optical observations of the WFC error box (Heise et al. 1997b), made on May 9 and 10, Bond (1997) found a variable object

Transcription

1 The Astrophysicl Journl, 497:L3 L6, 998 April The Americn Astronomicl Society. All rights reserved. Printed in U.S.A. OPTICAL FOLLOW-UP OF GRB T. J. Glm, P. J. Groot, J. vn Prdijs,, C. Kouveliotou, 3,4 R. G. Strom,,5 R. A. M. J. Wijers, 6 N. Tnvir, 6 J. Bloom, 7 M. Centurion, 8,9 J. Telting, 9 R. G. M. Rutten, 9 P. Smith, 0 C. Mckey, 0 S. Smrtt, 9 C. Benn, 9 J. Heise, nd J. in t Znd Received 997 November 0; ccepted 998 Februry 0; published 998 Mrch 7 ABSTRACT We report on the results of opticl follow-up observtions of the counterprt of the gmm-ry burst GRB , strting 7 hr fter the event. Multicolor U-, B-, V-, R c -, nd I c -bnd observtions were obtined during the first three consecutive nights. The counterprt ws monitored regulrly in R c until 4 months fter the burst. The light curve fter the mximum follows decline tht cn be fitted with power lw with exponent Devitions from smooth power-lw decy re moderte (rms 0.5 mg). We find no flttening of the light curve t lte times. The opticl fterglow fluence is significnt frction, 5%, of the GRB fluence. The opticl energy distribution cn be well represented by power lw, the slope of which chnged t the time of the mximum (the spectrum becme redder). Subject heding: gmm rys: bursts. INTRODUCTION With the Wide Field Cmers (WFCs; Jger et l. 993) on bord BeppoSAX (Piro, Scrsi, & Butler 995), it becme possible for the first time to determine rpidly the ccurte (3 rdius) gmm-ry burst (GRB) positions (Cost et l. 997, 997b; In t Znd et l. 997; Heise et l. 997); such positions, s it turned out, re the necessry ingredient for successful counterprt serches (see lso Tkeshim et l. 998 nd Smith et l. 998). The detection of fding opticl counterprt to GRB 9708 provided the first rcsecond locliztion of GRB (Groot et l. 997; vn Prdijs et l. 997; Glm et l. 997, 998). Its coincidence with fint extended object (Groot et l. 997b; Metzger et l. 997; Shu et l. 997), which is probbly the host glxy, suggests cosmologicl distnce for the GRB. A second opticl trnsient (OT) ws found for GRB (Bond 997). The mesurement of lower bound to the redshift ( z 0.835; Metzger et l. 997b) of this opticl counterprt hs unmbiguously settled the cosmologicl distnce scle to GRB sources. On My UT, the Gmm-Ry Burst Monitor (GRBM) on BeppoSAX recorded modertely bright GRB (Cost et l. 997c), which ws lso recorded with its Wide Field Cmers. The fluences, recorded with the GRBM ( kev) nd WFC ( 6 kev), were (.8 0.3) # 0 ergs cm nd (0.7 0.) # 0 ergs cm, respectively (Piro et l. 997). Astronomicl Institute Anton Pnnekoek, University of Amsterdm, nd Center for High Energy Astrophysics, Kruisln 403, 098 SJ Amsterdm, The Netherlnds. Deprtment of Physics, University of Albm t Huntsville, Huntsville AL Universities Spce Reserch Assocition. 4 NASA Mrshll Spce Flight Center, Code ES-84, Huntsville AL Netherlnds Foundtion for Reserch in Astronomy (NFRA), Postbus, 7990 AA Dwingeloo, The Netherlnds. 6 Institute of Astronomy, University of Cmbridge, Mdingley Rod, Cmbridge CB3 0HA, Englnd, UK. 7 Cltech, 05-4, Psden, CA Instituto de Astrofisic de Cnris, Vi Lcte, E-3800 L Lgun, Tenerife, Cnry Islnds, Spin. 9 Isc Newton Group of Telescopes/NFRA, Aprtdo de Correos 3, E Snt Cruz de l Plm, Tenerife, Cnry Islnds, Spin. 0 Kitt Pek Ntionl Observtory. Spce Reserch Orgniztion of the Netherlnds, Sorbonneln, 3584 CA Utrecht, The Netherlnds. L3 The burst ws lso recorded (Kouveliotou et l. 997) with BATSE (Fishmn et l. 989), with totl kev fluence of (3. 0.) # 0 ergs cm nd pek flux density ( kev) of ( ) # 0 ergs cm s. From opticl observtions of the WFC error box (Heise et l. 997b), mde on My 9 nd 0, Bond (997) found vrible object h m s t R.A , decl (J000), which shows n increse by mg in the V bnd. BeppoSAX Nrrow Field Instrument observtions reveled n X-ry trnsient (Piro et l. 997b) whose position is consistent with tht of the opticl vrible. Extended emission is not ssocited with this opticl counterprt (Fruchter et l. 997; Shu et l. 997b; Pin et l. 998), nd Ntrjn et l. (997) show either tht GRB originted from n intrinsiclly very fint dwrf glxy or tht it occurred t lrge distnce from host glxy (5 h 70 kpc). Here we report on the results of the opticl photometry of GRB , mde between 0.3 nd 0 dys fter the burst.. OPTICAL PHOTOMETRY We hve used the 4. m Willim Herschel Telescope (WHT) t L Plm to mke opticl observtions in U, B, V, R c, nd I c of the WFC error box with both the Prime Focus Cmer (PF; field of view 9.0 # 9.0) nd the Auxiliry Port Cmer (AUX; field of view 0.9 rdius). All WHT PF exposures lsted 600 s, except the one mde on August 6.9 UT, which lsted 800 s. The WHT AUX exposures were 3600 s ech. As prt of the Kitt Pek Ntionl Observtory Queue observtions, observtions were lso mde with the 3.5 m Wisconsin-Indin- Yle-NOAO (WIYN) telescope, using the WIYN Imger SKB t the Nsmyth focus. Ten 6.8 # 6.8 R-bnd imges were obtined between My 9.5 nd 9.3 UT t severl pointings in order to cover the whole error box. All exposures lsted 300 s. A log of the observtions is shown in Tble. We hve corrected the U- nd B-bnd imges for the nonlinerity t low counts of the LORAL CCD chip, used for the My 0,, nd WHT observtions (this effect is negligible in the V, R c, nd I c observtions). We mde photometric clibrtion (U, V, R c, nd I c ) using WHT observtions on My This mteril my be found t The L Plm webpge is not mintined by the Astrophysicl Journl.

2 L4 GALAMA ET AL. Vol. 497 TABLE Log of Observtions of GRB Dte (UT) Telescope U B V R c I c My WIYN ) ) ) ) My WHT PF ) My WHT PF ) ) ) ) My WHT PF My WHT PF ) ) ) ) My WHT PF My WHT AUX ) ) ) ) Jul WHT AUX ) ) ) ) Aug WHT PF ) ) ) ) 9.99 UT of the Lndolt (99) fields PG nd PG For V, R c, nd I c, we find good greement with the results of Sokolov et l. (997). To include B-bnd clibrtion nd void smll clibrtion differences with respect to Sokolov et l. (997), we used the B, V, R c, nd I c reference strs (,, nd 4 in Fig. [Pl. L]) s given by Sokolov et l. (997) (their str 3 ws close to sturtion nd therefore ws not used). Strs, 5, 6, nd 7 were used s secondry photometric stndrds for the U bnd (see Fig. ). We hve corrected for U- bnd tmospheric extinction of.3 times the nominl extinction (s Crlsberg Automtic Meridin Circle V-bnd extinction mesurements indicte). 3 3 This mteril my lso be found t The L Plm webpge is not mintined by the Astrophysicl Journl. Fig.. Top pnel: the R c -bnd light curve of GRB For the differentil light curve (filled symbols), the dt hve been used from Tble (filled strs), Sokolov et l. (997; filled circles), Pedersen et l. (998; filled squres), Grci et l. (997; filled tringles), Schefer et l. (997; filled dimonds), nd Chevlier et l. (997; cross). The open circles represents nondifferentil dt from Djorgovski et l. (997), Cstro-Tirdo et l. (998), Mignoli et l. (997), Kelemen (997), Fruchter et l. (997), nd Metzger et l. (997c). The four open squres represent wide R filter dt by Pedersen et l. (998). The power-lw fit to the second-order corrected dt with exponent is represented by the solid line. A fit to the second-order corrected dt of power lw plus constnt, FR F t C, R0 with exponent is indicted by the dshed line. The con- stnt C corresponds to mgnitude Rc Bottom pnel: the fit to the power-lw decy subtrcted from the second-order corrected dt of Tble (filled strs), Sokolov et l. (997; filled circles), nd Pedersen et l. (998; filled squres). 3. R c -BAND DIFFERENTIAL OPTICAL LIGHT CURVE In Figure, we present the R c -bnd opticl light curve, where we hve included results of differentil photometry (reltive to locl reference strs; filled symbols) nd clibrted photometry (open symbols). The differentil mgnitudes in Tble were determined with respect to strs,, nd 4. The other differentil dt re from Sokolov et l. (997; determined with respect to strs,, 3, nd 4), Pedersen et l. (998), Grci et l. (997), Schefer et l. (997), nd Chevlier et l. (997). The mgnitudes in the lst four dt sets were determined with respect to str 4 only. In the bsolute clibrtion of these differentil dt sets, we used the R c mgnitudes of these reference strs s given by Sokolov et l. (997). Note tht this involves correction to the dt of Pedersen et l. (998), Schefer et l. (997), nd Chevlier et l. (997) by 0. mg for the difference by tht mount in R c for the reference str 4 used by these uthors. The difference, in our dt, in the R c mgnitudes of the OT, resulting from the use of strs,, nd 4, or only str 4 s reference str, hs n rms of 0.07 mg. For dt sets with sufficiently lrge number of points nd covering time intervl of t lest 50 dys on the power-lw decy prt of the light curve (i.e., those of Sokolov et l. 997, Pedersen et l. 998, nd our dt), we mde second-order correction by mgnitude clibrtion offset, determined from the weighted verge offset with respect to the power-lw decy prt of the light curve. The corrections re , , nd for our dt (Tble ), Sokolov et l. (997), nd Pedersen et l. (998), respectively. We hve ssumed conservtive minimum error in the differentil mgnitudes of 0.05 mg. The R c -bnd light curve shows mximum between My 0. nd 0.8 UT,.3.9 dys fter the gmm-ry burst. We fitted the rising nd decying prts of the light curve with power lws, FR t. For the fit to the rising prt, we use both the nondifferentil photometry nd the differentil dt (without the second-order correction), ssuming minimum error of 0.0 mg. For the fit to the decying prt, we only use the second-order corrected dt. The R c -bnd light curve rises for 0.! log t! 0.0 with exponent ( x r 5.0/6). After the mximum (log t 0.7), we find ( x r 49/0). Including in the fit the nondifferentil dt nd the other differentil dt, i.e., without the second-order correction (ssuming minimum error of 0.0 mg), gives ( x r 74.6/38). Since hint of flttening of the light curve ws reported (Pedersen et l. 998), we hve lso fitted power lw plus constnt, F R FR 0 t C, to the decy prt of the second-order corrected dif- ferentil R c -bnd light curve. We find ;

3 No., 998 OPTICAL FOLLOW-UP OF GRB L5 the constnt C corresponds to R ( x c r 43/9; note tht this corresponds to Rc in the clibrtion of Pedersen et l. 998). We lso determined the opticl fluence S of the OT ( Å), tking for its spectrl slope b or 3 (see 4). We find S ( ) # 0 ergs cm ( b ) or S (.90 0.) # 0 ergs cm ( b 3 ) for the fluence collected until dy 0 fter the event. Extrpolting the power-lw decy beyond dy 0, we find totl opticl fluence S ( ) # 0 ergs cm ( b ) nd S (.6 0.4) # 0 ergs cm ( b ) BROADBAND OPTICAL FLUX DISTRIBUTION Between My nd UT, the R c -bnd flux incresed by mg hr. In estimting the spectrl energy distribution, we corrected the My 9.9 UT observtions for this brightening; i.e., we ssumed tht in ll pssbnds the rte of brightening is the sme nd reduced the mgnitudes to single epoch (My 9.93 UT). The My 0.98 UT observtions occurred t mximum light (see Fig. ), so we hve not pplied ny corrections. Corrections to the My dt were clculted using power-lw fit to the decy prt of the R c -bnd light curve (see 3). The corrections re minor (!0.04 mg) nd mrginlly (!0.0) ffect the spectrl slope. We fitted our B, V, R c, nd I c photometry with power lw, F n b n, using the photometric clibrtion of Bessell (979, Tble IV). (We chose to exclude the U-bnd dt to be ble to compre our results directly with Sokolov et l This exclusion does not ffect the spectrl slope significntly.) Before the fits were mde, the dt were corrected for interstellr extinction A V. Using the IRAS dtbse, we hve determined the 00 mm cirrus flux in the direction of the OT, I 00! 0.08 MJy sr ( j), corresponding to A V! 0.0 (Lureijs 989). However, to compre our results with those of Djorgovski et l. (997), we lso used their vlue for the interstellr extinction ( AV 0.08, corresponding to their AB 0.). The effect of vrying the interstellr extinction on the spectrl slope is smll; the mximum correction to the exponent b (with respect to AV 0.08) is 0.0 (for AV 0). We hve lso tken the B, V, R c, nd I c mesurements of Sokolov et l. (997), nd the ner simultneous U nd R c mesurements of Cstro-Tirdo et l. (998), nd determined b, correcting for n extinction of A V In Figure 3, we hve plotted the spectrl slopes s function of time, including the results of Djorgovski et l. (997) nd Metzger et l. (997b). 5. DISCUSSION The differentil R c light curve shows tht devitions from smooth power-lw decy re moderte (rms 0.5 mg; bottom pnel in Fig. ). The mgnitude Rc for possible underlying glxy is consistent with the limits derived by Shu et l. (997b) nd Pin et l. (998), i.e., Rc 5.5. We note, however, tht the introduction of constnt to the power-lw fit hs not improved the x substntilly; we conclude tht the evidence for flttening of the light curve t lte times is not yet convincing. The opticl fluence S ( Å, b ; 3) is bout 5% of the BATSE kev nd 0% of the WFC 6 kev GRB fluences. A similr comprison with the X-ry fterglow (Piro et l. 997; SX 7.3 # 0 ergs cm, 0 kev, from n extrpoltion of the X-ry light curve for t Fig. 3. The spectrl slope b s function of time in dys fter the burst (we used AV 0.08; see 4). The dt re from this Letter (open strs), Sokolov et l. (997; crosses), Djorgovski et l. (997; filled circle), Metzger et l. (997b; open circle), nd Cstro-Tirdo et l. (998; open squres). The mximum of the light curve occurred between dy.3 nd dy.9. 7 s) shows tht the opticl fterglow fluence is 0% of the X-ry fterglow fluence. The opticl spectrum of the OT becomes redder before reching mximum light (Fig. 3); fterwrd, the spectrl slope remins constnt. We distinguish three phses of the light curve: phse I is the rising phse ( , b ), phse II is t mximum light ( 0, b ), nd phse III is the decying phse ( , b. 0.06). Wijers, Rees, & Mészáros (997) give reltion between the spectrl slope b nd the temporl slope for the simplest firebll remnnt model ( forwrd blst wve with single Lorentz fctor G; their eq. []) nd for bemed firebll; i.e., G nd the energy per unit solid ngle E re functions of ngle (their eq. [4]). For phse III, we find very good greement with , implying b for the bemed cse ( mrginl greement with b for the simple cse). Phses I nd II re not in greement with the simple nd bemed firebll models tht predict less steep or even inverted spectrl slopes b. However, these erly phses might be explined by continuous distribution of Lorentz fctors (Rees & Mészáros 998). Lower G (redder) mteril tkes longer to sweep up significnt mount of externl mtter, nd hence its emission is delyed with respect to the higher G (bluer) mteril. The lower G mteril ctches up grdully with the decelerted higher G mteril. Hence, the rise of the light curve nd the evolution from blue to red cn be explined with the building up of the emission from mteril with lower G, while the powerlw decy (phse III) sets in when most of the mteril hs been mixed to single G. The WHT is operted on behlf of the English PPARC nd Dutch NFRA t the Spnish observtory Roque de Los Muchchos on L Plm, Spin. The 3.5 m WIYN telescope t Kitt Pek Ntionl Observtory, Ntionl Opticl Astronomy Observtories, is operted by the Assocition of Universities for Reserch in Astronomy (AURA), Inc., under coopertive greement with the Ntionl Science Foundtion. The WIYN

4 L6 GALAMA ET AL. Vol. 497 Observtory is joint fcility of the University of Wisconsin t Mdison, Indin University, Yle University, nd the Ntionl Opticl Astronomy Observtories. T. Glm is supported through grnt by NFRA under contrct C. Kouveliotou cknowledges support from NASA grnt NAG We thnk the BeppoSAX tem for the lert. REFERENCES Bessell, M. S. 979, PASP, 9, 589 Bond, H. 997, IAU Circ Cstro-Tirdo, A. J., et l. 998, Science, 79, 0 Chevlier, C., et l. 997, IAU Circ Cost, E., et l. 997, IAU Circ b, IAU Circ c, IAU Circ Djorgovski, S. G., et l. 997, Nture, 387, 876 Fishmn, G. J., et l. 989, in Proc. GRO Sci. Workshop, ed. W. N. Johnson (Greenbelt: GSFC), 3 Fruchter, A., et l. 997, IAU Circ Glm, T. J., et l. 997, Nture, 387, , in Conf. Proc. Fourth Huntsville Symp. on Gmm-Ry Bursts ed. C. Meegn, R. Preece, & T. Koshut (Woodbury: AIP), in press Grci, M. R., et l. 997, ApJL, submitted (stro-ph ) Groot, P. J., et l. 997, IAU Circ b, IAU Circ Heise, J., et l. 997, IAU Circ b, IAU Circ In t Znd, J., et l. 997, IAU Circ Jger, R., Heise, J., In t Znd, J., & Brinkmn, A. C. 993, Adv. Spce Res., 3(), 35 Kelemen, J. 997, Inf. Bull. Vrible Strs, 4496, Kouveliotou, C., et l. 997, IAU Circ Lndolt, A. U. 99, AJ, 04, 340 Lureijs, R. J. 989, Ph.D. thesis, Groningen Univ. Metzger, M. R., et l. 997, IAU Circ b, Nture, 387, c, IAU Circ Mignoli, M., et l. 997, IAU Circ. 666 Ntrjn, P., et l. 997, NewA, (5), 47 Pedersen, H., et l. 998, ApJ, 496, 3 Pin, E., et l. 998, ApJ, 49, L03 Piro, L., Scrsi, L., & Butler, R. C. 995, Proc. SPIE, 57, 69 Piro, L., et l. 997, preprint (stro-ph ). 997b, IAU Circ Rees, M. J., & Mészáros, P. 998, ApJ, 496, L Shu, K. C., et l. 997, Nture, 387, b, ApJ, 489, 7 Schefer, B., et l. 997, IAU Circ Smith, D., et l. 998, in Conf. Proc. Fourth Huntsville Symp. on Gmm- Ry Bursts, ed. C. Meegn, R. Preece, & T. Koshut (Woodbury: AIP), in press Sokolov, V. V., Kopylov, A. I., Zhrikov, S. V., Cost, E., Feroci, M., Nicstro, L., & Plzzi, E. 997, preprint (stro-ph ) Tkeshim, T., et l. 998, in Conf. Proc. Fourth Huntsville Symp. on Gmm- Ry Bursts, ed. C. Meegn, R. Preece, & T. Koshut (Woodbury: AIP), in press vn Prdijs, J., et l. 997, Nture, 368, 686 Wijers, R. A. M. J., Rees, M. J., & Mészáros, P. 997, MNRAS, 88, L5

5 Fig.. WHT prime focus 3.5 # 3.5 R c -bnd imge of the field of GRB Bond s vrible (OT) nd the six strs used for clibrtion re indicted. Note tht Sokolov s str 3 (our str 5) is not indicted since we did not use it s secondry B, V, R c, nd I c photometric stndrd. The U-bnd mgnitudes of the four U-bnd comprison strs, strs, 5, 6, nd 7, re , , , nd , respectively. Glm et l. (see 497, L4) PLATE L