GRB 070420

M. Stamatikos (NASA/ORAU), D. N. Burrows (PSU),
G. Cusumano (INAF-IASFPA), P. A. Evans (U Leicester),
C. Gronwall (PSU), V. Mangano (INAF-IASFPA),
C. B. Markwardt (GSFC/UMD), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Perri (ASDC) and
G. Stratta (ASDC) report on behalf of the Swift Team:

At 06:18:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070420 (trigger=276321).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 121.253, -45.561 which is 
   RA(J2000) = 08h 05m 01s
   Dec(J2000) = -45d 33' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multiply-peaked
structure with a duration of at least 70 sec starting at T-50s. 
The peak count rate was ~4000 counts/sec (15-350 keV), at ~7 sec 
after the trigger. 

XRT began observing the field at 06:19:52 UT, 99 seconds after the BAT 
trigger. XRT found a bright, fading, previously uncatalogued  X-ray source 
in the field of view at the following coordinates:
  RA(J2000) =  08h 04m 55.3s
  Dec(J2000)= -45d 33' 23.2"
with an uncertainty of 4 arcseconds radius (90% containment). This is a 
ground calculated position based on prompt downlinked data. This position 
lies 62 arcseconds from the center of the BAT error circle. 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 108 seconds after the BAT trigger.  An
afterglow candidate has been found in the initial data products.  The
2.7'x2.7' region for the list of sources generated on-board covers
100% of the XRT error circle. A source is detected at   
 RA(J2000) = 08h 04m 55.25s 
 Dec(J2000)= -45d 33' 21.0"   
with an estimated uncertainty of +/-0.5 arcsec.  This position is 
2.0 arcsec from the XRT position.  The estimated magnitude in the 
White filter is 17.3. No correction has been made for the expected 
extinction correspondingto E(B-V) of 0.52.

P. D'Avanzo, L.A. Antonelli, L. Calzoletti, S. Campana, G.  
Chincarini, M.L. Conciatore, S. Covino, S. Cutini, P. D'Avanzo, V.  
D'Elia, F. Dalessio, F.  Fiore, D. Fugazza, P. Goldoni, D. Guetta, C.  
Guidorzi, G.L. Israel,  D. Malesani, N. Masetti, A. Melandri, E.  
Meurs, L. Nicastro, E. Palazzi, E. Pian, S. Piranomonte, D. Rizzuto,  
L.  Stella, G. Stratta, G. Tagliaferri, G. Tosti, V. Testa, S.D.  
Vergani,  F. Vitali report on behalf of the REM team:


We observed the field of the GRB 070420 (Stamatikos et al. GCN 6330
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 )  
with the robotic 60-cm REM telescope located at La Silla (Chile). A   
set of observations was performed automatically in the optical and  
near infrared filters (V, R, I and J, H, K, z) starting on 2007 April  
20 at 06:19:32 UT, 42s after the Swift trigger and 79s after the GRB  
time.

Although the observations were performed under thin clouds and at  
very high airmass (~ 7.5) we clearly detect the NIR and optical  
afterglow at the coordinates reported by UVOT (GCN 6330
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 ).

Preliminary analysis gives an approximate magnitude about 5 minutes  
after the burst time was H  = 13.1 +/- 0.2.

Observations were then soon interrupted due to excessive airmass.

This message may be cited.

Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 070420 detected by SWIFT
(trigger 276321) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 34s after the GRB trigger
(8s after the notice) when the GRB was still active.
The elevation of the field decreased from
from 7 degrees above horizon and weather conditions
were good but with some thin clouds in the sky.

The date of trigger : t0 = 2007-04-20T06:18:13.824

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We do not detect the OT mentioned by Stamatikos et
al. (GCNC 6330) with a limiting magnitude of:
t0+34s to t0+94s : R > 16.1

The second image is 30.0s exposure in tracking mode.
We detect the OT:
t0+100s to t0+130s : R = 16.2

The third image is 30.0s exposure in tracking mode:
t0+136s to t0+166s : R = 16.4

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

It seems that this afterglow is very red compared to
values published by Stamatikos et al. (GCNC 6330)
and by D'Avanzo et al. GCNC 6331).

N.B. Galactic coordinates are lon=260.8667 lat= -7.4088
and the galactic extinction in R band is 0.7 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

C. Guidorzi (Uni-Bicocca/INAF-OAB), R.J. Smith, D.F. Bersier, I.A. Steele,
C.G. Mundell, S.N. Fraser, C.J. Mottram, A. Melandri,
M.F. Bode (Liverpool JMU), A. Gomboc (Ljubljana), P. O'Brien,
E. Rol, N. Tanvir (U. Leicester) report on behalf of the RoboNet
GRB collaboration:

"The 2-m Faulkes Telescope South (Australia) started observing the field of
the Swift burst GRB 070420 (Stamatikos et al. GCN Circ. 6330
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 ) about 3 hours
after the trigger time in BVRi filters.

We detect the UVOT optical transient reported by Stamatikos et al
(GCN Circ. 6330
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 ) and confirmed in IR by REM (D'Avanzo et al., GCN Circ. 
6331
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 )
in V, R and i' band filters at the following position:

RA(J2000) = 08:04:55.18
Dec(J2000)= -45:33:20.0

with an error of 0.7", and 1.2" away from the UVOT position.


Start Time   End Time    Exposure  Filter   Mag
(min)        (min)         (s)
--------------------------------------------------------
175.5        185.9        4x150      R      19.7 � 0.4
211.3        221.8        4x150      i'     19.3 � 0.5
--------------------------------------------------------

Magnitudes are calibrated with respect to USNOB1 R2 and I magnitudes.
Further observations are ongoing."

S. Immler (USRA/GSFC), F. E. Marshall (NASA/GSFC), S. T. Holland
(CRESST/USRA/GSFC), and M. Stamatikos (NASA/ORAU) report on
behalf of the Swift/UVOT team:

The UVOT position sent out as a GCN notice gave an invalid position while
GCN 6330
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  had the correct position. We regret any confusion this might have
caused. The revised UVOT position of the afterglow is
RA(J2000) = 08h04m55.17s, Dec(J2000) = -45d33'20.0".
We estimate a statistical accuracy of 0.2 arcsec radius (90% confidence)
but with a systematic uncertainty of the aspect solution of about 0.5 
arcsec.

The Swift/UVOT began taking data in the field of GRB070420 90s after
the BAT trigger (Stamatikos et al., GCN 6330
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 ). The afterglow is detected
in all UVOT optical filters. The large reddening, E(B-V) = 0.52, might 
explain
the non-detection in the UV. The flux is decaying with t**-0.7 in the White
for times between T+500 and T+10000 seconds. The photometry results are
given for the 7 UVOT filters below.

V                T+(s)          Exp(s)       Mag

White          109            99           17.4+/-0.1
                  7125           197          20.8+/-0.1
V                    90           10            17.9+/-0.1
B                  693          10            18.8+/-0.1
U                  669          20            21.1+/-0.1
UVW1         645          20            >20.4 (3-sigma)
UWM2         620          20            >20.8 (3-sigma)
UVW2             723          20            >20.8 (3-sigma)

G. Stratta (ASDC), M. Perri (ASDC), D.N. Burrows (PSU) and
M. Stamatikos (NASA/ORAU) report on behalf of the Swift XRT
team:

We have analyzed the first four orbits of Swift-XRT data on
GRB 070420 (Stamatikos et al. GCN 6330
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 ), with a total exposure
of 573 s in Windowed Timing (WT) mode and of 7.5 ks in Photon
Counting (PC) mode. This provides a refined XRT position at
RA,Dec(deg)=121.230194683,-45.5562815181  which is:

RA(J2000)=08h 04m 55.22s
Dec(J2000)=-45d 33' 22.6"

with an estimated error radius of 3.5 arcseconds (90% confidence).
This position is 62.3 arcseconds from the BAT position
(Stamatikos et al. GCN 6330
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 ), 0.8 arcseconds from the initial XRT
position (Stamatikos et al. GCN 6330
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 ) and 2.6 arcseconds from the
UVOT refined optical counterpart position
(Holland et al. GCN 6336
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 ).

The 0.3-10 keV X-ray light curve presents a steep decay between
T+106 s and T+300 s followed by a plateau phase up to about
T+2ks (end of the first orbit). The last three orbits of data
taken in PC mode show a new steepening of the light curve,
from T+5ks to T+18.5 ks (end of the third orbit).
The light curve from T+106 s to T+18.5 ks is well fit by a
double broken power law model with decay indices:
d1=-4.4+/-0.2, d2=-0.2+/-0.1 and d3=-1.5+/-0.1 and with the
first break at about t1=230s and the second break at about
t2=3200s.

The 0.3-10.0 keV X-ray spectrum from the XRT/WT data during the
decay phase is well fit by an absorbed power law with a photon
index of 2.6 +/- 0.1 and a total column density of
NH=(4.1+/-0.3)e21 cm**-2. The X-ray spectrum from the XRT/PC
data during the flat decay phase (corrected from the pileup
effects) can be fit by an absorbed power law with a photon index
of 2.0+/-0.2 and a total column density of
NH=(5.0+/-1.0)e21 cm**-2. We note that the Galactic column
density in the direction of the source is 3.7e21 cm**-2
(Dickey & Lockman 1990).

Assuming the X-ray emission continues to decline with decay index
d3, we predict a 0.3-10.0 keV XRT count rate of 1.3e-2 count/s at
T+24h and 4.7e-3 count/s at T+48h, which corresponds to an
observed 0.3-10.0 keV flux of 7e-13 erg/cm**2/s and
2.5e-13 erg/cm**2/s, respectively.

This circular is an official product of the Swift Team.

M. Jelínek (IAA-CSIC Granada), P. Tristram (MOA,
Univ. of Canterbury), A. de Ugarte Postigo, J.
Gorosabel, (IAA-CSIC), F. Hale (Earth & Sky,
Tekapo), A. Gilmore, P. Kilmartin (Univ.  of
Canterbury) and A. J. Castro-Tirado (IAA-CSIC), 
on behalf of a larger collaboration,

report:

"Following the detection by Swift of the GRB070420
(Stamatikos et al. GCNC 6330), we obtained V(26),
R (14) and I (1) -band images at the 0.6m B&C
telescope at the Mt. John Observatory on Lake
Tekapo. They were taken starting on Apr 20.33 UT
(i.e. 1.6 hr post-burst) and continued until Apr
20.51 UT (~6 hours after the burst).

Our preliminary V-band lightcurve shows a simple
powerlaw decay between 1.6 and 3.5 hours after the
burst with alpha~-1.2+-0.2 which is somewhat stee-
per than the value reported from UVOT (Immler et al.
GCNC 6336).

At 4 hours after the GRB we see a sudden rebrigh-
tening of the order of 1 magnitude in 40min above
the value extrapolated from the previous decay.

Further analysis is in progress."

This message can be quoted.

[GCN OPS NOTE(22apr07): Per author's request,
the "(Stamatikos et al. GCNC 6330)" was changed to
"(Immler et al GCNC 6336)".]

A. Parsons (GSFC), L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings
(GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/ORAU), G. Sato
(GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC) on behalf of 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 070420 (trigger #276321)
(Stamatikos, et al., GCN Circ. 6330
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 ). The BAT ground-calculated position
is RA, Dec = 121.245, -45.564 deg which is 
   RA(J2000) = 8h 4m 58.7s 
   Dec(J2000) = -45d 33' 50.6" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 8%.
 
The light curve is described by a slow rise to double-peaks at T+2 and T+8
sec, with subsequent gradual decay spanning from T-50 to T+70 sec. The T90
(15-350 keV) is 77 +- 4 sec (estimated error including systematics).
 
The time-averaged spectrum from T-29 to T+100 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.16 +- 0.24, and
Epeak of 114 +- 42 keV (chi squared 50.74 for 56 d.o.f.). For this model the
total fluence in the 15-150 keV band is 1.40 +- 0.04 x 10-5 erg/cm2 and the
1-sec peak flux measured from T+1.40 sec in the 15-150 keV band is 7.1 +-
0.8 ph/cm2/sec.  A fit to a simple power law gives a photon index of 
1.59 +- 0.05 (chi squared 61.25 for 57 d.o.f.).  All the quoted errors 
are at the 90% confidence level.

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and
T. Cline on behalf of the Konus-Wind team report:

The long GRB 070420 (Stamatikos et al., GCN 6330
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 ; Parsons et al., GCN 
6342
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 ), triggered Konus-Wind at T0=22693.706 s UT (06:18:13.706).

The burst started at T-T0 ~-50 s and had a duration of ~120 s.
As observed by Konus-Wind the burst
had a fluence of 2.64(-0.88, +0.11)x10^-5 erg/cm2,
and a 256-ms peak flux measured from T0-0.144 s
1.28(-0.29, +0.19)x10^-6 erg/cm2/s
(both in the 20 keV - 1 MeV energy range).

The spectrum of the most intense part integrated from T0 to T0+16.640 s
is well fitted (in the 20 keV - 1 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ E^(-alpha) * exp(-(2-alpha)*E/Ep)
with alpha = 1.08(-0.13, +0.12)
and Ep = 167(-16, +20) keV (chi2 = 40/52 dof).
The fluence of this part is 1.06(-0.19, +0.04)x10^5 erg/cm2.

The spectrum integrated from T0 to T0+65.792 s
is well fitted (in the 20 keV - 1 MeV range)
by a power law with exponential cutoff model
with alpha = 1.23(-0.18, +0.17)
and Ep = 147(-19, +29) keV (chi2 = 54/52 dof).

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

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB070420_T22693/

A. Pelangeon & J-L. Atteia (LATT-OMP) report:

We have used the spectral parameters of the most intense
part of GRB 070420 provided by Golenetskii et al. (GCNC 6344)
to compute the spectral pseudo-redshift** of this burst
detected by SWIFT-BAT (Stamatikos et al., GCNC 6330).

We find a pseudo-redshift pz= 1.56 +/- 0.35


** cf. http://www.ast.obs-mip.fr/grb/pz

B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 070420
(GCN 6330
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 , Stamatikos et al.) with a mid-exposure time of
2007-04-21 01:15 UT (~18.9 hrs post-burst) and again
at 2007-04-23 00:52 UT (~66.6 hrs post-burst).  For each
set of observations, total summed exposure times amounted
to 36 minutes in I and 30 minutes in J.

The afterglow of GRB 070420 (GRB 6330, Stamatikos et al.) is detected
in the first epoch images and is observed to fade between the
first and second epochs by approximately 1 magnitude.  This indicates a 
decay rate of alpha ~ 0.7 (afterglow flux proportional to t^-alpha),
which is consistent with early-time UVOT observations (GCN 6336
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 , Immler et al.).

We note that there is a source with constant brightness
only 1 arcseconds West and 0.7 arcseconds North of the afterglow of GRB 
070420. This source has magnitudes I~20 and J~18.5 (magnitudes are 
calibrated using USNO-B1.0 values in the optical and 2MASS stars in the IR.).
The source appears to have a stellar profile and is most likely 
just a coincident star.