -0.097 | 0.060 | 0.027 | 0.093 | 0.030 | 229 | 41 | 34
59 | 2020-05-12T21:47:43.340 | 0.015 | 0.002 | 0.015 | 0.002 | 0.015 | 0.002 | 0.015 | 0.002 | 28 | 2 | 3
60 | 2020-05-13T07:12:57.543 | 0.074 | 0.018 | 0.083 | 0.009 | 0.075 | 0.018 | 0.092 | 0.013 | 56 | 30 | 43
61 | 2020-05-14T14:49:22.000 | 0.346 | 0.030 | 0.350 | 0.030 | 0.397 | 0.029 | 0.347 | 0.029 | 1242 | 250 | 167
62 | 2020-05-16T01:50:23.542 | 0.099 | 0.008 | 0.123 | 0.008 | 0.101 | 0.006 | 0.134 | -0.014 | 292 | 63 | 65
63∗ | 2020-05-16T10:26:32.309 | 0.019 | 0.031 | 0.080 | 0.020 | 0.019 | 0.031 |  $\cdots$ |  $\cdots$ | 108 | 5 |  $\cdots$
64 | 2020-05-16T11:16:17.000 | 0.130 | 0.030 | 0.130 | 0.030 | 0.130 | 0.026 | 0.130 | 0.026 | 425 | 91 | 53
65 | 2020-05-16T18:12:52.080 | 0.108 | 0.033 | 0.138 | 0.033 | 0.111 | 0.030 | 0.209 | 0.031 | 674 | 191 | 141
66 | 2020-05-17T03:18:10.320 | 0.026 | 0.037 | 0.035 | 0.033 | 0.034 | 0.029 | 0.032 | 0.037 | 59 | 15 | 5
67+ | 2020-05-18T01:54:21.550 | 0.054 | 0.000 | 0.054 | 0.000 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 59 |  $\cdots$ |  $\cdots$
68 | 2020-05-18T05:17:57.715 | 0.014 | 0.024 | 0.044 | 0.020 | 0.014 | 0.024 | 0.047 | 0.024 | 101 | 6 | 13
69 | 2020-05-18T09:27:59.151 | 0.110 | -0.001 | 0.110 | -0.001 | 0.110 | -0.001 | 0.110 | -0.001 | 50 | 2 | 4
70 | 2020-05-18T11:00:41.150 | 0.008 | 0.032 | 0.221 | -0.155 | 0.011 | 0.029 | 0.028 | 0.032 | 77 | 7 | 8
71 | 2020-05-18T16:28:18.300 | 0.073 | 0.012 | 0.073 | 0.012 | 0.073 | 0.012 | 0.073 | 0.012 | 71 | 11 | 6
72G | 2020-05-19T18:57:36.300 | 0.030 | 0.020 | 0.046 | 0.016 | 0.031 | 0.019 | 0.035 | 0.020 | 359 | 68 | 50
73G | 2020-05-20T14:10:49.780 | 0.125 | 0.038 | 0.222 | -0.004 | 0.125 | 0.038 | 0.373 | -0.010 | 1013 | 208 | 171
74S1G | 2020-05-20T21:47:07.480 | 0.770 | 0.040 | 0.775 | 0.035 | 0.773 | 0.038 | 0.770 | 0.040 | 5060 | 1676 | 1219
75∗ | 2020-05-24T22:05:03.480 | 0.038 | 0.002 | 0.189 | 0.002 | 0.047 | -0.007 |  $\cdots$ |  $\cdots$ | 194 | 15 |  $\cdots$

Note. — 1 Start time, end time and total exposure time of each observation.

2 HE, ME and LE have exposures of 1650 ks, 1479 ks and 1339 ks for SGR
J1935+2154 observations respectively.

3 HE, ME and LE have exposures of 1796 ks, 1339 ks and 1123 ks for blank sky
observations used in this work.

4 Includes 8 observations from P0314005001 to P0314005008.

5 Includes many observations starting with P0101293001.

Note. — 1 The minimum net counts of each NaI detector of HE required for a
trigger.

2 The minimum number of HE detectors required for a trigger.

3 The minimum net counts for all detectors of ME or LE required for a trigger.

4 Common good time intervals of different telescopes for the blank sky
observations.

5 Number of false triggers (i.e. FT) found in the searching of the blank sky
data.

6 False alarm probability derived (formula 1) from the search results of the
blank sky data.

7 The equivalent Gaussian significance of the false alarm probability.

Note. — 1 Individual burst samples detected by LE, ME and HE respectively.

2 Common burst sample which is jointly detected by all three telescopes.

3 The standard deviation in the logarithm scale.

4 Common duration of the three telescopes.

Note. — 1 Common duration of LE, ME and HE.

2 Burst start time relative to the trigger time.

3 Duration of LE, ME and HE, respectively.

4 Burst start time of LE, ME and HE relative to the trigger time,
respectively,

5 Net counts of LE, ME and HE.

∗ LE data is unavailable due to instrumental effects (e.g., bright Earth, bad
events with higher grade).

+ LE data and ME data are unavailable due to instrumental effect (e.g., bright Earth, bad events with higher grade).

S1 The HE data of the burst suffered from data saturation.

S2 The LE and HE data of the burst suffered from data saturation.

G Bursts were also detected by Fermi/GBM.

## Acknowledgements

We thank the anonymous reviewer for very helpful comments and suggestions. We
are grateful to Shri Kulkarni for suggesting the one-month observation after
the initial ToO of SGR J1935+2154. This work is supported by the National Key
R&D Program of China (2021YFA0718500), the Strategic Priority Research Program
on Space Science, the Chinese Academy of Sciences (Grant No. XDA15360300,
XDA15052700, XDB23040400), and the National Natural Science Foundation of
China under Grants (No. U1838113, U2038106, U1938201, 12133007, 11961141013).
This work made use of the data from the Insight-HXMT mission, a project funded
by China National Space Administration (CNSA) and the Chinese Academy of
Sciences (CAS).

## References

  * Bochenek et al. (2020) Bochenek, C. D., Ravi, V., Belov, K. V., et al. 2020, Nature, 587, 59–62, doi: 10.1038/s41586-020-2872-x
  * Briggs et al. (2013) Briggs, M. S., Xiong, S., Connaughton, V., et al. 2013, Journal of Geophysical Research (Space Physics), 118, 3805, doi: 10.1002/jgra.50205
  * Cai et al. (2021) Cai, C., et al. 2021, Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stab2760
  * Cao et al. (2019) Cao, X., Jiang, W., Meng, B., et al. 2019, Sci. China Phys. Mech. Astron. 63, 249504 (2020), doi: 10.1007/s11433-019-1506-1
  * Chen et al. (2019) Chen, Y., Cui, W., Li, W., et al. 2019, Sci. China Phys. Mech. Astron. 63, 249505 (2020), doi: 10.1007/s11433-019-1469-5
  * Cheng et al. (2020) Cheng, Y., Zhang, G. Q., & Wang, F. Y. 2020, MNRAS, 491, 1498, doi: 10.1093/mnras/stz3085
  * CHIME/FRB Collaboration et al. (2020) CHIME/FRB Collaboration, Andersen, B. C., Bandura, K. M., et al. 2020, Nature, 587, 54, doi: 10.1038/s41586-020-2863-y
  * Collazzi et al. (2015) Collazzi, A. C., Kouveliotou, C., van der Horst, A. J., et al. 2015, ApJS, 218, 11, doi: 10.1088/0067-0049/218/1/11
  * Enoto et al. (2017) Enoto, T., Wada, Y., Furuta, Y., et al. 2017, Nature, 551, 481, doi: 10.1038/nature24630
  * Fishman et al. (1994) Fishman, G. J., Bhat, P. N., Mallozzi, R., et al. 1994, Discovery of intense gamma-ray flashes of atmospheric origin, NASA STI/Recon Technical Report N
  * Fletcher & Fermi GBM Team (2020) Fletcher, C., & Fermi GBM Team. 2020, GRB Coordinates Network, 27659, 1
  * Gehrels et al. (2004) Gehrels, N., Chincarini, G., Giommi, P., et al. 2004, ApJ, 611
  * Gendreau et al. (2016) Gendreau, K. C., Arzoumanian, Z., Adkins, P. W., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9905, Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, ed. J.-W. A. den Herder, T. Takahashi, & M. Bautz, 99051H, doi: 10.1117/12.2231304
  * Guidorzi et al. (2020a) Guidorzi, C., Marongiu, M., Martone, R., et al. 2020a, Astronomy & Astrophysics, 637, A69, doi: 10.1051/0004-6361/202037797
  * Guidorzi et al. (2020b) Guidorzi, C., Orlandini, M., Frontera, F., et al. 2020b, Astronomy & Astrophysics, 642, A160, doi: 10.1051/0004-6361/202039129
  * Guo et al. (2020) Guo, C.-C., Liao, J.-Y., Zhang, S., et al. 2020, Journal of High Energy Astrophysics, 27, 44, doi: 10.1016/j.jheap.2020.02.008
  * Göğüş et al. (1999) Göğüş, E., Woods, P. M., Kouveliotou, C., et al. 1999, The Astrophysical Journal, 526, L93–L96, doi: 10.1086/312380
  * Göğüş et al. (2000) —. 2000, The Astrophysical Journal, 532, L121–L124, doi: 10.1086/312583
  * Kirsten et al. (2020) Kirsten, F., Snelders, M. P., Jenkins, M., et al. 2020, Nature Astronomy, 5, 414–422, doi: 10.1038/s41550-020-01246-3
  * Kouveliotou et al. (1998) Kouveliotou, C., Dieters, S., Strohmayer, T., et al. 1998, Nature, 393, 235, doi: 10.1038/30410
  * Li et al. (2021) Li, C. K., Lin, L., Xiong, S. L., et al. 2021, Nature Astronomy, 5, 378\.  https://arxiv.org/abs/2005.11071
  * Li et al. (2018) Li, D., Wang, P., Qian, L., et al. 2018, IEEE Microwave Magazine, 19, 112, doi: 10.1109/MMM.2018.2802178
  * Li et al. (2018) Li, T., et al. 2018, Sci. China Phys. Mech. Astron., 61, 031011, doi: 10.1007/s11433-017-9107-5
  * Li et al. (2020) Li, X., Li, X., Tan, Y., et al. 2020, Journal of High Energy Astrophysics, 27, 64, doi: 10.1016/j.jheap.2020.02.009
  * Liao et al. (2020a) Liao, J.-Y., Zhang, S., Chen, Y., et al. 2020a, Journal of High Energy Astrophysics, 27, 24, doi: 10.1016/j.jheap.2020.02.010
  * Liao et al. (2020b) Liao, J.-Y., Zhang, S., Lu, X.-F., et al. 2020b, Journal of High Energy Astrophysics, 27, 14, doi: 10.1016/j.jheap.2020.04.002
  * Lin et al. (2020a) Lin, L., Göğüş, E., Roberts, O. J., et al. 2020a, The Astrophysical Journal, 902, L43, doi: 10.3847/2041-8213/abbefe
  * Lin et al. (2020b) Lin, L., Göğüş, E., Roberts, O. J., et al. 2020b, The Astrophysical Journal, 893, 156, doi: 10.3847/1538-4357/ab818f
  * Lin et al. (2013) Lin, L., Göǧüş, E., Kaneko, Y., & Kouveliotou, C. 2013, The Astrophysical Journal, 778, 105, doi: 10.1088/0004-637x/778/2/105
  * Liu et al. (2019) Liu, C. Z., Zhang, Y. F., Li, X. F., et al. 2019, Sci. China Phys. Mech. Astron. 63, 249503 (2020), doi: 10.1007/s11433-019-1486-x
  * Lyutikov (2003) Lyutikov, M. 2003, MNRAS, 346, 540, doi: 10.1046/j.1365-2966.2003.07110.x
  * Meegan et al. (2009) Meegan, C., Lichti, G., Bhat, P. N., et al. 2009, ApJ, 702, 791, doi: 10.1088/0004-637X/702/1/791
  * Mereghetti et al. (2020) Mereghetti, S., Savchenko, V., Ferrigno, C., et al. 2020, The Astrophysical Journal, 898, L29, doi: 10.3847/2041-8213/aba2cf
  * Nemiroff et al. (1997) Nemiroff, R. J., Bonnell, J. T., & Norris, J. P. 1997, J. Geophys. Res., 102, 9659, doi: 10.1029/96JA03107
  * Palmer & BAT Team (2020a) Palmer, D. M., & BAT Team. 2020a, GRB Coordinates Network, 27665, 1
  * Palmer & BAT Team (2020b) —. 2020b, GRB Coordinates Network, 27660, 1
  * Popov & Postnov (2010) Popov, S. B., & Postnov, K. A. 2010, in Evolution of Cosmic Objects through their Physical Activity, ed. H. A. Harutyunian, A. M. Mickaelian, & Y. Terzian, 129–132.  https://arxiv.org/abs/0710.2006
  * Ridnaia et al. (2021) Ridnaia, A., Svinkin, D., Frederiks, D., et al. 2021, Nature Astronomy, 5, 372, doi: 10.1038/s41550-020-01265-0
  * Scargle et al. (2013) Scargle, J. D., Norris, J. P., Jackson, B., & Chiang, J. 2013, ApJ, 764, 167, doi: 10.1088/0004-637X/764/2/167
  * Stamatikos et al. (2014) Stamatikos, M., Malesani, D., Page, K. L., & Sakamoto, T. 2014, GRB Coordinates Network, 16520, 1
  * Tavani et al. (2020) Tavani, M., Casentini, C., Ursi, A., et al. 2020, An X-Ray Burst from a Magnetar Enlightening the Mechanism of Fast Radio Bursts.  https://arxiv.org/abs/2005.12164
  * Thompson & Duncan (1995) Thompson, C., & Duncan, R. C. 1995, MNRAS, 275, 255, doi: 10.1093/mnras/275.2.255
  * Wu et al. (2022) Wu, B., Zhang, Y., Li, X., et al. 2022, The removal method and generation mechanism of spikes in Insight-HXMT/HE telescope.  https://arxiv.org/abs/2202.08997
  * Xiao et al. (2020) Xiao, S., Xiong, S. L., Liu, C. Z., et al. 2020, Journal of High Energy Astrophysics, 26, 58, doi: 10.1016/j.jheap.2020.02.003
  * Younes et al. (2017) Younes, G., Kouveliotou, C., Jaodand, A., et al. 2017, ApJ, 847, 85, doi: 10.3847/1538-4357/aa899a
  * Younes et al. (2020) Younes, G., Güver, T., Kouveliotou, C., et al. 2020, ApJ, 904, L21, doi: 10.3847/2041-8213/abc94c
  * Zhang et al. (2020) Zhang, C. F., Jiang, J. C., Men, Y. P., et al. 2020, The Astronomer’s Telegram, 13699, 1
  * Zhang et al. (2018) Zhang, S., Zhang, S., Li, T., et al. 2018, Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray, doi: 10.1117/12.2311835
  * Zhang et al. (2020) Zhang, S.-N., Li, T., Lu, F., et al. 2020, Science China Physics, Mechanics & Astronomy, 63, doi: 10.1007/s11433-019-1432-6
  * Zhang et al. (2020) Zhang, S. N., Li, X. B., Li, C. K., et al. 2020, The Astronomer’s Telegram, 13729, 1