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Possible Radio Afterglow of a 1989 Gamma-Ray ... - Williams College

Possible Radio Afterglow of a 1989 Gamma-Ray ... - Williams College

RADIO AFTERGLOW OF

RADIO AFTERGLOW OF 1989 GRB 7 source. Beginning with arguments based on the shape of our light curve alone, consider the following: 1. Time-varying radio signals from flare stars usually show much more rapid variations (e.g., Bastian, Bookbinder, & Dulk 1990). 2. Intraday variations in the flux from AGNs show much lower amplitude changes (≤35%; see Wagner & Witzel 1995), though the variability of weaker sources is less well known. Our proposed VLA observations will provide a good test of this alternative explanation. 3. The large change in the flux density and the lack of symmetry in the light curve both argue against a gravitational lens event (though a lensing event with amplification greater than 3 would be interesting in its own right!). Fig. 1.—Flux density of 16V38, a possible GRB afterglow (bold line); flux of a comparison source (light lines). sufficient quality to allow us to look for variations in the flux of 16V38 within a single 12 hour run. We conclude that 16V38 did more than triple its flux density in a day, then faded rapidly. The light curve of 16V38 in 1989 December–1990 January qualitatively resembles the light curves of other radio afterglows of GRBs. 2.2. Related Observations The field of 16V38 is a well-studied area, part of the Leiden- Berkeley Deep Survey (Windhorst, van Heerde, & Katgert 1984). As a consequence, other radio observations are available. Windhorst et al. (1985) made 20 cm observations in 1981 December. Their source 084244.01 lies only 2 from 16V38; it is described as resolved, with S20 p 540 60 mJy. The lower resolution employed by Windhorst et al. (1985), only 15, may have blended two sources seen at 3.6 cm (16V38 and 16V39 in the catalog of Windhorst et al. 1993). Four years later, this area was reobserved at l p 6 cm by Donnelly, Partridge, & Windhorst (1987). The upper limit placed on S 6 was ≤340 110 mJy. We have requested time at the VLA in its high-resolution A configuration to reobserve 16V38 in late 2000 at l p 20 and 3.6 cm to see if there is an underlying radio source of ≈500 mJy flux density. 3. ALTERNATIVE EXPLANATIONS While the radio light curve is suggestive, no GRB was detected near our source in 1989 December (note that BATSE did not become operational until 1991). Can we be sure the radio signal we recorded was indeed the afterglow of a GRB? We consider other possibilities: the radio signal we detected was due to a flare star, time-varying radio emission from a Galactic X-ray source, intraday variability of an active galactic nucleus (AGN), or gravitational lensing of a background We also examined available optical images of the field around 16V38 to see if a star or nonstellar object lay within the radio error circle of ≈0.5 radius. In 2000 May, S. Kulkarni kindly provided a relatively deep, Gunn R-band, Keck image of the vicinity of 16V38 (Fig. 2). No object to R ≈ 25 mag is present inside the radio error circle. This strengthens the argument against AGN and Galactic X-ray sources. In the case of flare stars, we need to be a bit more careful—many flare stars are M dwarfs and could in principle lie close enough to us that proper motion could alter their position by up to 0.4 over the ≈10 years between our radio observations and the Keck optical image (for the extreme case of a star at 10 pc and a transverse proper motion of 100 km s 1 ). We searched both the original Palomar Sky Survey (POSS) and the Second Generation POSS and found no suitable candidate. 4. DISCUSSION We conclude that none of the alternative explanations are consistent with the data we have. Two open questions are whether there is an underlying radio source of ≈500 mJy and whether there is an optical counterpart fainter than ≈25 mag in R. VLA observations planned at 20 and 3.6 cm will resolve the first question and provide subarcsecond positional accuracy for follow-up optical observations if 16V38 is again detected. If the radio signals we detected 11 years ago are indeed the radio afterglow of an anonymous GRB, 16V38 offers the opportunity to study the aftermath of a GRB more than a decade after it occurred. 4.1. Statistical Inferences Since we have only a single possible event, we cannot set definitive limits on the number of GRB afterglows. Perna & Loeb (1998) predict 4 2 N ≈ 2 # 10 (v /10) (1) b 2001 PASP, 113:6–9

8 SEATON & PARTRIDGE Fig. 2.—Keck R-band image of the vicinity of 16V38 (kindly provided by S. Kulkarni). The radius of the error circle is approximately 4 times the uncertainty in the radio position of 16V38. radio afterglows in a 10 GHz search to sensitivity comparable to ours, or 2 2 N ≈ 0.5(v b/10) deg . (2) Here v b is the opening angle of the presumably beamed emission; Perna & Loeb (1998) argue for vb ≥ 6 or 2 N ≤ 1.3 deg . The field in which 16V38 was found covers ≈0.019 deg 2 . However, our group has comparable radio observations on four other fields, covering in all ≈0.1 deg 2 . None shows evidence for sources with abrupt changes in flux density like 16V38. However, if the Perna & Loeb estimates are right, it is not improbable that we found one GRB afterglow in our five radio surveys. D. B. S.’s research was supported by the William Keck Foundation through a grant to the Keck Northeast Astronomy Consortium. R. B. P.’s research was supported by NSF grant AST 96-16971. We thank Dale Frail for useful conversations and especially Shri Kulkarni for providing the Keck image shown in Figure 2. 2001 PASP, 113:6–9

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