Gamma-ray Burst 180418A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

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Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 06:44:22 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     826428,   Seg_Num: 0
GRB_RA:          170.112d {+11h 20m 27s} (J2000),
                 170.355d {+11h 21m 25s} (current),
                 169.449d {+11h 17m 48s} (1950)
GRB_DEC:         +24.956d {+24d 57' 22"} (J2000),
                 +24.856d {+24d 51' 21"} (current),
                 +25.230d {+25d 13' 49"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       447 [cnts]    Image_Peak=66 [image_cnts]
TRIGGER_DUR:     0.064 [sec] 
TRIGGER_INDEX:   10418     E_range: 25-100 keV
BKG_INTEN:       20820 [cnts]
BKG_TIME:        24235.19 SOD {06:43:55.19} UT
BKG_DUR:         8 [sec]
GRB_DATE:        18226 TJD;   108 DOY;   18/04/18
GRB_TIME:        24246.07 SOD {06:44:06.07} UT
GRB_PHI:         167.52 [deg]
GRB_THETA:        14.46 [deg]
SOLN_STATUS:     0x20000003
RATE_SIGNIF:     22.84 [sigma]
IMAGE_SIGNIF:    8.81 [sigma]
MERIT_PARAMS:     +1  +0  +0  -4  +2  -1  +0  +0 +64  +0 
SUN_POSTN:        26.19d {+01h 44m 46s}  +10.83d {+10d 49' 54"}
SUN_DIST:        130.05 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       55.76d {+03h 43m 02s}  +14.72d {+14d 43' 04"}
MOON_DIST:       104.98 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.87, 69.48 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      160.82, 18.98 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 321.98,19.58 [deg].  
COMMENTS:           
COMMENTS:        NOTE: This BAT event is temporally(0.0<100sec) coincident with the FERMI_GBM event (trignum=545726651).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Wed 18 Apr 18 06:44:31 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      47
TRIGGER_NUM:     545726651
GRB_RA:          175.467d {+11h 41m 52s} (J2000),
                 175.705d {+11h 42m 49s} (current),
                 174.815d {+11h 39m 16s} (1950)
GRB_DEC:         +25.100d {+25d 06' 00"} (J2000),
                 +24.998d {+24d 59' 54"} (current),
                 +25.377d {+25d 22' 39"} (1950)
GRB_ERROR:       10.30 [deg radius, statistical plus systematic]
GRB_INTEN:       297 [cnts/sec]
DATA_SIGNIF:     13.60 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        18226 TJD;   108 DOY;   18/04/18
GRB_TIME:        24246.28 SOD {06:44:06.28} UT
GRB_PHI:         188.00 [deg]
GRB_THETA:        75.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.90
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      97%  GRB
2nd_MOST_LIKELY:   1%  Generic Transient
DETECTORS:       0,0,0, 0,0,0, 0,1,0, 0,0,1, 0,0,
SUN_POSTN:        26.19d {+01h 44m 46s}  +10.83d {+10d 49' 54"}
SUN_DIST:        133.45 [deg]   Sun_angle= -10.0 [hr] (East of Sun)
MOON_POSTN:       55.76d {+03h 43m 03s}  +14.72d {+14d 43' 06"}
MOON_DIST:       109.28 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      216.42, 74.26 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      165.44, 21.14 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2018/bn180418281/quicklook/glg_lc_medres34_bn180418281.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 297.03,10.42 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(0.0<100sec) coincident with the SWIFT_BAT event (trignum=826428).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 06:47:53 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     826428,   Seg_Num: 0
GRB_RA:          170.112d {+11h 20m 27s} (J2000),
                 170.355d {+11h 21m 25s} (current),
                 169.449d {+11h 17m 48s} (1950)
GRB_DEC:         +24.956d {+24d 57' 22"} (J2000),
                 +24.856d {+24d 51' 21"} (current),
                 +25.230d {+25d 13' 49"} (1950)
GRB_DATE:        18226 TJD;   108 DOY;   18/04/18
GRB_TIME:        24246.32 SOD {06:44:06.32} UT
TRIGGER_INDEX:   10418
GRB_PHI:         167.52 [deg]
GRB_THETA:        14.46 [deg]
DELTA_TIME:      6.00 [sec]
TRIGGER_DUR:     0.064 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     22.84 [sigma]
IMAGE_SIGNIF:    8.81 [sigma]
LC_URL:          sw00826428000msb.lc
SUN_POSTN:        26.19d {+01h 44m 47s}  +10.83d {+10d 49' 57"}
SUN_DIST:        130.05 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       55.79d {+03h 43m 11s}  +14.73d {+14d 43' 35"}
MOON_DIST:       104.95 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.87, 69.48 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      160.82, 18.98 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Lightcurve.  
COMMENTS:           
COMMENTS:        The next comments were copied from the BAT_POS Notice:  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 321.98,19.58 [deg].

GCN Circular #22646
V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), P. A. Evans (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU), M. Stamatikos (OSU/NASA/GSFC) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 06:44:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180418A (trigger=826428). Swift could not immediately slew to the burst due to an observing constraint. The BAT on-board calculated location is RA, Dec 170.112, +24.956 which is
```
   RA(J2000) = 11h 20m 27s
   Dec(J2000) = +24d 57' 22"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 1.5 sec. The peak count rate was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger.
Due to an observing constraint, Swift will not slew until T0+49.6 minutes. There will be no XRT or UVOT data until this time.
Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 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.)
GCN Circular #22647
GRB 180418A: KAIT Optical Afterglow Candidate
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 180418A (D'Elia et al., GCN 22646 starting at 06:46:41 UT, 125 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. Inside the Swift/BAT position error circle we detected an afterglow candidate at position:
```
RA:  11:20:29.20 (j2000)
Dec: 24:55:58.8  (j2000)
```
The clear band magnitude declined from Mag ~15.8 to ~17.2 during our observations. Observation are on going, multi-band follow-ups are encouraged.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 07:35:35 UT
NOTICE_TYPE:     Swift-XRT Nack-Position
TRIGGER_NUM:     826428,   Seg_Num: 0
POINT_RA:        170.071d {+11h 20m 17s} (J2000)
POINT_DEC:       +24.974d {+24d 58' 26"} (J2000)
IMG_START_DATE:  18226 TJD;   108 DOY;   18/04/18
IMG_START_TIME:  27327.50 SOD {07:35:27.50} UT,   3081.4 [sec] since BAT Trigger Time
COUNTS:          3    Min_needed= 20
STD_DEV:         0.00   Max_StdDev_for_Good=28.44 [arcsec]
PH2_ITER:        1    Max_iter_allowed= 4
ERROR_CODE:      1
COMMENTS:        SWIFT-XRT Nack Position.  
COMMENTS:        No source found in the image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 07:38:17 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     826428,   Seg_Num: 0
POINT_RA:        170.071d {+11h 20m 17s} (J2000)
POINT_DEC:       +24.974d {+24d 58' 27"} (J2000)
POINT_ROLL:      311.124d
IMG_START_DATE:  18226 TJD;   108 DOY;   18/04/18
IMG_START_TIME:  27332.24 SOD {07:35:32.24} UT,   3086.2 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        2.279
N_STARS:         14
X_OFFSET:        246 [pixels]
Y_OFFSET:        778 [pixels]
X_MAX:           1205 [pixels]
Y_MAX:           1737 [pixels]
DET_THRESH:      12
PHOTO_THRESH:    6
SL_URL:          sw00826428000msufc3086.fits
SUN_POSTN:        26.23d {+01h 44m 54s}  +10.84d {+10d 50' 41"}
SUN_DIST:        129.98 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       56.29d {+03h 45m 10s}  +14.85d {+14d 50' 49"}
MOON_DIST:       104.43 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.80, 69.44 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      160.78, 18.98 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.  
COMMENTS:        There is a bright star (mag=4.90) 11.87 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 07:38:30 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     826428,   Seg_Num: 0
POINT_RA:        170.071d {+11h 20m 17s} (J2000)
POINT_DEC:       +24.974d {+24d 58' 27"} (J2000)
POINT_ROLL:      311.124d
IMG_START_DATE:  18226 TJD;   108 DOY;   18/04/18
IMG_START_TIME:  27332.24 SOD {07:35:32.24} UT,   3086.2 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        2.279
N_STARS:         14
X_OFFSET:        246 [pixels]
Y_OFFSET:        778 [pixels]
X_MAX:           1205 [pixels]
Y_MAX:           1737 [pixels]
DET_THRESH:      12
PHOTO_THRESH:    6
SL_URL:          sw00826428000msufc3086.fits
SUN_POSTN:        26.23d {+01h 44m 54s}  +10.84d {+10d 50' 41"}
SUN_DIST:        129.98 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       56.29d {+03h 45m 10s}  +14.85d {+14d 50' 51"}
MOON_DIST:       104.43 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.80, 69.44 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      160.78, 18.98 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.  
COMMENTS:        There is a bright star (mag=4.90) 11.87 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 07:39:44 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     826428,   Seg_Num: 0
POINT_RA:        170.071d {+11h 20m 17s} (J2000)
POINT_DEC:       +24.974d {+24d 58' 27"} (J2000)
ROLL:            311.124d
IMG_START_DATE:  18226 TJD;   108 DOY;   18/04/18
IMG_START_TIME:  27332.24 SOD {07:35:32.24} UT,   3086.2 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     545729753
X_OFFSET:        565 [pixels]
Y_OFFSET:        1097 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       725
Y_GRB_POS:       1257
BINNING_INDEX:   1
IM_URL:          sw00826428000msuni3107.fits
SUN_POSTN:        26.23d {+01h 44m 55s}  +10.85d {+10d 50' 42"}
SUN_DIST:        129.98 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       56.31d {+03h 45m 13s}  +14.85d {+14d 51' 01"}
MOON_DIST:       104.42 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.80, 69.44 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      160.78, 18.98 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        There is a bright star (mag=4.90) 11.87 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 07:39:57 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     826428,   Seg_Num: 0
POINT_RA:        170.071d {+11h 20m 17s} (J2000)
POINT_DEC:       +24.974d {+24d 58' 27"} (J2000)
ROLL:            311.124d
IMG_START_DATE:  18226 TJD;   108 DOY;   18/04/18
IMG_START_TIME:  27332.24 SOD {07:35:32.24} UT,   3086.2 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     545729753
X_OFFSET:        565 [pixels]
Y_OFFSET:        1097 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       725
Y_GRB_POS:       1257
BINNING_INDEX:   1
IM_URL:          sw00826428000msuni3107.fits
SUN_POSTN:        26.23d {+01h 44m 55s}  +10.85d {+10d 50' 42"}
SUN_DIST:        129.98 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       56.31d {+03h 45m 14s}  +14.85d {+14d 51' 03"}
MOON_DIST:       104.42 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.80, 69.44 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      160.78, 18.98 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.  
COMMENTS:        There is a bright star (mag=4.90) 11.87 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 18 Apr 18 07:45:07 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     826428,   Seg_Num: 0
GRB_RA:          170.1206d {+11h 20m 28.94s} (J2000),
                 170.3630d {+11h 21m 27.12s} (current),
                 169.4570d {+11h 17m 49.67s} (1950)
GRB_DEC:         +24.9325d {+24d 55' 57.0"} (J2000),
                 +24.8321d {+24d 49' 55.6"} (current),
                 +25.2065d {+25d 12' 23.3"} (1950)
GRB_ERROR:       4.1 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  18226 TJD;   108 DOY;   18/04/18
IMG_START_TIME:  27352.00 SOD {07:35:52.00} UT,   3105.9 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:        26.23d {+01h 44m 56s}  +10.85d {+10d 50' 47"}
SUN_DIST:        130.04 [deg]   Sun_angle= -9.6 [hr] (East of Sun)
MOON_POSTN:       56.36d {+03h 45m 26s}  +14.86d {+14d 51' 47"}
MOON_DIST:       104.43 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      213.94, 69.48 [deg] galactic lon,lat of the burst
ECL_COORDS:      160.84, 18.96 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        TAM values, flux and significance fields are not valid.  
COMMENTS:        This position was automatically generated on the ground using  
COMMENTS:        Photon Counting data telemetered via TDRSS (SPER data).  
COMMENTS:        See http://www.swift.ac.uk/sper/docs.php for details.  
COMMENTS:        The probability that this is a serendipitous source in the  
COMMENTS:        SPER window is 2.8% < P(seren) < 6.6%.

GCN Circular #22648
C. Guidorzi, R. Martone (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell=20 (U. Bath), A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU) on behalf of a=20 large collaboration report:
The LCO 2-m Faulkes Telescope North (Hawaii) began observing Swift short=20 GRB 180418A (D'Elia et al. GCN 22646) on April 18, 07:18:19 UT (34=20 minutes since the GRB) with the SDSS-I filter. We clearly detect the=20 optical afterglow (Zheng et al. GCN 22647) with the following magnitude:
```
Mid time since GRB=A0=A0=A0=A0 Exp=A0=A0=A0=A0=A0=A0=A0 Filter=A0=A0=A0=A0=
=A0=A0=A0=A0 Magnitude
(hrs)=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 (s)
------------------------------------------------------------
0.62=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 5x60=A0=A0=A0=A0=
=A0=A0 SDSS-I=A0=A0=A0=A0=A0=A0=A0=A0 19.2 +- 0.1
------------------------------------------------------------
```
as calibrated against nearby SDSS objects.
GCN Circular #22649
J.P. Osborne (U. Leicester), D.N. Burrows (PSU), J.A. Kennea (PSU), G. Cusumano (INAF-IASF PA), M. Perri (ASDC) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:
The XRT began observing the field of GRB 180418A at 07:35:27.5 UT, 3081.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 170.12059, 24.93276 which is equivalent to:
```
   RA(J2000)  = 11h 20m 28.94s
   Dec(J2000) = +24d 55' 57.9"
```
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 88 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. We cannot determine whether the source is fading at the present time.
A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.07 x 10^20 cm^-2 (Willingale et al. 2013).
GCN Circular #22650
M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team.
Using 1679 s of XRT Photon Counting mode data and 4 UVOT images for GRB 180418A, 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 = 170.12153, +24.93309 which is equivalent to:
```
RA (J2000): 11h 20m 29.17s
Dec (J2000): +24d 55' 59.1"
```
with an uncertainty of 1.8 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.
GCN Circular #22652
Eleonora Troja (GSFC), 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), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jes=FAs Gonz=E1lez (UNAM), Carlos Rom=E1n-Z=FA=F1iga (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report:
We observed the field of GRB 180418A (D'Elia, et al., GCN 22646) with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio Astron=F3mico Nacional on Sierra San Pedro M=E1rtir from 2018/04 18.28 to 2018/04 18.43 UTC (1.8 minutes to 3.64 hours after the BAT trigger), obtaining a total of 2.57 hours exposure in the r and i bands.
At the position of the optical afterglow (Zheng & Filippenko, GCN 22647), in comparison with the SDSS DR9 catalog, we obtain the following detection:
=A0 r=A0=A0=A0 =3D 19.95 +/- 0.01
This magnitude is in the AB system and is not corrected for Galactic extinction in the direction of the GRB.
We thank the staff of the Observatorio Astron=F3mico Nacional in San Pedro M=E1rtir.
Further observations are planned.
GCN Circular #22655
Z. Liu (NAOC / U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and V. D'Elia report on behalf of the Swift-XRT team:
We have analysed 6.7 ks of XRT data for GRB 180418A (D'Elia et al. GCN Circ. 22646), from 3.1 ks to 31.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22650).
The light curve can be modelled with a power-law decay with a decay index of alpha=1.09 (+0.18, -0.16).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.02 (+0.28, -0.26). The best-fitting absorption column is 8.0 (+7.1, -5.8) x 10^20 cm^-2, in excess of the Galactic value of 1.1 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     8.0 (+7.1, -5.8) x 10^20 cm^-2
Galactic foreground: 1.1 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     2.02 (+0.28, -0.26)
```
If the light curve continues to decay with a power-law decay index of 1.09, the count rate at T+24 hours will be 3.4 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-13 (1.3 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00826428.
This circular is an official product of the Swift-XRT team.
GCN Circular #22656
E. Bissaldi (Politecnico & INFN Bari) and P. Veres (UAH) report on behalf of the Fermi GBM Team:
"At 06:44:06.28 UT on 18 April 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 180418A (trigger 545726651 / 180418281), which was also detected by the Swift/BAT (D'Elia et al. 2018, GCN 22646). The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time is 78 degrees.
The GBM light curve consists of a signle FRED-like peak, with a duration (T90) of about 2.5 s (50-300 keV). The time-averaged spectrum from T0-0.06 s to T0+0.77 s is adequately fit by a simple power law function with index -1.52 +/- 0.03.
The event fluence (10-1000 keV) in this time interval is (7.6 +/- 0.4)E-07 erg/cm^2. The 64-ms peak photon flux measured starting from T0+0.26 s in the 10-1000 keV band is 7.56 +/- 1.13 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
GCN Circular #22657
A. Sota, Y. Hu, J. C. Tello, I. Carrasco and A. J. Castro-Tirado (IAA-CSIC), on behalf of a larger collaboration, report: "Following the detection of GRB 180418A with the Neil Gehrels Swift Observatory (D'Elia et al. GCNC 22646), I-band observations at the 1.5m telescope at Observatorio de Sierra Nevada (Granada, Spain) have been gathered on Apr 18 starting at 19:49 UT (13 hr post burst). At the position of the optical afterglow (Zheng and Filippenko, GCNC 22647; Guidorzi et al., GCNC 22648; Troja et al., GCNC 22652), no counterpart is detected down to I = 21.0 in the coadded image (5x300s)."
GCN Circular #22658
D. M. Palmer (LANL), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), V. D'Elia (ASDC), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), J. P. Norris (BSU), 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+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 180418A (trigger #826428) (D'Elia et al., GCN Circ. 22646). The BAT ground-calculated position is RA, Dec = 170.132, 24.925 deg which is
```
  RA(J2000)  =  11h 20m 31.6s
  Dec(J2000) = +24d 55' 28.9"
```
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.
The mask-weighted light curve shows a single FRED-like pulse that starts at ~T0, peaks at ~T+0.4 s, and ends at ~T+3.5 s. T90 (15-350 keV) is 2.29 +- 0.83 sec (estimated error including systematics).
The time-averaged spectrum from T+0.10 to T+3.50 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.44 +- 0.12. The fluence in the 15-150 keV band is 3.2 +- 0.2 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+0.23 sec in the 15-150 keV band is 3.0 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
The duration and hardness of this burst show it to be intermediate between the short and long burst populations.
Using a 16-ms binned light curve, the lag analysis finds a lag of 0.1000 +/- 0.026 s for the 100-350 keV to 25-50 keV band, and 0.088 +/- 0.026 s for the 50-100 keV to 15-25 keV band. These values are also intermediate between canonical short and long GRBs.
The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/826428/BA/
GCN Circular #22659
W. Fong (Northwestern Univ.), N. R. Tanvir (U. Leicester), A. J. Levan (U. Warwick), and R. Chornock (Ohio Univ.) report on behalf of a larger collaboration:
"We observed the location of the short-duration GRB 180418A (D'Elia et al., GCN 22646) with the Gemini Multi-Object Spectrograph (GMOS) mounted on the Gemini-North 8-meter telescope. We obtained 4x120-sec each in the griz-bands at a mid-time of 2018 April 18.451 UT (4.09 hr post-burst) in 0.7-1" seeing over an airmass range of 1.2-1.5. We detect the optical afterglow (Zheng et al., GCN 22647, Guidorzi et al., GCN 22648, Troja et al., GCN 22652) in all bands with the following preliminary magnitudes:
```
g = 22.0 +/- 0.1
r = 21.6 +/- 0.2
i = 21.2 +/- 0.2
z = 21.9 +/- 0.4
```
All magnitudes are reported in the AB system, calibrated to SDSS, and not corrected for Galactic extinction. Notably, the most nearby source in projection is 13.3" from the optical afterglow position, although it is difficult to tell whether the source is point-like or extended in our images. There are otherwise no clearly-extended sources within 30" of the optical afterglow position (corresponding to 185 kpc at z~0.5) to 3-sigma limits of g>23.4 AB mag, r>22.5 AB mag.
In addition, we obtained 4x900-sec of GMOS spectroscopy of the optical afterglow at a mid-time of 2018 April 18.382 UT (2.44 hr post-burst) at an airmass of 1.1. No prominent features can be identified, beyond a blue continuum. Analysis is ongoing.
Further observations are planned. We thank the Gemini staff, and in particular observers Jason Chu and Michael Hoenig, for their assistance with these observations."
GCN Circular #22660
Daniele Malesani (DAWN/NBI and DARK/NBI), Kasper Elm Heintz (Univ. Iceland and DAWN/NBI), Maria Stone (Univ. Turku), and James Stone, report on behalf of a larger collaboration:
We observed the optical afterglow (Zheng & Filippenko, GCN 22647; Guidorzi et al., GCN 22648; Troja et al., GCN 22652; Fong et al., GCN 22659) of GRB 180418A (D'Elia et al., GCN 22646) with the 2.5-m Nordic Optical Telescope (NOT) equipped with StanCam.
Observations were carried out in the Bessel R filter, but calibrated against SDSS r-band Pan-STARRS local photometry. A low S/N source is detected at a position consistent with previous reports.
At a mid time of April 18.936 UT (15.72 hr after the trigger), we measure for the counterpart r = 22.70 +- 0.15 AB.
Compared to the Gemini-North measurement (Fong et al., GCN 22659), we infer a decay slope alpha = 0.75 +- 0.17, assuming F(t) propto t^-alpha.
GCN Circular #22661
L. P. Xin, J. Z. Yan, J. Y. Wei, Y. L. Qiu, J. S. Deng, J. Wang, X. H. Han, X. M. Meng, C. Wu, D. TURPIN, H. L. Li report:
We began to observe GRB 180418A (D'Elia et al., GCN 22646) with Xinglong 0.8-m TNT telescope, China, at 2018-04-18, 14:23:11(UT), about 7.8 hours after the burst.
We obtained 10 R-band images with an exposure time of 300 sec for each frame. The optical afterglow (Zheng & Filippenko, GCN 22647; Guidorzi et al., GCN 22648; Troja et al., GCN 22652; Fong et al., GCN 22659; Malesani et al., GCN 22660 ) was not detected in our stacked image with 10*300 sec. The 3 sigma upper limit is estimated to be about R~20 mag calibrated by USNO B1.0 R2 mag at about 8.2 hours after the burst,
We acknowledge the excellent support from Xinglong staff YunPeng Wang.
GCN Circular #22662
The field of GRB 180418A (Swift trigger 826428; D'Elia et al., GCN #22646) was observed 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 01:27 UT on 19th April, 18.7 hours after the GRB trigger. They were performed at an average seeing of 1.5" and at an average airmass of 1.7.
A source is detected in the g'r'i' bands at a position consistent with the X-ray and optical afterglow (Zheng et al., GCN #22647; Goad et al., GCN #22650). The source is not clearly point-like, and it is therefore possible that some host galaxy light is contributing to the flux. Based on 22 min of exposure in g'r'i'z' and 20 min in JHK, the estimated AB magnitudes are:
```
g' = 23.60 +/- 0.14 mag
r' = 23.24 +/- 0.13 mag
i' = 22.88 +/- 0.21 mag
z' > 22.96 mag
J > 21.16 mag
H > 20.67 mag
K > 18.82 mag
```
Given magnitudes are calibrated against SDSS for g'r'i'z' and 2MASS field stars for JHK, and they are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V)=0.01 mag in the direction of the burst (Schlafly & Finkbeiner 2011).
I thank Sam Kim for his excellent support from La Silla.
GCN Circular #22663
FROM: Kuntal Misra at ARIES,India
Kuntal Misra, Abhishek Paswan, Mridweeka Singh, S. B. Pandey, T. S. Kumar, Amitesh Omar (ARIES, Nainital, India) report
We observed the field of GRB 180418A (D=E2=80=99Elia et al. GCN 22646) with the facilities at the Devasthal Observatory, operated by ARIES India, located in the Central Himalayan region. Observations were triggered around 2018-04-18T14:41 UT (nearly 8 hours after the burst) with the 1.3 m Devasthal Fast Optical Telescope (DFOT) in r and i bands and with the newly commissioned 3.6 m Devasthal Optical Telescope (DOT) in griz bands. The afterglow is not detected in co-added images in r and i bands of 3x300 sec each taken with DFOT. However, we detect a faint source consistent with the position of the optical afterglow (Zheng et al. GCN 22647, Guidorzi et al. GCN 22648, Troja et al. GCN 22652, Fong et al. GCN 22659, Malesani et al. 22660) at a magnitude of 22.3+/-0.1 in r band with the ARIES Devasthal Faint Object Spectrograph Camera (ADFOSC) mounted on the 3.6 m DOT. The magnitudes are calibrated with respect to the nearby stars and are not corrected for Galactic extinction.
Further analysis is in progress and more observations are planned to get deeper images.
We thank the DOT operation team for successfully conducting the observations.
GCN Circular #22664
Eleonora Troja (GSFC), 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), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jes=FAs Gonz=E1lez (UNAM), Carlos Rom=E1n-Z=FA=F1iga= (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report:
We re-observed the field of GRB 180418A (D'Elia, et al., GCN 22646) with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio Astron=F3mico Nacio= nal on Sierra San Pedro M=E1rtir from 2018/04 19.13 to 2018/04 19.41 UTC (20.47 to 27.09 hours after the BAT trigger), obtaining a total of 4.17 hours exposure in the r and i bands.
The optical afterglow (Zheng & Filippenko, GCN 22647) is detected in all bands and significantly faded with respect to our first night of observations (Troja et al., GCN 22652). In comparison with the SDSS DR9 catalog, we obtain the following detections:
```
 =A0 r =3D 23.63 +/- 0.22
 =A0 i =3D 23.14 +/- 0.16
```
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=F3mico Nacional in San Pedro M=E1rtir.
GCN Circular #22665
M. H. Siegel (PSU) and V. D'Elia (ASDC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 180418A 3086 s after the BAT trigger (D'Elia et al., GCN Circ. 22646). A fading source consistent with the XRT position (Goad et al. GCN Circ. 22650) is detected in the initial UVOT exposures, confirming the previously reported optical afterglows (Zheng & Filippenko, GCN Circ. 22647; Guidorzi et al., GCN Circ. 22648; Troja et al., GCN Circ. 22652; Fong et al., GCN Circ. 22659; Malesani et al., GCN Circ. 22660; Schady et al., GCN Circ. 22662; Misra et al., GCN Circ. 22663).
The preliminary UVOT position is:
```
   RA  (J2000) =  11:20:29.21 = 170.12172 (deg.)
   Dec (J2000) = +24:55:59.2  =  24.93311 (deg.)
```
with an estimated uncertainty of 0.49 arc sec. (radius, 90% confidence).
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 (fc)        3087         3236          147         19.19+-0.11
white             3087         4470          344         19.45+-0.09
white            49806        72093         4850        >22.62
v                 3243         4835          347        >19.53
b                 4064         4264          196        >20.19
b                15510        16114          590        >20.82
u (fc)            3859         4059          196         19.35+-0.24
u                14598        31858          894        >20.73
uvw1              3655         3855          196         19.19+-0.28
uvw1              9720        27635         1206         20.02+-0.21
uvm2              3450         3649          196         18.93+-0.27
uvm2              8814        26975         2165         20.65+-0.26
uvw2              4476         4676          196        >19.69
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #22666
P. Schady and T.-W. Chen (both MPE Garching) report:
We re-observed the field of GRB 180418A (Swift trigger 826428; D'Elia et al., GCN #22646) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) on 20th April, at a mid-time of 02UT. Observations were performed at an average seeing of 1.2" and at an average airmass of 1.7.
Compared to the previous night of GROND observations (Schady, GCN #22662), the target has decayed by ~0.8dex in the r'-band. Based on around 60 minutes of data, the estimated AB magnitudes from the second epoch of GROND observations are:
```
g' = 24.5 +/- 0.2 mag
r' = 24.0 +/- 0.1 mag
i' = 23.4 +/- 0.2 mag
z' > 23.5 mag
J > 21.7 mag
H > 21.1 mag
K > 19.5 mag
```
Given magnitudes are calibrated against SDSS and 2MASS field stars, and they are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V)=0.01 mag in the direction of the burst (Schlafly & Finkbeiner 2011).
We thank Sam Kim for his excellent support from La Silla.
GCN Circular #22668
Changsu Choi (CEOU/SNU), Yongjung Kim (CEOU/SNU), Woojin Park (KHU), Suhyun Shin (CEOU/SNU) and Myungshin Im (CEOU/SNU), on behalf of a larger collaboration
We observed the field of GRB 180418A (Swift BAT trigger 826428; D'Elia et al., GCN #22646) using SQUEAN installed at 2.1m Otto Struve telescope of McDonald Observatory in US, on 18th and 19th April (UT).
The optical counter part of GRB180418A was clearly detected by observation on 18th April, starting from 2018-04-18T08:18:27 to 08:31:38 UT which is 0.065 days to 0.0742 after BAT trigger. No obvious source is found in the images taken on 2018-04-19 (UT).
The preliminary photometry results are given below.
All magnitudes are reported in AB magnitudes. The photometric calibration is based on the stars in SDSS catalog in the observed field. No galactic extinction correction is applied.
```
Date           t-T0      Filter Exp.      Magnitude Error
(UT, start)  (mid, days)            (s)     (3" aperture)
2018-04-18T08:18:27 0.065 r       60*3    20.44 0.07
2018-04-18T08:22:53 0.068 i 60*3 20.34 0.04
2018-04-18T08:25:02 0.070 r 60*6 20.47 0.06
2018-04-18T08:27:48 0.072 z 60*3 20.61 0.12
2018-04-18T08:31:38 0.075 r 60*3 20.57 0.08
```
GCN Circular #22670
T. Horiuchi, H. Hanayama, M. Honma (IAO, NAOJ), R. Itoh, K. Shiraishi, K. Murata, Y. Tachibana and N. Kawai (Tokyo Tech) report on behalf of the MITSuME and OISTER collaboration:
We observed the field of GRB 180418A (D'Elia et al.,GCN 22646) with the optical three color (g', Rc and Ic) CCD camera attached to the Murikabushi 1m telescope of Ishigakijima Astronomical Observatory.
The observation started on 2018-04-18 13:55:32 UT, (~7 hours after the burst). We detected the previously reported afterglow (W. Zheng and A. Filippenko, GCNC22647) in the R band.
The photometric results of the OT are listed below. We used UCAC-4 catalog for flux calibration.
```
#T0+[hour]  MID-UT    T-EXP[sec] g'   Rc   Ic
-------------------------------------------------------------------
8.6    15:22:01      10380  >22.8  22.4+/-0.5 >22.0
-------------------------------------------------------------------
T0+ : Elapsed time after the burst [hour]
T-EXP: Total Exposure time [sec]
```
GCN Circular #22671
Klotz A., Noysena.K., Atteia J.L. (CNRS-OMP-IRAP), Boer, M., Eymar, L. (CNRS-ARTEMIS), Gendre B. (UVI - Etelman Obs.) report:
We imaged the field of GRB 180418A detected by SWIFT (trigger 826428) with the TAROT robotic telescope (D=25cm) located at the European Southern Observatory, La Silla observatory, Chile.
The observations started 28s after the GRB trigger (15s after the notice). The elevation of the field decreased from 6 degrees above horizon and weather conditions were good.
The first image is trailed with a duration of 60.0s (see the description in Klotz et al., 2006, A&A 451, L39). Despite the very elevation we can measure the very early optical light variation of the OT reported by Zheng & Filippenko (GCNC 22647). We used NOMAD-1 1149-0186465 as reference star r(AB)=15.23 (according SDSS DR9). Preliminary analysis gives:
```
start end  r(AB) 1sig
(sec) (sec)
   28   36  14.30 0.04
   36   43  14.24 0.03
   43   51  14.40 0.03
   51   58  14.61 0.04
   58   66  14.98 0.05
   66   73  15.42 0.06
   73   81  15.52 0.09
   81   88  15.83 0.15
```
The following images are taken in tracking mode. We used the same reference star for PSF fitting:
```
start end  r(AB) 1sig
(sec) (sec)
  100  130  15.96 0.06
  141  171  16.50 0.11
  181  211  16.58 0.13
  221  251  17.10 0.31
  262  292  16.90 0.25
  302  392  17.00 0.29
```
Magnitudes are not corrected for galactic dust extinction.
We note that the maximum brightness of the OT seems to occured at 40s r(AB)=14.24 followed by a decay of about alpha=2.7 until 70s. Then the decay is compatible with alpha=1.1 until 6 min (end of TAROT observations because the elevation was too low).
GCN Circular #22697
J. Bright (Oxford), K. P. Mooley (NRAO, Caltech; Jansky Fellow), R. P. Fender (Oxford)
We observed the field of GRB 180418A (D'Elia et al., GCN 22646) with the AMI Large Array at 15.5 GHz. Our observations on 2018 Apr 18.88, Apr 20.89 and Apr 22.86 (UT) (0.61 d, 2.61 d and 4.58 post-burst) do not reveal any radio source at the XRT location (Goad et al., GCN 22650), with preliminary 3sigma upper limits of 99 uJy, 81 uJy and 93 uJy respectively.
We thank the MRAO staff for scheduling these observations. Results from the AMI short-GRB program are posted on the AMI-GRB database available at http://4pisky.org/ami-grb/.
photometric redshift z = 1.55 +0.22 -0.45
of host-galaxy: A. E. Nugent et al. 2022, ApJ, 940, 57

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