GRB 070809
GCN Circular 7889
Subject
GRB 070809: Putative host galaxy and redshift
Date
2008-06-21T03:06:46Z (17 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley, J. S. Bloom, M. Modjaz, A. A. Miller, J. Shiode, J.
Brewer, D. Starr, and R. Kennedy (UC Berkeley) report:
GRB 070809 was a short-hard burst (Barthelmy et al., GCN 6788) detected
by Swift. In our observations on 2007-08-10 and 2007-08-11 we detected
a very faint optical afterglow candidate and presented evidence (at
about the 3-sigma level) for fading between the two nights. (GCNs 6739,
GCN 6774).
On the night of 2008-02-10, we re-imaged the field using Keck I (+LRIS),
again in R and g filters simultaneously, for a combined 2550s (R) and
2820s (g). We unambiguously confirm the fading behavior of the optical
transient, with no detection in either filter to R > 25.0, g > 26.3. We
rule out the presence of a host galaxy coincident with the transient
location to the same level. (An color image of the field is posted to
http://lyra.berkeley.edu/~dperley/070809/070809host.png. A comparison
between the early- and late-time imaging is available at
http://lyra.berkeley.edu/~dperley/070809/070809compare.png)
Several other short bursts with no strictly coincident host galaxy have
been found to be close in physical projection to relatively low-redshift
galaxies (e.g. Bloom et al. 2006, 2007, Stratta et al. 2007, Troja et
al. 2008). This burst is not an exception, with an edge-on spiral
galaxy centered at an offset of 5.6" to the northwest. Photometry of
this source gives magnitudes of R = 21.7 +/- 0.3, g = 22.7 +/- 0.2 (the
relatively large uncertainties are due to the extended nature of the
source and the variable background due to the presence of a nearby
bright star).
On the night of 2008-06-07 we obtained 2x900s of spectroscopy of this
source using Keck I (+LRIS), with a PA aligned with the orientation of
the galaxy. The galaxy is well contained within the 1 arcsec slit. Two
relatively bright emission lines are detected - one at 4542.0 A and one
at 6100.1 A. Associating these lines with [O II] and [O III],
respectively, the redshift of this galaxy is z=0.2187. No other
emission lines are significantly detected.
The line flux of the [O II] doublet is ~2 x 10^-16 erg/s/cm^2
(correcting for Galactic extinction of E(B-V) = 0.09), corresponding to
an uncorrected star formation rate (Kewley et al. 2002) of ~0.15
M_sun/year. As the galaxy is edge-on this is likely to be well below
the actual value. The velocity dispersion of the galaxy along the slit
axis is 110 +/- 20 km/s, which over the observed radius of 1.8" (=6.3
kpc) gives a mass of 1.8 x 10^10 M_sun. These values suggest a
relatively small spiral galaxy.
While this galaxy is not particularly massive or luminous, the close
proximity (20 kpc in projection at z=0.2187) and lack of a coincident
host is strongly suggestive of association given previous short bursts.
However, we issue several caveats:
- Some short bursts have been shown to have secure hosts at z~1 (e.g.
060801, 070429B and 070714B: Berger et al. 2007, GCN 6836, GCN7140,
Cenko et al. 2008.; see Berger 2008 for a review), and our limiting
magnitudes do not rule out relatively underluminous galaxies at this
redshift.
- One other probable galaxy is present somewhat closer to the afterglow,
a very faint (R = 24.6, g = 25.7) source 2.3" away from the OT position.
The probability of chance association of the afterglow position with
this source (if it is a galaxy) based on galaxy count/offset statistics
is higher than the probability of association with the spiral (~10% vs.
~5%), though they are comparable. The source is at unknown redshift.
- It is possible (but unlikely, GCN 6788) that this event is not a short
burst, in which case a even higher-redshift origin would not be surprising.
At a redshift of z=0.2187, the isotropic energy release for this burst
would be E_iso = 1.1 * 10^49 erg in the observed 15-150 keV band.
References:
Bloom et al. 2006 - ApJ 638,354
Bloom et al. 2007 - ApJ 654,878
Stratta et al. 2007 - A&A 474,827
Troja et al. 2008 - MNRAS 385L,10
Cenko et al. 2008 - arXiv 0802.0874
Berger et al. 2007 - ApJ 664,1000
Berger 2008 - arXiv 0805.0306
GCN Circular 6788
Subject
GRB 070809: The Swift evidence for the SHB nature of this burst
Date
2007-09-13T21:00:38Z (18 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
S.D. Barthelmy (GSFC), T. Sakamoto (GSFC), J. Norris (SLAC), N. Gehrels (GSFC)
report:
GRB 070809 (Marshall GCN 6728 and Krimm GCN 6732) is very likely
a short burst. We base this on four results:
1) The spectral lag in 25-50 to 100-350 keV bands is consistent with zero:
+11 ms +38-61 ms for 8 ms binning.
2) The T90 was 1.3 +- 0.1 sec which is in the 'short' range
of BAT burst durations (see figure 9 of the BAT1 catalog paper; Sakamoto et al.,
accepted in ApJS, arXiv:0707.4626)
3) The fluence hardness ratio S(50-100 keV)/S(25-50 keV) is
1.24 +- 0.27 (90% error), which is consistent with the BAT short GRB
population (see figure 10 of the BAT1 catalog paper; Sakamoto et al.,
accepted in ApJS, arXiv:0707.4626).
4) The XRT light curve showed a flat phase often seen with long bursts.
However, some short bursts also have such a phase (GRB 050724 and 051221A).
GCN Circular 6774
Subject
GRB 070809: Confirmation of optical transient
Date
2007-09-08T22:39:45Z (18 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley (UC Berkeley), C. C. Thoene (DARK, UCB), and J. S. Bloom
(UCB) report:
After our initial observations (GCN 6739) of the possibly short GRB
070809 (Palmer et al., GCN 6728), we obtained a second series of imaging
the following night using the Keck I telescope (+LRIS).
We imaged the field for a total of 640s (R) and 880s (g) simultaneously
in a series of images starting at 06:20:17 UT (2007-08-11) at very high
airmass.
The point-like source reported in GCN 6739 visible the first night has
significantly faded, and is detectable only at the 3 sigma level in
R-band and is undetected in g-band. Using an absolute calibration
relative to Landolt standard stars, we calculate magnitudes of this
source of:
UT (mid-time) t_burst (hr) magnitude
2007-08-10 06:35 11.21 R = 24.1 +/- 0.2
g = 25.7 +/- 0.2
2007-08-11 06:31 35.14 R = 25.0 +/- 0.3
g > 26.0 (3-sigma)
We caution that the source is very close to a bright (R~13) star and
slightly blended with a diffraction spike in our imaging. This and the
very high airmass of the observations may create additional
uncertainties in the photometry.
In addition, we acquired a 2x600s spectrum of the nearby edge-on galaxy
noted in GCN 6739 during twilight starting at 05:46 UT (2007-08-11),
covering the wavelength range from about 3800 to 9200 AA. No obvious
emission lines are detected.
GCN Circular 6751
Subject
GRB 070809: Swift/UVOT Corrected Photometry
Date
2007-08-10T20:54:59Z (18 years ago)
From
Margaret Chester at PSU <chester@astro.psu.edu>
M.M. Chester (PSU) and F.E. Marshall (NASA/GSFC) report on behalf of the
Swift UVOT team:
We have identified an additional exposure of GRB 070809 made by the
Swift/UVOT that covers the XRT error circle, and can now provide an
optical upper limit at a much earlier time, 74 seconds after the BAT
trigger. In addition, an error in the photometry analysis previously
reported (Chester et al., GCN Circ. 6744) has been identified, and
corrected upper limits are reported below.
Filter Tstart Tstop Exp Magnitude
(s) (s) (s) (3-sigma UL)
v (FC) 74 474 394 19.1
v 6344 18560 1251 20.2
b 5932 24593 614 20.4
u 5727 24363 1911 20.6
uvw1 5523 23449 2164 20.6
uvm2 5317 19316 2018 21.0
uvw2 6139 17646 1279 20.6
GCN Circular 6744
Subject
GRB 070809: Swift/UVOT Upper Limits
Date
2007-08-10T16:10:59Z (18 years ago)
From
Margaret Chester at PSU <chester@astro.psu.edu>
M.M. Chester (PSU) and F.E. Marshall (NASA/GSFC) report on behalf
of the Swift UVOT team:
The Swift/UVOT observed the field of GRB 070809 (Marshall et al., GCN
Circ. 6728) beginning 2007-08-09, 20:50:54, 5317 seconds after the BAT
trigger.
We do not find any new source in any of the UVOT observations inside the
Swift/XRT error circle (RA,DEC(J2000)= 203.7708, -22.1408).
The 3-sigma upper limits for detecting a source inside the XRT error
circle in the co-added frames are:
Filter Tstart Tstop Exp Magnitude
(s) (s) (s) (3-sigma UL)
v 6344 18560 1251 20.8
b 5932 24593 614 21.3
u 5727 24363 1911 21.7
uvw1 5523 23449 2164 21.7
uvm2 5317 19316 2018 21.7
uvw2 6139 17646 1279 21.6
The values quoted above are not corrected for the expected Galactic
extinction corresponding to a reddening of E_{B-V} = 0.09 mag in the
direction of the burst (Schlegel et al. 1998).
GCN Circular 6739
Subject
GRB 070809: Keck Imaging
Date
2007-08-10T11:11:05Z (18 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley (UC Berkeley), C. C. Thoene (DARK, UCB), J. Cooke (UC
Irvine), J. S. Bloom (UCB), and E. Barton (UCI) report:
Starting at 06:21:52 UT (2007-08-10), we imaged the field of GRB 070809
(GCN 6728), a short or intermediate-duration (GCN 6732) burst, with the
Keck I telescope (+LRIS). We obtained under very high airmass (>2.6)
and intervening cirrus 4x300 seconds of imaging in R band and 4x330
seconds in g band.
At the edge of the refined XRT error circle (GCN 6737) we detect a
single, faint source in both filters, with coordinates:
RA = 13:35:04.55
dec = -22:08:30.8
(err: 0.4")
Relative to the USNO B1.0 star at 13:35:09.80 -22:09:15.9 (R2=19.55) we
measure a preliminary magnitude of R~24.0. It is not possible to tell
whether the object is fading, but it does not appear visibly extended.
We note also the presence of an edge-on galaxy near the error circle, at
coordiates of 13:35:04.25, -22:08:26.8.
No other objects are present in or near the error circle to a limiting
magnitude of R~25.5.
An image of the field is available at
http://lyra.berkeley.edu/~dperley/070809/070809keckR.png .
GCN Circular 6737
Subject
GRB 070809: Swift-XRT refined analysis
Date
2007-08-10T08:44:26Z (18 years ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
P.A. Evans, K.L. Page (U. Leicester) and F. E. Marshall (NASA/GSFC) report on
behalf of the Swift-XRT team:
We have analysed the first 11 ks of Swift XRT data for GRB 070809
(Marshall et al. GCN Circ 6728), all of which is in Photon Counting (PC)
mode. We find a refined XRT position of RA, Dec = 203.7699, -22.1416
degrees, which is equivalent to:
RA (J2000) = 13h 35m 4.78s
Dec(J2000) = -22d 08' 29.8"
with an estimated uncertainty of 3.5 arcsec (radius, 90% containment
including systematics). This is 81.9" from the refined BAT position
(Krimm et al. GCN Circ 6732) and 4.2" from the XRT position reported in
GCN Circ 6728.
The XRT light curve shows fading behaviour which is well fitted by a
power-law with two breaks. The initial decay followed a slope of
alpha=2.3 (+0.7/-1.1), and then broke at ~T0+200 s to a shallow decay
with alpha=0.28 (+/-0.13). The shallow phase ended at approximately
T0+10000 s and the current decay follows a slope of alpha=1.35
(+0.80/-0.72).
The PC mode spectrum from T0+85-25000 s is well fitted by an absorbed
power-law with a column density of 6.7e20 (+7.0e20/-5.3e20) cm^-2,
consistent with the Galactic value of 6.40e20 cm^-2 (Kalberla et al.
2005). The spectral slope was gamma=1.49 (+0.25/-0.22), consistent with
the BAT value (GCN Circ 6732). The observed (unabsorbed) 0.3-10 keV flux
is 1.64e-12 (1.79e-12) erg cm^-2 s^-1.
If the source continues to decay with alpha=1.35, we predict a count
rate of 1.7e-3 counts s^-1 at T0+24 hours, which corresponds to an
observed (unabsorbed) flux of 1.09e-13 (1.20e-13) erg cm^2 s^-1.
This circular is an official product of the Swift/XRT team.
GCN Circular 6732
Subject
GRB 070809, Swift-BAT refined analysis
Date
2007-08-10T01:46:26Z (18 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
H. Krimm (GSFC/USRA), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
C. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
Using the data set from T-120 to T+183 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 070809 (trigger #287344)
(Marshall, et al., GCN Circ. 6728). The BAT ground-calculated position is
RA, Dec = 203.767, -22.119 deg which is
RA(J2000) = 13h 35m 4.2s
Dec(J2000) = -22d 7' 7"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 74%.
The mask-weighted light curve consists of a single peak starting at ~T-0.3
and ending at ~T+1.2 sec. T90 (15-350 keV) is 1.3 +- 0.1 sec (estimated error
including systematics).
The time-averaged spectrum from T-0.4 to T+1.1 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.69 +- 0.22. The fluence in the 15-150 keV band is 1.0 +- 0.1 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.08 sec in the 15-150 keV band
is 1.2 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence
level. We note that the T90-hardness_ratio for this burst falls in between
the SHB and LSB clusters in the T90-HR scatter plot for Swift-BAT.
GCN Circular 6731
Subject
GRB 070809: Optical Pre-imaging from Palomar
Date
2007-08-09T22:55:44Z (18 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley (UC Berkeley), P. Nugent (LBL), and J. S. Bloom (UCB) report:
We have created a stacked image through the co-addition of 8
unfiltered images taken by the NEAT collaboration and 13 images in
the RG610 filter taken by the Palomar-Quest Consortium at the Palomar
Oschin Schmidt telescope (obtained from 2002-2006) of GRB 070809
(Marshall et al., GCN 6728), possibly a short-hard burst.
No object is detected within the XRT error circle (GCN 6728) to a
limiting magnitude (3-sigma) of R~22.3.
This message may be cited.
GCN Circular 6729
Subject
GRB 070809: ROTSE-III Optical Limits
Date
2007-08-09T19:56:11Z (18 years ago)
From
Eli Rykoff at U of Michigan/ROTSE <erykoff@umich.edu>
E.S. Rykoff (U Mich), F. Yuan (U Mich), C. Akerlof (U Mich), H. Swan (U Mich),
B. E. Schaefer (Louisiana State), R. Quimby (U Texas), report on behalf of the
ROTSE collaboration:
ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded
to GRB 070809 (Swift trigger 287344; Marshall, et al., GCN 6728), producing
images beginning 10.2 s after the GCN notice time. An automated response took
the first image at 19:22:48.2 UT, 30.9 s after the burst, under excellent
conditions. We took 10 5-sec, 10 20-sec and 10 60-sec exposures. These
unfiltered images are calibrated relative to USNO A2.0 (R). Imaging is on going.
Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma
Swift/BAT error circle or the XRT error circle, for both single images and
coadding into sets of 10. Individual images have limiting magnitudes ranging
from 15.8-17.4; we set the following specific limits.
start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd?
--------------------------------------------------------------------
19:22:48.2 19:22:53.2 5 16.1 30.9 N
19:22:48.2 19:24:06.2 78 17.5 30.9 Y
19:24:19.9 19:29:06.8 286 18.1 122.6 Y
19:29:16.6 19:40:41.9 685 18.4 419.3 Y
GCN Circular 6728
Subject
GRB 070809: Swift detection of a short burst
Date
2007-08-09T19:48:41Z (18 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
F. E. Marshall (NASA/GSFC), D. N. Burrows (PSU), M. M. Chester (PSU),
J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), C. Guidorzi (Univ Bicocca&INAF-OAB),
S. T. Holland (CRESST/GSFC/USRA), J. A. Kennea (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (Univ. Bicocca & INAF-OAB), T. Sakamoto (NASA/ORAU),
G. Sato (GSFC/ISAS), B. Sbarufatti (INAF-IASFPA),
M. Stamatikos (NASA/ORAU) and R. L. C. Starling (U Leicester) report
on behalf of the Swift Team:
At 19:22:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070809 (trigger=287344). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 203.782, -22.134 which is
RA(J2000) = 13h 35m 08s
Dec(J2000) = -22d 08' 01"
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 2 sec. The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger.
It is currently unclear from the spectral data if this is a short
hard burst.
The XRT began observing the field at 19:23:28 UT, 71 seconds after the
BAT trigger. Using prompt downlinked data, we found a fading X-ray source
located at RA, Dec 203.7708, -22.1408 which is equivalent to
RA(J2000) = 13h 35m 04.99s
Dec(J2000) = -22d 08' 26.2"
with an uncertainty of 4.0 arcseconds (radius, 90% containment).
This location is 44.7 arcseconds from the BAT on-board position,
inside the BAT error circle.
UVOT took a finding chart exposure of nominal 400 seconds with the V
filter starting 74 seconds after the BAT trigger. However, the 8'x8'
region for the list of sources generated on-board does not cover the
ground-generated XRT error circle, so no statement can be made
regarding an optical counterpart.
Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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.)