6 GRB 990308

GRB 990308 was detected by GRO/BATSE on 1999 March 8.21 UT, on the RXTE All-Sky monitor and also weakly by the Ulysses GRB detector. An OA was detected using the QUEST camera on a 1.0 m Schmidt telescope in Venezuela (Schaefer et al. 1999). These optical measurements give a power law of $\alpha=2\pm2$ (Schaefer et al. 1999), and are the only data in which the OA is detected. Early non-detections by LOTIS and Super-LOTIS suggest $\alpha<1.3$, while later non-detections by the WIYN and Keck telescopes set the constraint $\alpha>1.1$. Taking all constraints into consideration, Schaefer et al. (1999) found a best fitting constant power-law, $\alpha = 1.2\pm0.1$.

 

 

Table 2: Aperture photometry of GRB hosts in AB magnitudes and corrected for foreground extinction according to Schlegel et al. (1998) using the Johnson V-band estimate for CL and Cousins I-band estimate for LP.

GRB	Obs. Date		E(B-V)	CL			LP
	absolute	days		int.	mag	S/N	int.	mag	S/N
980329	24/26 Aug. 2000	$\sim$880	0.073	8072	$27.2 \pm 0.1$	10	8156	$26.2 \pm 0.1$	9
980519	7 Jun. 2000	$\sim$750	0.240	8983	$27.0 \pm 0.2$	9	...	...	...
990308	19 Jun. 2000	$\sim$468	0.023	7842	$29.7 \pm 0.4$	5	...	...	...

We used the original QUEST data (Schaefer et al. 1999) to transform the OT position to the STIS clear image coordinate system via an intermediate transformation to a deep NOT image, due to the lack of common tie objects between the QUEST and STIS images. The QUEST camera periodically drops lines in the readout so that the CCD chips in the array keep in synchronization for objects at different declinations and this results in a small jump in right ascension at known positions on the images. No line drops occurred near the OT position on the QUEST images, so the relative astrometry does not suffer from this complication. The NOT/ALFOSC image is based on the combination of 7 R-band images with a total integration time of 6000s, obtained specifically for this purpose on 29-30 March 2001. The pixel scale in the NOT/ALFOSC image is $0\hbox{$.\!\!^{\prime\prime}$ }187$.

In Table 3 we present the individual GRB 990308 afterglow coordinates, transformed from the three QUEST images to the NOT image. Only point sources were used as tie objects. For each transformation, the standard deviation of the residuals of the fit in each axis is given, together with the number of objects included in the fit. The last column gives the expected centroid error of the afterglow coordinate, as estimated from the FWHM and the signal-to-noise ratio of the individual afterglow detections in the QUEST images. The last line gives the mean coordinate, with the error determined as the standard deviation of the individual measurements, normalized to the degrees of freedom and divided by the square root of the number of measurements. The estimated error of the mean coordinate is in good agreement with what is predicted from the signal-to-noise ratio and FWHM of the afterglow detections. In the STIS image the errors corresponds to 2.50 and 1.58 pixels in x and y respectively.

 

 

Table 3: Afterglow coordinates in the NOT image.

IMAGE	$x_{\rm {NOT}}$	$\sigma_{x}$	$y_{\rm {NOT}}$	$\sigma_{y}$	$N_{\rm {OBJ}}$	$\sigma_{\rm {centroid}}$
CCD 1	1086.46	0.31	1128.40	0.48	11	0.34
CCD 3	1085.85	0.16	1128.23	0.65	10	0.38
CCD 4	1085.29	0.20	1128.94	0.28	8	0.43
Mean	1085.87	$\pm$ 0.33	1128.53	$\pm$ 0.21		0.22

The transformation from the NOT image to the STIS image was based on 10 tie objects, all of which are relatively compact extended sources in the STIS image. The use of extended objects in the astrometric tie may potentially introduce differential colour error in the centroid determination for objects with a color gradient, as the passbands of the groundbased and STIS data are different. Such errors will appear as an increased scatter in the affine transformation. The normalized standard deviation of the residuals of the fit are 0.70 and 1.19 STIS pixels in x and y respectively. We may use these standard deviations as conservative estimates of the error imposed, when transforming the afterglow coordinate from the NOT image reference frame to the STIS image reference frame. The estimate of the error in pixels of the afterglow coordinate in the STIS image then becomes

$$\sigma_x = \sqrt{2.50^2 + 0.70^2} = 2.60 \equiv 0\hbox{$.\!\!^{\prime\prime}$ }066$$

$$\sigma_y = \sqrt{1.58^2 + 1.19^2} = 1.98 \equiv 0\hbox{$.\!\!^{\prime\prime}$ }050$$

The final STIS position and error estimate is given in Table 1.

At the locus of the OA we marginally detect a very faint point-like object (see Fig. 2) which we measure to have a STIS CL magnitude of $30.1\pm0.4$ with a detection significance of $S/N \sim5$ (foreground extinction corrected photometry given in Table 2). Including the faint extended emission north of this object gives $29.9 \pm 0.4$. We also detect an extended object $0\hbox{$.\!\!^{\prime\prime}$ }3$ to the south with an estimated magnitude of $29.8\pm0.4$ ($S/N \sim3$) and $0\hbox{$.\!\!^{\prime\prime}$ }8$ to the North a much larger disk-like object with a magnitude of $27.7\pm0.1$. Using the galaxy counts of Gardner et al. (2000) we find a relatively low probability ($\sim$0.02 within a radius of 1 $^{\prime\prime}$ of the OA position) that the three objects are projected neighbors.

Could the point-like component coincident with the OA location in fact be the OA itself? Assuming a constant power-law one can deduce the decay slope by interpolating the brightness at the time of the first V-band QUEST observation and the measured brightness in the STIS observations 468 days after the burst. This gives a power-law exponent of $\alpha \sim 1.35$, consistent with the best estimate of the power-law slope, $\alpha \simeq 1.2$ (based on all available data). If correct, this would be the latest trace to date of an OA, at t₀+468 days. Another possibility is that the point-like component is caused by some re-brightening mechanism, such as e.g. dust echoing.

In summary, we identify the object coincident with the OA localisation as the possible remnant OA (point-like) or the host (extended). If the point-like object turns out to be non-variable and therefore not the OA, then the OA must have been fainter than $\sim$30 mag. This implies that the late time decay slope must have been larger than 1.35. This scenario and the constraints from the early data could be explained by introducing a break in the light-curve. Specifically, an early $\alpha\sim1.3$ slope (as supported by the early LOTIS data) followed by a steeper slope fits this scenario well. A revisit of this field with HST+ACS is required to disentangle these ambiguities.