GRB 180613A

A. Cucchiara, D. Morris (U. Virgin Islands), C. Guidorzi,
R. Martone (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell
(U. Bath), report report on behalf of a larger collaboration:


"On June 13.84 UT  (T_0 +4.5 hours) we began observing the
center of the field of GRB 180613A (Tohuvavohu et al. GCN 22769
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 ,
Beardmore et al. GCN 22773
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 ) using the Las Cumbres Observatory
1m telescope located South Africa Astronomical Observatory.

We performed a series of 5x120s observations in SDSS-i' and
SDSS-r' band for a total of 20 minutes on sky.

In the full median stacked i'-band images we clearly identified the
NIR counterpart reported by Thoene et al. (GCN #2275
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 , #22820
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 ).
This object is also marginally detected in our r' band stack.

We estimate the following brightness for the optical counterpart:

i' = 19.3 +- 0.6

We caution the usage of this measurement due to the high sky
background near the GRB location caused by the presence of the
nearby by a nearby 5.6 mag star (Schady et al. GCN #22777
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 ).

This magnitude is calibrated against nearby UCAC4 sources,
and is not corrected for Galactic extinction."

C.C. Thoene (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC,
DARK/NBI), N.R. Tanvir (Univ. of Leicester),  D.A. Kann, L. Izzo (both
HETH/IAA-CSIC), J.P.U. Fynbo (DAWN/NBI) and D.B. Malesani (DAWN/NBI,
DARK/NBI) report on behalf of the Stargate consortium:

We re-observed the field of GRB 180613A (Tohuvavohu et al., GCN 22769
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 ) with
HAWK-I/VLT on 14 June 2018 at 23:37 UT (32.02 hr after the burst). The
observation consisted of a combined exposure of 36 min in the J-band. The
seeing of this observation was worse than during the first epoch (Thoene et
al., GCN 22775
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 ) with an average value of 0.8", but the exposure time was
significantly longer, allowing us to reach a deeper limit. In this second
epoch we do not detect any more the source found by Thoene et al. (GCN
22775
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 ) down to a 3-sigma limit of J(Vega) > 24.2 mag. This implies that the
source has faded, confirming that this source was indeed the afterglow of
GRB 180613A.

We also revise and provide the full photometry of our first HAWK-I epoch
(GCN 22775
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 , Thoene et al.) and obtain the following values:
J(Vega)=23.24+/-0.26 or J(AB)=24.15+/-0.26
H(Vega)=22.23+/-0.22 or H(AB)=23.62+/-0.22
Ks(Vega)=21.04+/-0.27 or Ks(AB)=22.89+/-0.27

These values result in an IR spectral slope of 2.02 +/- 0.64, which would
indicate a significant amount of extinction, explaining the faintness of
the afterglow. Due to the lack of further data, we are not able to draw any
definite conclusions in this matter.

We acknowledge the continued excellent support from the ESO staff, in
particular Frederic Vogt, Ivan Aranda and Eleonora Sani.

S. Sebzda (PSU) and A. Tohuvavohu (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180613A
156 s after the BAT trigger (Tohuvavohu et al., GCN Circ. 22769
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 ).
No optical afterglow consistent with the XRT position
(Beardmore et al. GCN Circ. 22773
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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

u_FC               156          405          246         >20.4
v                  461        24234         1434         >20.9
b                  411        13230         1022         >99.0
u                  156        18973         2109         >21.8
w1                 510        18472         2212         >21.2
m2                 486        24474         1627         >21.0
w2                1036        23321         1318         >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.09 in the direction of the burst
(Schlegel et al. 1998).

P. Schady (MPE, Garching) reports:

We note a mistake in the position reported for source #2 in Schady et al.
(GCN #22777
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 ), which should have been

RA = 14:06:04.69
Dec = -43:04:17.7

There was also a mistake in our reference to Thoene et al. which should be
GCN�#22775, rather than GCN #2275
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 .

We apologise for any confusion that this error may have caused.

P. Schady, T. Schweyer, T.-W. Chen (all MPE Garching) and J. Bolmer (ESO
Santiago) report:

We observed the field of GRB 180613A (Swift trigger 840853; Tohuvavohu et
al., GCN #22769
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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 23:30 UT on 2018-06-13, 7.9 hrs after the GRB
trigger. They were performed at an average seeing of 1.5" and at an
average airmass of 1.1.

We do not detect the HAWKI source reported by Thoene et al. (GCN #2275
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 ).
However, we note that the larger 4.9 arcsec uncertainty on the enhanced
XRT position (Beardmore et al., GCN #22773
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 ) now includes two, brighter
sources that are below the sensitivity of the DSS images.

Source #1 lies at the SW edge of the enhanced XRT 90% error circle, at

RA = 14:06:05.24
Dec = -43:04:21.6

with a preliminary magnitude r' = 23.1 +/- 0.1 mag.

Source #2 lies at the NW edge of the enhanced XRT error circle, at

RA = 14:06:05.24
Dec = -43:04:21.6

with r' = 22.9 +/- 0.1 mag. The above positions have an accuracy of 0.1"
in both directions.

Our g' band photometry is strongly affected by a nearby 5.6 mag star
located SW of the GRB position, but source #1 is detected in all other
GROND bands. Source #2 is detected in r', i' and z'. Our observations at
the position of the XRT afterglow have preliminary 3 sigma upper limits of

r' > 23.7 mag
i' > 22.8 mag
z' > 22.7 mag
J > 21.5 mag
H > 21.0 mag
K > 20.4 mag

All quoted magnitudes are in the AB system and are calibrated against
GROND zeropoints and 2MASS field stars. They are not corrected for the
Galactic foreground extinction corresponding to a reddening of E_(B-V)=
0.08 in the direction of the burst (Schlafly & Finkbeiner 2011).

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

Using the data set from T-239 to T+625 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180613A (trigger #840853)
(Tohuvavohu et al., GCN Circ. 22769
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 ).  The BAT ground-calculated position is
RA, Dec = 211.545, -43.082 deg which is
  RA(J2000)  =  14h 06m 10.9s
  Dec(J2000) = -43d 04' 56.5"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.

The mask-weighted light curve shows some weak emission that starts
at ~T+25 s and ends at ~T+80 s. T90 (15-350 keV) is 50.8 +- 8.4 sec
(estimated error including systematics).

Because this trigger occurred when BAT was in the process of resuming to
normal operation from the recent reboot. The entire detector array had the
lower level discriminators set to a value higher than the level of normal operations.
Therefore, the standard spectral analyses are unavailable.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/840853/BA/

C. C. Thoene (HETH/IAA-CSIC), N. Tanvir (Univ. of Leicester), A. de Ugarte
Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, L. Izzo (both
HETH/IAA-CSIC), G. Pugliese (API/U. Amsterdam), P. Schady (MPE) and D. Xu
(NAO/CAS) report on behalf of the Stargate consortium:


We observed the field of GRB 180613A (GCN 22769
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 , Tohuvavohu et al.) with
HAWKI at the VLT (ESO, Paranal, Chile) starting on June 13, 2018, at 22:51
UT (7.25 hours after the burst). Imaging observations were obtained in J, H
and Ks filters with exposure times of 11-12 min under thin clouds but
excellent seeing conditions (around 0.4 arcsec) and preliminary reductions
provided by the observatory.

The object reported in GCN 22772
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  (Kann & Izzo) is clearly detected and
likely a foreground star. Inside the refined XRT error circle (GCN 22771
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 ,
Evans) we detect a faint source in all three bands at the coordinates
(errors +/-0.5 arcsec):

RA 14:06:04.89
DEC -43:04:19.6

The source seems to be slightly extended to the NW and a point-like source
at the SW end of it. This could possibly be the underlying host +
afterglow. Further observations are needed to determine a possible fading
of the source. Preliminary photometry gives J = 22.86 +/- 0.25 mag using
aperture photometry centered on the point-like part of the source and
compared to field stars from the 2MASS catalogue.

We acknowledge the excellent support from ESO staff, in particular Fr��d��ric
Vogt, Ivan Aranda and Stephane Brillant and enjoyed the great new option to
chat on Skype with the telescope during the observations.

[GCN OPS NOTE(14jun18):  Per author's request, the "days" in the 3rd line
was changed to "hours".]

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A. Kennea
(PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A. Evans (U.
Leicester) and A. Tohuvavohu report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 180613A (Tohuvavohu et al.
GCN Circ. 22769
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 ), from 155 s to 19.0 ks after the  BAT trigger. The
data comprise 95 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. 22771
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.05 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.04 (+0.17, -0.16). The
best-fitting absorption column is  1.5 (+1.1, -0.9) x 10^21 cm^-2,
consistent with the Galactic value of 6.5 x 10^20 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.45 (+0.20,
-0.19) and a best-fitting absorption column of 2.0 (+1.1, -0.9) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.0 x 10^-11 (5.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.0 (+1.1, -0.9) x 10^21 cm^-2
Galactic foreground: 6.5 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.45 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
2.05, the count rate at T+24 hours will be 2.9 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.4 x
10^-15 (1.6 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00840853.

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 615 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 180613A, 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 = 211.52057, -43.07237 which is equivalent
to:

RA (J2000): 14h 06m 4.94s
Dec (J2000): -43d 04' 20.5"

with an uncertainty of 4.9 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.

D. A. Kann and L. Izzo (HETH/IAA-CSIC) report:

Inspection of archival DSS images of the enhanced XRT position (error 
radius 2.0", Evans, GCN #22771
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 ) of GRB 180613A (Tohuvavohu et al., GCN 
#22769
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 ) reveals a known source within the XRT error circle. This source 
is known in the USNO B1.0 catalog, and is also part of Gaia DR1/2 
(source ID 6109389046377848320, with possible parallax of 0.98 +/- 0.62 
mas, Gmag = 19.748). PS1 and SDSS do not cover this location.

It is not possible to determine from the DSS2 image if this source is 
stellar or slightly extended. We suggest this source may be the host 
galaxy of this GRB, but it could also be a foreground star, as the 
stellar density of this field is quite high.

[GCN OPS NOTE(13jun18): Per author's request, "Ecans" was changed to "Evans"
in the second line.]

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 180613A, we find an
enhanced XRT position of the afterglow: RA, Dec: 211.5204, -43.0722
which is equivalent to:
   RA (J2000)  = 14 06 04.89
   Dec (J2000) = -43 04 19.9
with an uncertainty of 2.0 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/840853.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

A. Tohuvavohu (PSU), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 15:36:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180613A (trigger=840853).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 211.562, -43.076 which is 
   RA(J2000) = 14h 06m 15s
   Dec(J2000) = -43d 04' 31"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the available
BAT light curve shows no significant structure. 

The XRT began observing the field at 15:38:47.7 UT, 148.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 211.52004,
-43.07158 which is equivalent to:
   RA(J2000)  = 14h 06m 04.81s
   Dec(J2000) = -43d 04' 17.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 111 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 in excess of the Galactic value (6.50 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.5
(+3.67/-3.01) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.63e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
154 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 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. 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
0.09. 

Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.psu.edu). 
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.)