GRB 180620B

V. Sharma, A. Vibhute and D. Bhattacharya (IUCAA), V. Bhalerao (IIT-B), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the Astrosat CZTI collaboration:

Analysis of Astrosat CZTI data showed the detection of a long GRB 180620B, which was also detected by Swift (Evans P. A. et al., GCN 22807
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 ), Fermi-GBM (Mailyan B. et al., GCN 22813
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 ), Konus-Wind (Svinkin D. et al., GCN 22825
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 ).

The source was clearly detected in the 40-200 keV energy range. The light curve shows multiple peaks of emission with strongest peak at 15:50:37.5 UT. The measured peak count rate is 259.6 cts/s above the background in combined data of four quadrants, with a total of 2643 cts. The local mean background count rate was 582.3 cts/s. Using cumulative rates, we measure a T90 of 22.3 s.

It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.

CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

A. A. Breeveld (UCL-MSSL), P. A. Evans (U. Leicester) and M. De Pasquale 
(U. Instanbul) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180620B 92 
s after the BAT trigger (Evans et al., GCN Circ. 22807
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 ).
A source consistent with the enhanced XRT position (Beardmore et al., 
GCN Circ. 22814
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 ) and the Grond position (Schweyer and Schady, GCN Circ. 
22819
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 ), and also detected by Izzo et al. (GCN Circ. 22823
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 ) is detected 
in the initial UVOT exposures in all filters except uvw2. The 
non-detection in the uvw2 filter is consistent with the redshift of 1.12 
(Izzo et al. GCN Circ. 22823
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 ).

Preliminary detections and 3-sigma upper limits using the UVOT 
photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for 
the early exposures are:

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

white               92          242          147         18.06 �� 0.08
v                  634          826           39         17.41 �� 0.22
b                  559          579           20         17.56 �� 0.18
u                  304          554          246         17.40 �� 0.09
uvw1              9950        18219         1394         20.17 �� 0.2
uvm2              6563        16382          804         20.6 �� 0.3
uvw2              4256         4443          183         > 19.7

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.01 in the direction of the 
burst (Schlegel et al. 1998).

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 long-duration GRB 180620B
(Swift-BAT detection: Evans et al., GCN Circ. 22807
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 ;
Fermi-GBM detection: Mailyan, GCN Circ. 22813
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 )
triggered Konus-Wind at T0=57039.332 s UT (15:50:39.332).

The burst light curve shows a multi-peaked structure
which starts at ~T0-156 s and has a total duration of ~260 s.
The emission is seen up to 0.8 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180620_T57039/

As observed by Konus-Wind, the burst
had a fluence of 1.19(-0.12,+0.14)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0-0.056 s,
of 1.52(-0.61,+0.72)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The spectrum integrated over the most intense
part of the burst (measured from T0 to T0+49.408 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.04(-0.31,+0.36)
and Ep = 118(-16,+23) keV (chi2 = 44/57 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 44/56 dof).

The spectrum near the maximum count rate
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
with  alpha = -0.74(-0.33,+0.37)
and Ep = 129(-15,+21) keV (chi2 = 66/57 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 66/56 dof).

Assuming the redshift z=1.1175 (Izzo et al., GCN Circ. 22823
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 )
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is ~3.9x10^52 erg,
the peak luminosity L_iso is ~1.0x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~273 keV.

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

L. Izzo (HETH/IAA-CSIC), M. Arabsalmani (CEA Saclay), D. B. Malesani (DAWN/NBI and DARK/NBI), K. Wiersema (Warwick), G. Pugliese (API/U. Amsterdam), S. D. Vergani (GEPI/Obs. Paris), A. de Ugarte Postigo (HETH/IAA-CSIC and DARK/NBI), K. E. Heintz (Univ. Iceland and DAWN/NBI), D.A. Kann (HETH/IAA-CSIC), P. Schady (MPE), N.R. Tanvir (Leicester), J.P.U. Fynbo (DAWN/NBI), C. C. Thoene (HETH/IAA-CSIC), J. Bolmer (MPE Garching), report on behalf of the Stargate Consortium:

We observed the optical afterglow of GRB 180620B (Evans et al. GCN 22807
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 ; Schweyer & Schady GCN 22819
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 ) using the ESO Very Large Telescope (VLT) UT2 equipped with the X-shooter spectrograph. Spectroscopic observations started on 09:27 UT on 21 June 2018, i.e. 0.73 days after burst. In the resulting spectrum, we find a series of strong absorption lines that we identify as Fe II, Mg II, Mg I, Zn II, Cr II, Al II and Al III at a common redshift of 1.1175. At this redshift, we also detect emission lines of [O II], [Ne III] and Balmer lines. Both absorption and emission lines show velocity structure. The absorption profiles are broad and spread over more than 800 km/s in velocity space.

We are grateful to the visiting observers, Jean-Christophe Loison and Pedro Machado, for giving up some of their observing time. We thank the ESO staff, Marcela Espinoza and Alain Smette, for excellent support.

T. N. Ukwatta (LANL), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), P. A. Evans (U Leicester),
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-179 to T+843 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180620B (trigger #843211)
(Evans et al., GCN Circ. 22807
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 ).  The BAT ground-calculated position is
RA, Dec = 357.514, -57.954 deg which is
  RA(J2000)  =  23h 50m 03.3s
  Dec(J2000) = -57d 57' 13.5"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 78%.

The mask-weighted light curve shows a multi-peaked structure that starts
at ~T-160 s and ends at ~T+160 s. The three main peaks occur at ~T0, ~T+40 s,
and ~T+100 s, respectively. Note that because the event data are only available
from ~T-180 s, there might be additional burst emission before the available
event data range. T90 (15-350 keV) is 198.8 +- 27.4 sec (estimated
error including systematics).

The time-averaged spectrum from T-159.376 to T+162.428 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.30 +- 0.20,
and Epeak of 123.9 +- 68.3 keV (chi squared 58.09 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.0 +- 0.03 x 10^-5 erg/cm2
and the 1-sec peak flux measured from T-0.33 sec in the 15-150 keV band is
3.6 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.60 +- 0.05 (chi squared 65.76 for 57 d.o.f.).  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/843211/BA/

Tassilo Schweyer and Patricia Schady (MPE Garching) report:

We observed the field of GRB 180620B (Swift trigger 843211; Evans et al.,
GCN #22807
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla
Observatory (Chile).

Observations started at 05:06 UT on 2018-06-21, 13.3 hours after the GRB
trigger. They were performed at an average seeing of 1.6" and at an
average airmass of 1.5.

We found a single, bright point source within the 1.4" Swift-XRT enhanced
error circle reported by Beardmore et al. (GCN #22814
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 ) at

RA (J2000.0) = 23:50:05.088
Dec (J2000.0) = -57:57:43.88

with an uncertainty of 0.14" in each coordinate.

Based on a total exposure time of around 30mins we estimate the following
AB magnitudes:

g' = 21.3 +/- 0.1 mag
r' = 20.8 +/- 0.1 mag
i' = 20.4 +/- 0.1 mag
z' = 20.3 +/- 0.1 mag
J = 19.7 +/- 0.1 mag
H = 19.6 +/- 0.2 mag
K = 19.1 +/- 0.4 mag

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.01 in the direction
of the burst (Schlafly & Finkbeiner 2011).

We thank Markus Rabus for the excellent support from La Silla.

A. Melandri (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), Z. Liu (NAOC / U. Leicester), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V.
D'Elia (ASDC) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 180620B (Evans et al. GCN
Circ. 22807
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 ), from 89 s to 56.0 ks after the  BAT trigger. The data
comprise 237 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 22814
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 ).

The late-time light curve (from T0+4.3 ks) can be modelled with a
power-law decay with a decay index of alpha=0.61 (+/-0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.34 (+/-0.05). The
best-fitting absorption column is  2.30 (+/-0.15) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.99 (+0.11, -0.10)
and a best-fitting absorption column of 1.6 (+/-0.3) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 7.9 sigma
Photon index:	     1.99 (+0.11, -0.10)

If the light curve continues to decay with a power-law decay index of
0.61, the count rate at T+24 hours will be 0.12 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.1 x
10^-12 (5.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00843211.

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

A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 3193 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 180620B, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 357.52122, -57.96243 which is equivalent
to:

RA (J2000): 23h 50m 5.09s
Dec (J2000): -57d 57' 44.8"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

B. Mailyan (UAH) reports on behalf of the Fermi GBM Team:

"At 15:50:36.05 UT on 20 June 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180620B (trigger 551202641 / 180620660),
which was also detected by the Swift/BAT and XRT (Evans et al. 2018, GCN
22807
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 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 137
degrees.

The GBM light curve shows multi-peaked structure
with a duration (T90) of about 46.7 s (50-300 keV).
Evans et al. 2018 report a shorter duration of
~30 s in the Swift/BAT. We suggest the discrepancy
may be due to extended hard emission of this GRB.

The time-averaged spectrum from T0 to T0+46 s
is adequately fit a power law function with an exponential
high-energy cutoff. The power law index is 0.85 +/- 0.1 and
the cutoff energy, parameterized as Epeak, is 149 +/- 11 KeV.
A Band function fits the spectrum equally well
with Epeak= 106 +/- 15 keV, alpha = -0.5 +/- 0.2 and
beta = -2.2 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(7.7 +/- 0.04)E-06 erg/cm^2. The 1-s peak photon flux measured
starting from T0 in the 10-1000 keV band is 6.8 +/- 0.5 ph/s/cm^2.

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

P. A. Evans (U Leicester), A. A. Breeveld (UCL-MSSL), A. Deich (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), M. H. Siegel (PSU) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 15:50:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180620B (trigger=843211).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 357.517, -57.957 which is 
   RA(J2000) = 23h 50m 04s
   Dec(J2000) = -57d 57' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 15:52:00.3 UT, 83.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 357.52114, -57.96306 which
is equivalent to:
   RA(J2000)  = 23h 50m 05.07s
   Dec(J2000) = -57d 57' 47.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 23 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 1.39
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 7.04e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 91 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	23:50:05.10 = 357.52126
  DEC(J2000) = -57:57:44.1  = -57.96224
with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.98 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 
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.)