GRB 180402A

E. Mazaeva (IKI),  L. Elenin (KIAM),  A. Pozanenko (IKI), A. Volnova 
(IKI), I. Molotov (KIAM)  report on behalf of larger GRB  follow-up 
collaboration:

We observed the field of the Swift  short burst GRB 180402A (Lien et 
al., GCN 22580
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 ) with 0.4-m telescope of ISON-NM observatory starting on 
Apr., 02 (UT) 09:45:39, i.e. 40 seconds after the burst trigger. We 
obtained 4 unfiltered images of 30 s exposure each. We do not detect any 
source within XRT error circle (Lien et al., GCN 22580
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 ). Upper limits 
are following.

Date      start (Ut)   t-T0         Exp.     UL (3sigma)
                       (mid.,sec)    (s)
2018-04-02 09:45:39   55            1*30     14.5
2018-04-02 09:45:39  133            4*30     15.0

The photometry is based on nearby USNO-B1.0 stars (R2 magnitudes).

C. Malacaria (NASA-MSFC/USRA) reports on behalf of the Fermi GBM Team:

"We report the refined spectral analysis of the
short-duration GRB 180402A
(Swift-BAT detection: Lien et al., GCN Circ. 22580
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 ;
Fermi-GBM detection: Malacaria, GCN Circ. 22585
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 ;
Konus-Wind observation: Svinkin, GCN Circ. 22588
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 ).

The time-averaged spectrum from T0-0.064 s to T0+0.192 s
is well fit by a power-law function
with an exponential high-energy cutoff.

The best-fit parameters are:
Alpha = -0.36 +/- 0.10, Epeak = 1319 +/- 161 keV.
The fluence (10-1000 keV) in this time interval is
(1.21 +/- 0.06)E-06 erg/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, M. Ulanov,
A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline
on behalf of the Konus-Wind team, report:

The short-duration, hard-spectrum GRB 180402A
(Swift-BAT detection: Lien et al., GCN Circ. 22580
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 ;
Fermi-GBM detection: Malacaria, GCN Circ. 22585
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 )
triggered Konus-Wind at T0=35100.353 s UT (09:45:00.353).

The burst light curve shows bright, multi-peaked emission episode,
which starts at T0-64 ms and has a total duration of ~180 ms,
preceded by a weaker pulse at ~T0-340 ms.

As observed by Konus-Wind, the burst
had a fluence of 2.14(-0.33,+0.53)x10^-6 erg/cm2,
and a 16-ms peak flux, measured from T0+0.086 s,
of 2.03(-0.45,+0.60)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

Since the most intense part of the burst was detected before
the trigger, the spectral analysis was performed using
the KW 3-channel light curve data.

Modeling the KW 3-channel spectrum of the main episode
(from T0-0.064 s to T0+0.116 s)
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -0.01(-0.43,+0.64), and Ep = 703(-161,+276) keV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180402_T35100

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

S.W.K. Emery (UCL-MSSL) and A. Y. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180402A
140 s after the BAT trigger (Lien et al., GCN Circ. 22580
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 ).
No optical afterglow consistent with the XRT position
(D'Elia et al., GCN Circ. 22586
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 )
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC          4328         4478          147         >20.8
u_FC               140          390          246         >19.7
white             4328         6805          407         >20.6
v                  423         5916          286         >19.2
b                 5101         6736          393         >20.2
u                  140         6531          845         >20.3
w1                4691         6326          393         >20.4
w2                5512         5712          197         >19.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.77 in the direction of the burst
(Schlegel et al. 1998).

V. D'Elia (ASDC), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.
Tohuvavohu (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAB) and A.Y. Lien report on behalf of the Swift-XRT
team:

We have analysed 5.4 ks of XRT data for GRB 180402A (Lien et al. GCN
Circ. 22580
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 ), from 83 s to 18.3 ks after the  BAT trigger. The data
comprise 24 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The best available XRT position  (using the
promptly downlinked event data, the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 251.9336,
-14.9686 which is equivalent to:

RA (J2000): 16 47 44.06
Dec(J2000): -14 58 06.9

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.94 (+/-0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.46 (+0.28, -0.26). The
best-fitting absorption column is  4.6 (+2.2, -1.7) x 10^21 cm^-2,
consistent with the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 5.6 x 10^-11 (7.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.6 (+2.2, -1.7) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.46 (+0.28, -0.26)

If the light curve continues to decay with a power-law decay index of
1.94, the count rate at T+24 hours will be 1.3 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x
10^-16 (9.1 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00821103.

This circular is an official product of the Swift-XRT team.

C. Malacaria (NASA-MSFC/USRA) reports on behalf of the Fermi GBM Team:

"At 09:44:59.37 UT on 02 April 2018, the Fermi Gamma-Ray Burst
Monitor triggered and located GRB 180402A
(trigger 544355104 / 180402406).

The event was also detected by the Swift/BAT
(Lien et al. 2018, GCN 22580
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 ).
The GBM on-ground location is consistent with the Swift position.

The LAT boresight angle is 90 degrees.

The duration (T90) is 0.448 +/- 0.326 s (8-1000 keV).
The time-averaged spectrum from T0-0.1 s to T0+0.2 s is best fit
by a Band function.
The best-fit parameters are: Alpha = 2.23 +/- 2.03,
Beta = -1.35 +/- 0.04, Epeak = 123.80 +/- 44.00 keV.

The event fluence (8-1000 keV) in this time interval is
(8.69 +/- 0.62)E-07 erg/cm^2.
The 1-sec peak photon flux measured starting from T0+0 s in the
10-1000 keV band is 17.5 +/- 1.4 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

T. N. Ukwatta (LANL), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180402A (trigger #821103)
(Lien et al., GCN Circ. 22580
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 ).  The BAT ground-calculated position is
RA, Dec = 251.923, -14.928 deg which is
  RA(J2000)  =  16h 47m 41.6s
  Dec(J2000) = -14d 55' 41.6"
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 37%.

The mask-weighted light curve shows a single short pulse that starts
at ~T0, peaks at ~0.1 s, and ends at ~0.3 s T90 (15-350 keV) is 0.18 +- 0.03 sec
(estimated error including systematics). The ground analysis confirmed
that the additional variation seen in the raw light curve are indeed from
MAXI J1820+070 (Lien et al., GCN Circ. 22580
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 ).

The time-averaged spectrum from T+0.05 to T+0.26 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
0.52 +- 0.30.  The fluence in the 15-150 keV band is 1.4 +- 0.2 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.34 sec in the 15-150 keV band
is 1.8 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/821103/BA/

WeiKang Zheng and Alexei V. Filippenko (UC Berkeley) report on
behalf of the KAIT GRB team:

The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, responded to Swift GRB 180402A (Lien et al.,
GCN 22580
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  starting at 09:47:14 UT, 135 s after the burst.
Observations were performed with an automatic sequence in the
clear (roughly R), V, and I filters, and the exposure time was 20 s
per image. We do not detect any optical afterglow candidate within
the XRT position error circle (Lien et al., GCN 22580
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 ).
The typical limiting magnitude of our single clear image is about
18.5 mag calibrated to the APASS catalog.

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier
Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), Eleonora
Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jes��s Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM),
Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and
Vicki Toy (UMD) report:

We observed the field of GRB 180402A (Lien, et al., GCN 22580
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 ) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the
1.5m Harold Johnson Telescope at the Observatorio Astron��mico Nacional on
Sierra San Pedro M��rtir from 2018/04 2.41 to 2018/04 2.50 UTC (1.8 minutes
to 2.33 hours after the BAT trigger), obtaining a total of 1.76 hours
exposure in the r and i bands.

For a source within the Swift-XRT error circle, in comparison with the
USNO-B1 catalog, we obtain the following upper limits (3-sigma):

  r > 23.3
  i > 23.3

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.

C. Guidorzi, R. Martone (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell 
(U. Bath), A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU) on behalf of a 
large collaboration report:

The LCO 1-m unit at McDonald Observatory (Texas) began observing Swift 
possibly short GRB 180402A (Lien et al. GCN 22580
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 ) on April 2, 10:02 UT 
(17.3 minutes since the GRB) with the SDSS-I filter. Within the XRT 
error circle we do not detect any uncatalogued optical afterglow down to 
i'>19.3 mag (5x120 s exposure, at a mid time of 23.7 min post GRB) as 
calibrated against nearby Pan-STARRS objects.

A. Y. Lien (GSFC/UMBC), A. P. Beardmore (U Leicester),
J.D. Gropp (PSU) and K. L. Page (U Leicester) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 09:44:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180402A (trigger=821103).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 251.905, -14.963 which is 
   RA(J2000) = 16h 47m 37s
   Dec(J2000) = -14d 57' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short peak
with a duration of about 0.3 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. 
There are some additional variation in the light curve, however, 
this variation is also consistent with the presence of the 
currently-active black hole transient MAXI J1820+070 in the FOV. 
Further analysis of the full downlinked data will be required
to determine the cause of the variation. 

The XRT began observing the field at 09:46:19.2 UT, 79.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 251.9335,
-14.9683 which is equivalent to:
   RA(J2000)  = 16h 47m 44.05s
   Dec(J2000) = -14d 58' 05.8"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 100 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.94
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
139 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers none of the
XRT error circle. The 8'x8' region for the list of sources generated on-board
covers 100% of the XRT error circle. The list of sources is typically complete
to about 18.0 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.77. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)