arXiv:astro-ph/9704260v1 26 Apr 1997 icvr a oncnre yteHTosrainon This observation 1995). HST al. the et by (Paradijs event confirmed the GRB soon at the was of 1997b) discovery gal source distant al. dis- a a et suggesting a as transient, (Groot of optical the report source of a extended position came optical an Next an of nu- 1997a). of covery by al. detection followed et a been (Groot and variable has observations, This optical 1997b). merous al. et (Costa across h -a aeayedda ro o only box error an with yielded position new source camera the a X-ray A of by the measurement 1997a). 1997 accurate al. 28, and et February fast (Costa on Beppo/SAX 970228 satellite GRB Italian-Dutch burst the of cloud Oort the 1992). also from Greiner authors originating & GRBs Some (Bickert of 1993). idea Thompson Lamb the & & entertain Bulik Duncan 1995a; Li, Lamb high & 1995b; invoking can (Bulik features models these corona neutron both galactic velocity by that shown reproduced (Meeegan been be distribution has also brightness It the 1995). well al. in as at roll-over BATSE, the of by by degree measured high as distribution the GRB by of suggested isotropy strongly un- was cosmological still scale The is distance 1995). (GRBs) Lamb bursts (Paczy´nski ray 1995; debate gamma der the a of et origin (Klebesadel the discovery 1973) their after decades three Almost INTRODUCTION 1 o.Nt .Ato.Soc. Astron. R. Not. Mon. .Czerny B. of models the the for of bursts 970228 motion gamma-ray GRB proper of the counterpart of optical discovery the of Implication 0Otbr2018 October 30 3 2 1 bevtr,Uiest fHlik,Bx1,004Helsink 00014 14, Box Ch Helsinki, of of University University Astrophysics, Observatory, and 00- Astronomy 18, of Bartycka Department Center, Astronomical Copernicus Nicolaus h ra-hog nti edcm ihtedetection the with came field this in break-through The 1 .Bulik T. , 000 0–0 00)Pitd3 coe 2018 October 30 Printed (0000) 000–000 , sdrcl once ihtebrtN osrit r on o G for found are constraints No burst the 6 with between connected directly is e words: Key ntefaeoko h otpplrgmarybrtmdl.W fin We models. 970228 burst detecte ray GRB analyze gamma was to We popular ejecta level. distance most the detection the the Keck of of framework the the motion below in faded proper th has of the source counterparts Beppo/S event optical optical the GRB and X-ray by the fading 970228 after a GRB of discovery of the determination to position accurate The ABSTRACT ftedsac otebrtis burst the to mod extende corona distance the Galactic of The the variability out. the ruled if are and models source cosmological point the a that of motion the explain 1 , 2 .Sikora M. , . cand pc 6 am-as uss -as rnins-otcl rnins- transients optical: - transients X-rays: bursts- gamma-rays: ∼ arcmin 2 1 ∼ n .Vilhu O. and 0kpc. 10 axy ,Finland i, 1 asw Poland Warsaw, 716 l. cg,54 ot li v,CiaoI 60637 IL Chicago Ave, Ellis South 5640 icago, > d ftelgtcre ndffrn aebnsso htemis- that show bands wave different follows Analysis spec- in sion 1997). curves a al. of et yielded the (Tonry telescope lines of the identifiable Keck obtain no 7 with using to trum April attempt object on An extended 1997b). performed the exis- al. sub- been A the et has component. shown (Sahu extended observation has an HST and which source sequent 1997a), point a al. of et tence (Sahu 26 March httepitojc a infiatpoe oinof motion proper significant a year has mas 540 object point the that (Wijer afterglow fireball M´esz´aros relativistic & a Rees 1997). with consistent be a ae eo eeto ii nteRbn sn the using variabilit band a also R 0 indicating the 6, a and in on 5 April limit on detection telescope Keck source below extended the faded that 1997) has al. at (Metzger reported been edsusteglci ooamdl.I eto we 4 section In models. corona galactic the the dis- sect on discuss in define important, limit we and are lower parallax 3 effects relativistic a the which derive from for and 970228 tances we GRB scale 2 to section distance distance In burst models. gamma-ray their the on observations . host the with object extended > 6 . c eetmt h muto nryrqie to required energy of amount the estimate We pc. 6 0 p rtevraiiyo h xeddsource extended the of variability the or kpc 100 oprsno h w ubeosrain a shown has observations Hubble two the of comparison A nti ae edsustedrc mlctoso these of implications direct the discuss we paper this In . 5 ′′ 3 cl,adudriigteietfiaino the of identification the undermining and scale, ≈ − t 1 − Crvoe l 97.Frhroe thas it Furthermore, 1997). al. et (Caraveo 1 Rihr 95 hc a enfudto found been has which 1995) (Reichart us.Aotamonth a About burst. e n h extended the and d l ue difficulties suffer els hs observations these oe ii on limit lower a d ore efind We source. d XStlieled Satellite AX B originating RBs stars:neutron ion ≈ s, y 2 B. Czerny, T. Bulik, M. Sikora, O. Vilhu overview the requirements for the cosmological model, and 2.2 Characteristic scales finally we discuss the results in section 5. The observed variability of the observed counterparts: the moving point source and the fading extended object in- troduces characteristic regimes in the distance scales. The 1 2 LIMITS ON THE DISTANCE SCALE proper motion of the point source on the sky is 0.54”year− . The inferred linear velocity perpendicular to the line of sight 2.1 Minimum distance to GRB 970228 is

5 D The possibility that burst are very local phe- vps ≈ 10− c . (3) nomenon is not frequently discussed since there is no specific ⊥ 1pc suggestion of how the gamma rays may actually be produced Thus the distance at which the apparent velocity of the point in the result of, for example, the cometary collisions. Also source on the sky reaches the speed of light is D1 = 100 kpc. the present knowledge about the structure of the Oort cloud The angular size of the extended source is ≈ 0.5”, and does not seem to be in agreement with the statistical proper- we take a conservative assumption that it varies on the ties of gamma bursts (Maoz 1973). However, the conclusion timescale of a month. A typical apparent linear velocity i.e. of that study was not firm so in any case it is interesting to the ratio of the linear size to the observed variability scale consider the lower limit for the distance to the source which is can be derived from the discovered proper motion. ex −4 D If the source is really as close to the Earth as the Oort v⊥ ≈ 10 c . (4) cloud its apparent motion consists of two components: (i) 1pc ex proper motion and (ii) the effect of parallax due to the or- This define a distance at which v⊥ = c, so D2 = 10 kpc. bital motion of Earth. The second effect is unavoidable and We can now consider three regions in the space of pos- it is difficult to suspect that the two independent movements sible distance to GRB 970228: A. the nonrelativstic region might simply cancel. D < D2, B. the mildly relativistic region D2

VEartht sin θV −GRB 2.5 π = = (1) D D The direction of the observed proper motion has been re- 3 GALACTIC HALO DISTANCES ported as “south-east” while the expected direction of par- In the framework of these models GRBs originate from allactic motion is almost exactly east at this time of year. −1 high velocity neutron stars, with characteristic velocities Therefore only a fraction equal 0.38”year out of the to- ∼ −1 −1 1000 km s . Such objects would leave the Galactic plane tal velocity of 0.54”year can be attributed to the paralax. −1 (the escape velocity from the Galaxy is ∼ 600km s ) and Taking π = 0.38, as observed, we obtain the minimum dis- form a Galactic corona (Lamb 1995; Li & Dermer 1992). Ob- tance to the source servations of isotropy of gamma-ray burst distribution on

Dmin = 6.6 pc. (2) the sky (Meeegan at al. 1995) provide a strong constraint on the distance to a typical burst. This constraint can be In principle the proper motion of the object may com- consistent only with models characterized by strong source pensate for the orbital motion of the Earth and the distance evolution (also called the delayed turn-on, furthermore DT) to the object can be as small as for example 0.1 pc. How- or within the beaming model (Li & Dermer 1992) where the ever, it requires that the peculiar velocity of GRB 970228 bursting direction is aligned with the direction of the initial be equal to that of the Earth with the accuracy of ∼ 2 %. 1 kick velocity, furthermore BM. The isotropy constrains the ∼ − ◦ ◦ Although the amplitude of the velocity ( 30 km s ) is of beaming angle to be 15 <θb < 25 (Bulik & Lamb 1995b; the same order as the dispersion velocity in the Sun vicin- Duncan & Li 1997). The estimates of the distance to the far- ity the precise coincidence of the direction with the orbital thest burst seen by BATSE varies from 80 kpc to 350 kpc motion of the Earth is improbable. (Duncan & Li 1997; Bulik & Lamb 1995b). Physical models The minimum value of 6.6 pc for the distance of of gamma ray bursts in the Galactic corona invoke mag- GRB 970228 is much stronger limit that the lower limit netic instabilities in super magnetic neutron stars (Duncan of 12000 A.U obtained from triangulation (Hurley 1997). & Thompson 1992; Podsiadlowski, Rees & Ruderman 1995; It clearly excludes the Oort cloud as the birth site of the Lamb, Bulik & Coppi 1995), or accretion of a planetesimal gamma bursts. on a neutron (Colgate & Leonard 1995). Within the framework of each of the scenarios a blast wave or a jet can follow a gamma ray burst event. Galactic nature of GRB 970228 ??? 3

Since GRB 970228 was a relatively bright burst we as- 4 EXTRAGALACTIC ORIGIN sume that it is located at the distance of 100 kpc or less, i.e. Cosmological origin of GRB explains in a natural way both in the mildly relativistic region. This estimate of Γ depends, the impressive isotropy of the burst distribution as well as of course, on the assumed distance and varies from ≈ 1.1 the roll-over of the brightness distribution for low fluxes for the distance of 50 kpc, 1.9 for 100 kpc to ≈ 3 for 300 which leads to V/Vmax ≈ 0.3 instead of 0.5 expected for kpc. The variability of the extended source requires Lorentz a homogeneous distribution in Euclidean space (Meeegan Γ factors in the range from 5 to 30. at al. 1995). Given the above calculated value of Γ we can estimate Within this frame, the most popular family of models that the extent of the visible emission of the jet comes from −1 ◦ are models based on sudden merger of two neutron stars the angle smaller than Γ ≈ 30 . Assuming that the visible or a neutron star and a black hole (Paczy´nski 1991). Such jet is directly associated with the gamma-ray burst we im- a merger is claimed to lead to ejection of a fraction of the mediately obtain that if the expansion is isotropic we only −2 disrupted star at ultra-relativistic speeds corresponding to see a fraction f = 0.25Γ ≈ 0.06 of the emitting surface. the bulk Lorentz factor, Γ > 100 (M´esz´aros & Rees 1993). However, since the source is moving we know that the expan- The ejecta are usually considered to be quasi-spherical or sion is not spherical. In the framework of the DT model this 2 mildly collimated into direction of the angular momentum places an interesting limit, requiring that there is 4Γ ≈ 16 vector of the system. We consider below the implication of times more sources of the galactic corona GRBs or that they the detected proper motion of the optical point source to burst 16 times more often. Such a limit does not arise within this general scenario. the framework of the BM model proposed by (Duncan & Identifying the proper motion of the optical source as Thompson 1992) since the direction of emission is not ran- a motion of relativistic ejecta observed at very small angle domly oriented. It has to be also noted that we find a tan- to the ejecta axis, we obtain that the opening angle of the talizing agreement of two numbers: the estimated angular ◦ ejecta must be, at most, of the order of the Doppler beam width of the jet (30 ) and the beaming angle required by ◦ ◦ (6 1/Γ) (otherwise, one would see on the sky the expanding the isotropy of the BATSE bursts (15 − 25 ) in the frame- source, rather than the propagating source). Let us assume work of this model. that the distance to the GRB 970228 is equal to 1 Gpc which The optical transient has been observed for ∆tobs ≈ corresponds to a rather moderate cosmological redshift of ∼ 1 month. In this time the ejecta must have traveled a dis- 0.16. It is a very conservative assumption since on the basis tance of of the gamma-ray brightness the source may be possibly 2 17 2 ≈ r ≈ c∆tobsΓ ≈ 10 Γ cm . (6) placed at z 0.3 (Wijers, Rees & M´esz´aros 1997), provided it is a standard cosmological candle. Even larger distance is A relativistic shock collects matter which slows it down while suggested by weakness or absence of the host galaxy. Thus, ploughing through space. The amount of matter collected by for a distance of 1 Gpc measured by HST the proper motion the ejecta can be estimated as 540”year−1 translates into the bulk Lorentz factor Γ ∼ 8500. π 2 2 For such large Lorentz factor the ejecta travel for a ∆M = a rρ ≈ a rρ , (7) 25 4 2 3 very large distance r > 10 (Γ/10 ) cm. Thus the ejecta −1 can not avoid interacting with the matter swept over such where a = Γ r is the perpendicular size of the a journey and the mass of the gas collected is at least cone sweeping by the ejecta. Thus we obtain ∆M = 4 4 4 51 4 3 ∆M ≈ 10 (Γ/10 ) M⊙. The energetic requirements of 10 Γ (ρ/1g cm− ) g. A relativistic shock will be efficiently −1 equation 9 can be applied to the case of cosmological models decelerated if it collides with a mass ∆M >meΓ where 4 as well. In particular when Γ = 10 , we find that me is the mass of the ejecta. Thus we obtain a constraint 65 4 6 30 on the mass of the ejecta E > 10 (Γ/10 ) (ρ/10− ) erg. (10)

51 5 −3 me > ∆MΓ = 10 Γ (ρ/1g cm ) g . (8) Such a requirement seems to rule out all of the cosmological models. The variability of the extended source pushes the The energetic requirement for a burst source to accelerate above requirement up by six orders of magnitude. me to a Lorentz factor Γ is 2 42 6 ρ E ≈ mec Γ > 10 Γ erg , (9) 10−30g cm−3 5 DISCUSSION AND CONCLUSIONS where we scaled the density to a typical value for the inter- The proper motion of the discovered point-like optical coun- galactic space. terpart imposes severe constraints on the origin of gamma For the expected range of 1 < Γ < 3 for the motion ray bursts. In the following discussion we make an assump- of the point source these models are on the edge of viabil- tion that the moving point source is directly related to the ity with energetic requirement varying from a comfortable 42 45 GRB. The relation of fading extended object to the GRB 10 ergs to rather constraining 10 ergs. The variability of event seems less clear. Thus we discuss various possibilities, the extended source increases these requirements by a factor 4 6 assuming that either it is or it is not directly related to the of at least 10 and up to 10 . Note that the above estimates GRB. are very conservative since the density in the Galactic halo The comparison of the observed proper motion with can be higher. the parallax expected at a given distance puts a firm limit on the minimum distance to the source equal 6.6 pc. This conclusion rules out all models connecting the gamma burst phenomenon with Oort cloud. 4 B. Czerny, T. Bulik, M. Sikora, O. Vilhu

The assumption of cosmological distances to gamma ray Bisnovatyi-Kogan, G. S. 1995, ApJSS 97, 185 bursts leads also to serious problems. Observed proper mo- Bulik, T., Lamb, D. Q. 1995a, Ap&SS 231, 373 tion requires huge Doppler factors (Γ ∼ 104) for the bulk Bulik, T., Lamb, D. Q. 1995b, Gamma Ray Bursts, Proceedings motion of the radiating gas. The time evolution of the ac- of the Third Huntsville Symposium, (New York: AIP), eds.: companying extended source require the Doppler factor even Kouvelitou, C. Briggs, M. F., Fishman, G. J., p. 345 Caraveo P.A., Mignani R., Bignami G.F., 1997, IUE Circ. 6629 by a factor of 10 higher. We have shown that the energy re- Colgate, S. A., Leonard, P. J. T. 1995, High Velocity Neu- quired to propagate such a highly relativistic gas is eleven tron Stars and Gamma-Ray Bursts (New York: AIP), eds.: orders of magnitude higher than that available in a collision Rotschild, R. E., Lingenfelter R. E., p. 269 of two neutron stars. Costa, E., et al. 1997a, IAU Circ. 6572 Also the models locating gamma ray bursts in the Costa, E., et al. 1997b, IAU Circ. 6576 Galactic halo may have difficulties. The energetic require- Duncan, R., Li, H. 1997, ApJ in press ments can be satisfied if the typical distance to a GRB is Duncan, R., Thompson, C. 1992 ApJ 392, L9 less than 100 kpc. Such a distance scale is marginally consis- Groot, P. J., et al. 1997a, IAU Circ. 6584 tent with the BATSE observations of isotropy of the GRB Groot, P. J., et al. 1997b, IAU Circ. 6588 distribution. The delayed turn-on models (DT) require that Hurley, K. 1997, ApJL submitted 2 Klebesadel, R. W., Strong, I. B., & Olson, R. A. 1973, ApJ 182, the sources burst 4Γ ≈ 16 more times, while in the beaming L8 model (BM) no such requirement arises. Lamb, D. Q. 1995, PASP 107, 1152 The Γ factors required by the variability of the extended Lamb, D. Q., Bulik, T., Coppi, P. S. 1995, High Velocity Neu- source make the energetic requirements prohibitive for the tron Stars and Gamma-Ray Bursts (New York: AIP), eds.: galactic halo models of GRBs. Rotschild, R. E., Lingenfelter R. E., p. 219 The apparent motion of the point source or the vari- Li, H., Dermer, C. D. 1992, Nature 359, 514 ability of the extended source do not constrain the models Li, H., Duncan, R., Thompson, C. 1993, Gamma-Ray Bursts: for distances in the nonrelativistic region A., i.e 6.6 pc < Proceedings of the 2nd Huntsville Workshop (New York:AIP), D < 10 kpc. For example we cannot rule out the moder- eds.: Fishman, G. J., Brainerd, J. J., Hurley, K. p. 600 ately local origin of the gamma bursts. If the bursts come Maoz, E. 1993, ApJ 414, 877 Metzger, M. R., Cohen, J. L. Blakeslee, J. P., Kulkarni, S. L., from the distances of order of 5–100 pc and the roll-over of Djorgovski, S. G., Steidel, C, C. 1997, IUE Circ. 6631 the brightness distribution is an apparent effect caused by Meegan, C. A. 1996, ApJS 106, 65 the detection threshold, like suggested by (Bisnovatyi-Kogan M´esz´aros, P., Rees, M. J. 1993, ApJ 405, 278 1995), who argues that such a geometrical distribution may Paczy´nski, B. 1991, Acta Astr. 41, 257 also be acceptable at the present stage. Paczy´nski, B. 1995, PASP 107, 1167 All these conclusions are based on assumptions that (i) van Paradijs, J. 1997, Nature 386, 686 the identification of the optical counterpart of the gamma Podsiadlowski, P., Rees, M. J., Ruderman, M. 1995, MNRAS 273, ray burst was correct and (ii) the measurement of the proper 755 motion of the point-like optical counterpart was undoubtful. Reichart, D. 1997, ApJ submitted Unfortunately, further observations of GRB 970228 are dif- Sahu, K., Livio, M., Petro, L., & Macchetto, F. D. 1997a, IAU Circ. 6606 ficult since the source approaches the Sun on the sky. This Sahu, K., Livio, M., Petro, L., Macchetto, F. D., van Paradijs, part of the sky will be visible again in a few months but J., Kouveliotou, C., Fishman, G., & Meegan, C. 1997b, IAU the source is rapidly fading at all frequencies so its future Circ. 6619 detection may be impossible. Tonry, J. L., Hu, M. W., Cowie, L. L. 1997, IAU Circ. 6620 Therefore the confirmation of our conclusions would Wijers, R.A.M.J., Rees, M. J., M´esz´aros, P. 1997, MNRAS sub- rather come from the detection of similar behavior of the mitted optical counterparts of other gamma bursts well localized by Beppo/SAX. Since several such bursts a year are expected This paper has been processed by the authors using the at least one or two should be bright enough to allow the Blackwell Scientific Publications LAT X style file. accurate measurements and similar analysis. E

ACKNOWLEDGEMENTS

We would like to thank the Danish Space Research Insti- tute where the idea of this paper has been born in a discus- sion over lunch. This work was supported in part by grants 2P03D00410 (BCZ), 2P02D00911 (TB), and 2P03D01209 (MS) of the Polish State Committee for Scientific Research.

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