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GCN Circular 9436

Subject
GRB 090529: TLS Afterglow Observations
Date
2009-05-30T00:12:12Z (15 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
D. A. Kann, U. Laux and B. Stecklum (TLS Tautenburg) report:

We observed the optical afterglow (Xin et al., GCN 9431) of the Swift GRB 
090529 (Sakamoto et al., GCN 9430) with the 1.34m Schmidt telescope of the 
Thueringer Landessternwarte Tautenburg, Germany. Observations began at 
twilight, which, together with the setting moon, caused an elevated sky 
background. Airmass was low and conditions were excellent until clouds 
came in during the last Rc image. We obtained 6 x 300 sec dithered images 
in the Ic band and 3 x 600 sec images in the Rc band. The afterglow is 
very faintly visible on several Ic frames, clearly visible on the first 
two Rc frames and hardly visible on the last Rc frame.

Using the USNOB1.0 catalog as a reference, we derive the following 
position (J2000) for the afterglow:

RA = 14:09:52.53
Dec. = +24:27:32.77

with an error of 0".5. This position lies 0".7 from the UVOT position 
(Schady et al., GCN 9435), in full agreement with it.

We find several sources of roughly similar magnitude within 10" of the 
afterglow. These may slightly affect the photometry.

To calibrate, we use the isolated star with USNO catalog # 1144-0210566 at 
position

RA = 14:09:47.50
Dec. = +24:27:44.5

which has R2 = 18.32 mag and I = 17.54 mag. (Note that it is 0.54 mag 
brighter in R1.)

We derive the following magnitudes 0.3 days after the GRB:

time (d)	Mag	dMag	Exposure	Filter

0.287871	20.98	0.13	6 x 300		Ic
0.304668	21.39	0.12	1 x 600		Rc
0.312191	21.40	0.12	1 x 600		Rc
0.319703	21.47	0.35	1 x 600		Rc

Compared to the magnitude found by Xin et al., GCN 9431, this indicates 
the afterglow has faded by two magnitudes. The normal Rc - Ic color as 
well as the confirmation by Swift UVOT of a rising afterglow (Schady et 
al., GCN 9435) implies that it is not overly reddened, an alternate 
explanation by Holland et al., GCN 9432.

No further observations are planned.

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