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The Astrophysical Journal, 523:L121-L124, 1999 October 1
1999. The American Astronomical Society. All rights reser ved. Printed in U.S.A.

OPTICAL AND RADIO OBSERVATIONS OF THE AFTERGLOW FROM GRB 990510: EVIDENCE FOR A JET F. A. Harrison,1 J. S. Bloom,1 D. A. Frail,2 R. Sari,3 S. R. Kulkarni,1 S. G. Djorgovski,1 T. Axelrod,4 J. Mould,4 B. P. Schmidt,4 M. H. Wieringa,5 R. M. Wark,5 R. Subrahmanyan,5 D. McConnell,5 P. J. McCarthy,6 B. E. Schaefer,7 R. G. McMahon,8 R. O. Markze,6 E. Firth,8 P. Soffitta,9 and L. Amati10
Received 1999 July 12; accepted 1999 August 2; published 1999 September 7

ABSTRACT We present multicolor optical and two-frequency radio obser vations of the bright BeppoSAX event GRB 990510. Neither the well-sampled optical decay nor the radio obser vations are consistent with simple spherical afterglow models. The achromatic steepening in the optical band and the early decay of the radio afterglow, which both occur at t 1 day, are evidence for hydrodynamical evolution of the source and can be most easily interpreted by models in which the gamma-ray burst ejecta are collimated in a jet. Employing a simple jet model to explain the obser vations, we derive a jet opening angle of v0 = 0.08(n/1 cm 3)1/8, reducing the isotropic gamma-ray energy release of 2.9 # 10 53 ergs by a factor of 300. Subject headings: cosmology: miscellaneous -- gamma rays: bursts -- radio continuum: general -- shock waves
1. INTRODUCTION

Gamma-ray burst (GRB) afterglow obser vations from X-ray through radio can be interpreted in the context of fireball models, where a shock produced by the interaction of relativistic ejecta with the circumburst environment expands into the surrounding medium, producing broadband synchrotron emission (e.g., Meszaros & Rees 1997; Sari, Piran, & Narayan 1998; ДД Waxman 1997). The optical light cur ve of GRB 970508, for example, exhibits a monotonic decay: Fn t a with a = 1.3 for 200 days (Fruchter et al. 1999a), well-described by the expansion of a spherical blast wave (Wijers, Rees, & Meszaros ДД 1997). Recently, the rapid decay of some events has been interpreted as evidence for jetlike or collimated ejecta (Sari, Piran, & Halpern 1999), but this explanation is not unique (Chevalier & Li 1999). For GRB 990123, the steepening of the optical light cur ve (Kulkarni et al. 1999a; Fruchter et al. 1999b) combined with the early radio decay (Kulkarni et al. 1999b) together provide the best evidence to date for deviations from spherical symmetr y. Due to sparse sampling, however, simultaneous steepening in all optical bands--the distinctive feature of hydrodynamic evolution of a jet--was not clearly obser ved. The bright BeppoSAX event GRB 990510 is distinguished by excellent sampling of the optical decay in multiple bands and by the early-time detection and continued monitoring of the radio afterglow. In this Letter we present the optical and radio light cur ves and argue that in concert they provide clear
1 Palomar Obser vator y 105-24, California Institute of Technology, Pasadena, CA 91125. 2 National Radio Astronomy Obser vator y, Socorro, NM 87801. 3 California Institute of Technology, Theoretical Astrophysics 103-33, Pasadena, CA 91125. 4 Research School of Astronomy, Australian National University, Private Bag, Weston Creek P.O., ACT 2611, Australia. 5 Australian Telescope National Facility, CSIRO, Epping, NSW 2121, Australia. 6 Obser vatories of the Carnegie Institute of Washington, 813 Santa Barbara Street, Pasadena, CA 91101-1292. 7 Department of Physics, Yale University, New Haven, CT 06520. 8 Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, England, UK. 9 Istituto Astrofisica Spaziale, CNR, Area di Ricerca Tor Vergata, Via Fosso del Cavaliere 100, 00133 Roma, Italy. 10 Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy.

evidence for a relatively simple jetlike evolution of the ejecta. The level of collimation implied for this event reduces, by a factor greater than 100, the energy required to produce the gamma-ray flash.
2. OBSERVATIONS

GRB 990510, imaged by the BeppoSAX Wide Field Camera (WFC) on May 10.37 (UT) (Dadina et al. 1999), was a long (75 s) relatively bright event with a fluence (E 1 20 keV) of 2.6 # 10 5 ergs cm 2, ranking it fourth among the BeppoSAX WFC localized sample and in the top 10% of BATSE bursts (Kippen et al. 1999; Amati et al. 1999).11 After announcement of the WFC position by the BeppoSAX team, numerous groups began the search for an optical transient, eventually discovered by Vreeswijk et al. (1999a). The optical transient is coincident with a fading X-ray source seen in the BeppoSAX Narrow Field Instruments (Kuulkers et al. 1999). Spectra taken with the VLT (Vreeswijk et al. 1999b) identify numerous absorption lines, determining a minimum redshift of 1.619 0.002. Adopting this as the source redshift implies an isotropic gamma-ray energy release of 2.9 # 10 53 ergs (we employ a standard Friedmann cosmology with H0 = 65 km s 1 Mpc 1, Q 0 = 0.2, and L = 0 throughout). We commenced optical obser vations of the 3 radius BeppoSAX WFC error circle using the Mount Stromlo Observator y (MSO) 50 inch (1.3 m) telescope 3.5 hr after the GRB. We continued monitoring with the MSO 50 inch, the Yale 1 m telescope on Cerro Tololo in Chile, and the 40 inch (1 m) telescope at the Las Campanas Obser vator y (LCO) in Chile. Radio obser vations began at the Australia Telescope Compact Array (ATCA), in Narrabri, Australia, about 17 hr following the GRB event. Tables 1, 2, 3, and 4 present the BVRI optical data taken by our collaboration (quoted errors are 1 j statistical uncertainties). The VR and I light cur ves, along with points from numerous other groups reported in the literature (Galama et al. 1999; Kaluzny et al. 1999; Stanek et al. 1999a; Pietrzynski & Udalski 1999a, 1999b; Covino et al. 1999; Lazzati, Covino, & Ghisellini 1999; Pietrzynski & Udalski 1999c; Marconi et al. 1999), are plotted in Figure 1. We have calibrated the re11 GCN circulars are available at http://lheawww.gsfc.nasa.gov/docs/gamcosray/legr/bacodine/gcn_main.html.

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TABLE 1 B- B a n d P h o t o m e t r y o f G R B 9 9 0 5 1 0 Date (1999 May UT) 10.971 11.058 11.131 11.154 11.180 11.207 11.266 11.292 11.320 12.125 12.171 12.221 12.300 12.996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnitude 19.86 17.88 17.95 18.84 18.90 18.98 19.23 19.39 20.11 20.01 20.06 20.89 21.22 21.22 0.05 0.05 0.05 0.06 0.06 0.06 0.06 0.06 0.06 0.08 0.09 0.09 0.12 0.17 Telescope Yale Yale Yale Yale Yale Yale Yale Yale Yale Yale Yale Yale Yale Yale 1 1 1 1 1 1 1 1 1 1 1 1 1 1 m m m m m m m m m m m m m m TABLE 3 R- B a n d P h o t o m e t r y o f G R B 9 9 0 5 1 0 Date (1999 May UT) 10.514 10.522 10.529 10.775 10.783 10.791 10.992 11.071 11.094 11.194 11.280 11.333 11.508 11.512 11.516 12.138 12.183 12.233 12.975 13.238 14.308 ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ Magnitude 17.54 17.61 17.60 18.53 18.61 18.55 18.90 19.07 19.20 19.24 19.35 19.40 19.67 19.71 19.76 20.49 20.59 20.47 21.04 21.42 22.01 0.02 0.02 0.02 0.07 0.07 0.04 0.04 0.04 0.04 0.04 0.05 0.06 0.07 0.06 0.09 0.08 0.09 0.12 0.14 0.14 0.18 Telescope MSO 50 MSO 50 MSO 50 MSO 50 MSO 50 MSO 50 Yale 1 m Yale 1 m Yale 1 m Yale 1 m Yale 1 m Yale 1 m MSO 50 MSO 50 MSO 50 Yale 1 m Yale 1 m Yale 1 m Yale 1 m Yale 1 m Yale 1 m inch inch inch inch inch inch

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inch inch inch

ported magnitudes to the Landolt bandpass system (approximately Johnson-Cousins). For calibration, we obser ved a number of Landolt stars on May 11 under photometric conditions with the MSO 50 inch telescope. The uncertainty in the zero point of the calibration introduces a magnitude error of 0.03 in all bands. From Figure 1, it is evident that the light cur ve steepens contemporaneously in all bands between day 1 and 2. To characterize the shape, we fit the data with the following analytic four-parameter function: Fn (t) = f (t/t ) a1[1 ex p ( J )]/J;
( a1 a 2 )

J(t, t, a1 , a 2 ) = (t/t )

.

(1)

The functional form has no physical significance, but provides a good description of the data and has the property that the asymptotic power-law indices are a1 and a2 at early and late times, respectively. Fitting the V, R, and I data (excluding B due to larger statistical uncertainties) simultaneously yields t* = 1.20 0.08 days, a1 = 0.82 0.02, and a2 = 2.18 0.05, where the errors are formal 1 j errors and do not reflect the covariance between parameters. The x2 for the fit is acceptable: 65 for 82 degrees of freedom. We have removed five out of the 92 total data points with uncertain calibrations. Due to calibration uncertainty, we cannot determine if the light cur ve exhibits variability on timescales shorter than the trend described by the functional fit. The difference in fit parameters from those found by Stanek et al. (1999b) is due to the slightly
TABLE 2 V-Band Photometry of GRB 990510 Date (1999 May UT) 10.514 10.522 10.529 10.775 10.783 10.791 10.979 11.011 11.508 11.512 11.516 12.146 12.367 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnitude 17.84 17.88 17.95 18.84 18.90 18.98 19.23 19.39 20.11 20.01 20.06 20.89 21.22 0.02 0.02 0.01 0.06 0.08 0.05 0.04 0.05 0.09 0.08 0.07 0.07 0.14 Telescope MSO 50 inch MSO 50 inch MSO 50 inch MSO 50 inch MSO 50 inch MSO 50 inch Yale 1 m LCO 40 inch MSO 50 inch MSO 50 inch MSO 50 inch Yale 1 m LCO 40 inch

different function used. Using the same function, we find consistency with their results to better than 2 j in all parameters. To derive the extinction-corrected normalizations, obtained by fitting with the shape described above, we use the astros metric position from Hjorth et al. (1999) [R.A. = 13h38m07.11, decl. = 80 29 48 . 2 (J2000)] and the dust maps from Schegel, Finkbeiner, & Davis (1998). The resulting Galactic extinction in the direction of the transient is E(B V) = 0.20. In the standard Landolt bandpass system, assuming RV = AV /E( B V) = 3.1, we obtain AB = 0.87, AV = 0.67, AR = 0.54, and AI = 0.40. After correction, the magnitudes corresponding to the flux f* in equation (1) are V* = 19.03 0.01, I* = 18.42 0.01, 0.01. The errors are the formal 1 j errors and R* = 18.81 from the fit, with an additional 0.03 mag due to the uncertain zero-point calibration. Obser vations of the field around GRB 990510 with ATCA began on 1999 May 10 at 22:36 UT. All obser vations (Table 5) use a bandwidth of 128 MHz and two orthogonal linear polarizations for each wavelength pair. A radio afterglow is clearly detected, starting 3 days after the event (Fig. 2). The error bars provided in the table are statistical (radiometric) errors only. At early times, variation due to interstellar scintillation will dominate the error in flux determination from the source (see the legend to Fig. 2).
3. EVIDENCE FOR A JET

The majority of other well-studied GRBs, in particular GRB 970228 and GRB 970508, have afterglow light cur ves that decay monotonically for the first month or more, and these have been interpreted in the context of spherical fireball models (e.g., Tavani 1997; Wijers et al. 1997; Reichart 1997; Granot,
TABLE 4 I-Band Photometry of GRB 990510 Date (1999 May UT) 10.999 12.154 11.034 12.042 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnitude 18.40 20.04 18.61 19.83 0.04 0.09 0.05 0.10 Telescope Yale Yale LCO LCO 1 1 4 4 m m 0 inch 0 inch


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HARRISON ET AL.
TABLE 5 ATCA Radio Flux Measurements Date (1999 May UT) 11.09 11.09 13.68 15.61 17.58 19.59 19.59 25.32 46.81 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Frequency (GHz) 4.8 8.6 8.7 8.7 8.7 4.8 8.6 8.7 8.7 Flux Density (mJy) 110 104 227 202 138 177 127 82 1 69 74 30 31 32 36 31 32 28 Integration (hr) 7.5 7.5 9.0 8.0 6.6 11.4 11.4 10.6 11.7

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Angular Resolution (arcsec) 4.2 1.9 1.9 1.8 2.1 3.1 1.7 2.2 4.0 # # # # # # # # # 1.8 1.3 1.3 1.4 1.2 2.6 1.5 1.2 3.6

Note.--The date indicates the obser vation center.

Fig. 1.--Optical light cur ves of the transient afterglow of GRB 990510. In addition to photometr y from our group (filled symbols; see Table 1), we have augmented the light cur ves with data from the literature ( open symbols). The photometric zero points in Landolt V-band from our group are consistent with that of the OGLE group (Pietr zynski & Udalski 1999b), and the I-band zero point is from the OGLE group. Some R-band measurements were based on an incorrect calibration of a secondar y star in the field (Galama et al. 1999), and we have recalibrated these measurements.

Piran, & Sari 1999). In the optical, spherical models with typical parameters predict flux rising quickly (within hours) to a maximum value fm (at time tm), after which it decays as a power law, t a with a 1. At later times, the decay becomes somewhat faster (a change in a of 0.25), as the cooling break sweeps across the band (Sari, Piran, & Narayan 1998). In the radio band, above the self-absorption frequency, the behavior is similar, but with typical values of tm 1 week. The obser ved optical and radio decay of GRB 990510 is quite distinct, showing frequency-independent steepening in the optical and early decline in the radio on a timescale of 1 day--behavior clearly inconsistent with spherical models. An achromatic break or steepening in light cur ves is expected if the emitting surface has a nonspherical geometr y. At any given time, due to relativistic beaming, only a small portion of the emitting surface with opening angle 1/g is visible. At early times (when v0 1/g), the obser ved light cur ve from a collimated source is identical to that of a sphere. As the fireball evolves and g decreases, the beaming angle will eventually exceed the opening angle of the jet, and we expect to see a deficit in the emission--i.e., a break in the light cur ve. At a comparable or later time (Rhoads 1999; Sari, Piran, & Halpern 1999; Panaitescu, Meszaros, & Rees 1998) the jet will begin ДД to spread laterally, causing a further steepening. To model the light cur ve, we adopt the afterglow analysis for a jet source given in Sari et al. (1999). At early times (g 1 v0 1), the light cur ve is given by the spherical solution; F(n0 ) t a with a = 3( p 1)/ 4 if the electrons are not cooling, and a = 3 p/ 4 1/ 2 if they are. From the GRB 990510 early-time optical slope, a1 = 0.82, and we derive p = 2.1 assuming the electrons producing the optical emission are in the slow cooling regime and p = 1.76 other wise. The latter

value would result in the electron energy being unbounded, and we conclude that p = 2.1. At late times (g ! v0 1), when the evolution is dominated by the spreading of the jet, the model predicts a = p, independent of the cooling regime. Indeed, our measured value of a 2 = 2.18 0.05 is consistent with this expectation. The optical data allow us to infer p and the epoch of the break (related to the opening angle of the jet). However, in order to fully characterize the afterglow we also need to determine (1) na, the self-absorption frequency, (2) Fm and tm, and (3) the cooling frequency nc at a given epoch. The optical obser vations show that even at early times the optical flux is decaying and is therefore above nm. The radio, however, is well below nm, and by combining the ATCA and optical data we can derive Fm, tm, and nm. Following Sari et al. (1999), we have fitted a t 1/3 power law to the four radio points and obtained F8.7 GHz 20 4 mJy(t/t1 ) 1/3, where t1 = 3.3 days is the time of

Fig. 2.--Obser ved and predicted radio light cur ves at 8.6 GHz. Detections are indicated by the crosses, with error bars indicating the rms noise in the image. The true flux uncertainty is dominated by the signal modulation due to refractive interstellar scintillation (e.g., Frail et al. 1997). Using the Galactic scattering model of Taylor & Cordes (1993) and the formalism from Goodman (1997), we calculate a scintillation timescale of 2 hr in the first few weeks after the burst. Although our typical 8 hr integrations average over the scintillation, we expect modulation of the mean flux density of order 50%. Predictions for the evolution of the radio flux density (solid line) are based on the jet model of Sari et al. (1999) (see text for more details). The dotted line shows the model prediction for a spherical fireball. The dotdashed line illustrates the obser ved optical behavior.


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the second radio detection. Using this and the optical data at t1, we get nm (t1 ) = 280 GHz and Fm (t1 ) = 650 mJy. After the jet begins to spread, nm decays as t 2, and we expect nm to arrive at radio frequencies at 19 days, producing a break in the radio light cur ve to the t p slope seen in the optical. In the above, we have assumed that na is below 8.7 GHz. A x2 analysis constrains the 4.8-8.7 GHz spectral slope to be between 1.3 and 0.4 (95% confidence), consistent with the n1/3 slope expected if na ! 8.7 GHz and inconsistent with the n2 expected if na 1 8.7 GHz. Figure 2 shows the radio light cur ve along with the prediction for both spherical (dotted line) and collimated (solid line) ejecta. The relatively sharp transition in the GRB 990510 decay to the asymptotic value a 2 = p expected when both the jet edge becomes visible and when lateral spreading begins suggest both transitions occur at similar times in this event. Using the gamma-ray energy of 2.9 # 10 53 ergs, we find a Lorentz factor at the jet break time of 12(n/1 cm 3) 1/8. This implies an opening angle of v0 = 0.08(n/1 cm 3)1/8, and for a 2 two-sided jet the energy is reduced by a factor 2 /v0 300 to 51 3 1/4 12 1 # 10 (n /1 cm ) ergs.
4. CONCLUSION

With one of the best-sampled optical light cur ves and simultaneous early-time radio obser vations, GRB 990510 provides the clearest signature obser ved to date for collimation of the ejecta in GRB sources. The achromatic steepening in the optical light cur ve as well as the early decay, after t 1 day, of the radio emission is inconsistent with other obser ved af12 The estimates of Rhoads (1999) will give a smaller opening angle and therefore a lower energy; here we have used the estimates in Sari et al. (1999).

terglows that have been modeled with spherically symmetric ejecta. The GRB 990510 afterglow emission can be remarkably well fit by a simple model for the jet evolution. It is interesting to ask if the obser vations to date are consistent with all GRB engines having an energy release of 1052 ergs, with the wide obser ved luminosity distribution being due to variation in the degree of collimation. Of GRBs with measured redshifts for which the gamma-ray energy release can be calculated, only GRB 990123 and GRB 990510 show breaks in the optical light cur ves on timescales less than 1 week, and interestingly these are among the highest fluence BeppoSAX events to date. GRB 990123 has an implied isotropic energy release of 3.4 # 10 54 ergs, which reduces by a factor of 100 if the light-cur ve break occurring at t 2 days is interpreted as the signature of a jet. As argued here, the energy required for GRB 990510 in the context of the jet model is 1051 ergs. In contrast, GRB 970508 and GRB 970228 show no evidence for a jet in the optical (although GRB 970508 may in radio); however, their isotropic energy release is quite modest: only 8 # 10 51 and 5 # 10 51 ergs, respectively. The candidates for the largest energy release, highest gamma-ray fluence where no evidence for collimation is seen, are GRB 971214 (z = 3.2) with Eg = 3 # 10 53 ergs (Kulkarni et al. 1998) and GRB 980703 (z = 0.966) with Eg = 1 # 10 53 ergs (Djorgovski et al. 1998). Light-cur ve obser vations of these events are, however, limited to t 2 weeks, and so collimation may still reduce the energy of these bursts by factors of 40, still consistent with a total energy release 1052 ergs. We thank Scott of LCO, MSSSO, This work was su S. G. D.), NASA Foundation (S. G. Barthelmy for operating the GCN, the staffs and ATCA, and the entire BeppoSAX team. pported by grants from NSF (S. R. K. and (F. A. H. and S. R. K.), and the Bressler D.).

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