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GRB 990712
TITLE: GCN GRB
OBSERVATION REPORT
NUMBER: 565
SUBJECT: HST Observations of GRB 990712
DATE: 00/02/24 20:51:00 GMT
FROM: Andrew S. Fruchter at STScI <fruchter@stsci.edu>
A. Fruchter, K. Sahu, R. Gibbons, L. Petro and H. Ferguson
(STScI) on behalf of the larger HST GRB collaboration report:
We have observed the field of GRB 990712 with the HST STIS CCD on
August 29, 1999, 48 days after outburst. Observations with
total exposures times of 3720s were taken in both the (50CCD)
clear aperture and (LP) long pass filters. At the position
of the previously located OT (GCN reports starting with GCN 387),
we find a disk galaxy with two apparent knots along its major
axis. Using the astrometry of Sahu et al. (2000, Ap J.
accepted) we are able to place the position of the OT on our
image to a one-sigma accuracy no better than 0."15, where
the source of the error is nearly equally divided between that of
the plate solution in Sahu et al.'s original image and the
measurement of the relative position of the OT to the astrometric
calibrators in that image. The best estimate position of
the OT is offset from the fainter blue knot by only 0."07;
however, a brighter red knot lying 0."24 from the best
estimate position can not be ruled out as the true location of
the OT.
The images are displayed at http://www.stsci.edu/~fruchter/GRB/990712 .
These results are being published now because comments from
members of the community have made it clear that there is great
interest even in inconclusive HST results. However, this
object is scheduled to be reobserved in late April or early May
of this year. The resulting images, when subtracted from
those reported here, should allow an unambiguous identification
of the OT, and an accurate determination of its magnitude and
color at this first HST epoch.
 |
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| The HST STIS 50CCD (clear aperture) image of GRB 990712. The image was taken on August 29, 1999, or 48 days after outburst. Two bright knots of emission are clearly visible along the major axis of the host galaxy. . | A color image of the host of GRB 990712, made by scaling B to 50CCD-LP, G to 50CCD+LP, and R to LP. The one sigma error circle for the position of the optical transient, based on the astrometry of Sahu et al. (2000) is shown in red |
| On this page
the HST GRB Collaboration presents the
Hubble Space Telescope images of GRB 990712. The Gamma-Ray Burst of
July 7, 1999 was first located by the Beppo-Sax satellite. The optical transient was
discovered in images taken 4 hours later by Kailash Sahu
and collaborators at the SAAO 1m telescope at Sutherland, South
Africa. A redshift of z=0.43 was determined by Vreeswijk
et al. using the VLT. A complete list of GCN circulars for this burst can be found at
the UCO Lick REACT web pages. We observed the field of GRB 990712 with the HST STIS CCD on August 29, 1999, 48 days after outburst. Observations with total exposures times of 3720s were taken in both the (50CCD) clear aperture and (LP) long pass filters. At the position of the previously located OT (GCN reports starting with GCN 387), we find a disk galaxy with two apparent knots along its major axis. Using the astrometry of Sahu et al. (2000, Ap J. accepted) we are able to place the position of the OT on our image to a one-sigma accuracy no better than 0."15, where the source of the error is nearly equally divided between that of the plate solution in Sahu et al.'s original image and the measurement of the relative position of the OT to the astrometric calibrators in that image. The best estimate position of the OT is offset from the fainter blue knot by only 0."07; however, a brighter red knot lying 0."24 from the best estimate position can not be ruled out as the true location of the OT. Because the OT is located on a relatively bright (V~23) galaxy, its magnitude cannot be accurately determined without knowing the underlying surface brightness of the galaxy. However, an upper limit to the brightness of the OT can be estimated by point source subtraction from the knots. If the OT is contained under the fainter blue source then the maximum allowable point source subtraction has a magnitude R = 25.6 +/- 0.2 with estimated spectral index of f(nu)={-1 +/- 0.5}, where the error in the power-law index is dominated by systematic errors due to the present uncertainty in the calibration of the STIS LP filter. This magnitude and color is consistent with an extrapolation of the power-law decay of the OT reported by Sahu et al., and would effectively rule out a SN component to this afterglow. However, if the OT is in fact associated with the brighter, redder object, then the maximum allowable point source has R = 24.5 +/- 0.25 with f(nu)=nu^{-3 +/- 0.5}. A SN with absolute magnitude about 1 mag fainter than 1998bw could be then accommodated. |
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