Theory of Gamma-Ray Burst Emission in Light of BSAX Results

arXiv:astro-ph/9802193v1 13 Feb 1998 ( X-ray, of wealth the —————————————————— by theoret- challenged All are models puzzles. ical and complexities emerging its now all is with phenomenon GRB complete the more of A dif- picture physics. confronting GRB of of way problems new ficult a old opened to shattered and satellite discovery beliefs, X-ray BSAX an boxes. of error slews GRB carrying ever and fastest models the out theoretical ignoring in able flares radio delayed [31]). few (e.g., possibly a or lasting transients de- minutes/hours UV/optical to rapid search- be for hope to ing only believed was the counterparts Be- GRB discovery, tect initial BSAX [25]). the the (e.g., by fore acceleration produced particle component of impulsive decay hard models’ faster the much ‘Pre-BSAX a of predicted waves events. shock GRB main hours-days the lasting after emission Cer- hard high-energy of surprise. prediction [14,51] by theoretical no us [8,14, was cought there research tainly, discovery GRB The of 35]. field the revolutionized Surprise of Year A 1. a BSAX. by good detected very GRBs in of is majority the model T for shock after keV) emission. synchrotron GRB (2–500 GRB The X-ray afterglow first unsolved. and the still initial c of the are discovery concerning expansion the issues fireball after of obtained theories data wave shock Relativistic b Results BSAX a of Light in Tavani M. Emission Burst Gamma-Ray of Theory .Bat .Gom .Fiore. Scarsi, F. Nuclear L. & in Giommi P. eds. published Bradt, Supplement, H. be Proceedings To B Rossi-Physics Lincei. and dei Beppo-SAX Nazionale from Results XTE Sky: X-Ray ⋆ oubaAtohsc aoaoy oubaUiest,NwYork, New University, Columbia Laboratory, Astrophysics Columbia FT/N,vaBsii1,I213Mln (Italy). Milano I-20133 15, Bassini via IFCTR/CNR, ae rsne tteSymposium the at presented Paper ) ebifl ics h hoeia mlctoso eetde recent of implications theoretical the discuss briefly We h neut fteBA emwsremark- was team BSAX the of ingenuity The BSAX by afterglows X-ray of discovery The oe(tl) 12 coe 97 Accademia 1997, October 21-24 (Italy), Rome , a,b h Active The alne ytewat fXry pia n radio and optical X-ray, of wealth the by hallenged itne,d hyipyanwkn fexplosive of kind new a imply they do distances, [30]). models (e.g., star neutron coalescing of estimates mistic lw[7 n osbeotclcutrat[4 of [34] ( counterpart 970508 GRB optical possible and [37] glow at emitted is [8,37]). fluence (e.g., total times the late fraction of a X-ray 10% of least than case at larger the BSAX, in by Also detected detected that [19]. afterglows event of main 10% the than for larger is of 940217 emission pulses. gamma-ray GRB GRB delayed the initial of of fluence The timescales decay the than soitdwt hstp fbrt a eanac- timescale remain a can for bursts Particles celerated of type afterglows. this GRB with associated of interpreted manifestation be a naturally now as can can emission 940217 range GRB gamma-ray keV delayed of 50–300 The deter- the misleading. ‘durations’ be in GRB BATSE by how mined shows 940217 GRB clearly of case [19] the in EGRET by ini- discovered of ? timescales pulses cooling tial the longer from more expected much of than iceberg lasting events. the processes of impulsive radiation tip complex main the GRBs the classical some- Are after days, (hours, weeks) in times dissipated late times is at range energy X-ray GRB the of fraction stantial issues. open the discuss re- of briefly problems some We here moment. many the and at unsolved data, main radio and optical eosrain loriemn usin htare that questions many raise also observations he h oa nryifre rmteXryafter- X-ray the from inferred energy total The SXsoe o h rttm htasub- a that time first the for showed BSAX etoso am-a uss(Rs yBSAX. by (GRBs) bursts gamma-ray of tections reetwt iersle ra-adspectra broad-band time-resolved with greement lw SXdt otiuet drs several address to contribute data BSAX glow. fGBsucsaea extragalactic at are sources GRB If h dlyd am-a emission gamma-ray ‘delayed’ The ∼ > Y107(USA). 10027 NY 10 52 rs xed h otopti- most the exceeds ergs) ∼ 10 2 − 10 3 ie longer times ⋆ 1 2 phenomenon ? (see also the speculations of ref. host or a foreground galaxy [26]). After the de- [32] to be compared with those of ref. [30]). layed rise, the optical lightcurve follows a power- Renormalizing the logN–logP distribution of law decay of index ∼ 1.17±0.04 for several weeks GRBs (see Fig. 1) in terms of redshift for an (e.g., [34]). The nature of the OT associated with assumed luminosity function can be done for GRB 970228 is currently controversial, with a GRB 970508 associated with the (lower limit) nebulosity near a fading pointlike OT [46,39,5,16]. optical transient redshift z = 0.83 [26]. Unless The OT was still detectable by HST ∼ 6 months GRB 970508 is anomalous, it can be shown that after the event near V ≃ 28 [16]. Its inferred the standard candle assumption cannot satisfac- optical lightcurve shows clear deviations from a torily describe the GRB brightness distribution, power-law decay of index ∼ 1.14±0.05 [16]. The contrary to the conclusions of previous studies OT associated with GRB 971214 shows an ini- (e.g., [10]). Cosmological models of GRBs have to tial power-law decay of index ∼ 1.4 ± 0.2 [17], be formulated now without the appeal of a nat- and a possible flattening near R = 25.6 about ural energy scale provided by the neutron star 10 days after the event [21]. What is the nature of coalescence model. What is the origin of the the faint nebulosities associated with GRB 970228 spread in luminosity (and spectral characteristics and GRB 971214 ? If these are distant star- as shown in ref. [44]) of GRB sources ? forming galaxies, why would GRBs be preferen- BSAX discoveries opened the way for rapid tially hosted near the cores of these galaxies rather follow-up observations in the optical and radio than farther away as coalescing neutron star mod- bands. At present, four out of eight GRBs de- els predict ? If the comoving GRB formation tected by BSAX (GRB 970228, 970402, 970508, rate follows that of star forming galaxies (strongly 971214) unambiguously show the existence of fad- peaked near z ≃ 1 [23]), why are only the bright, ing X-ray sources within the WFC error boxes harder and longer GRBs showing the strongest (10 − 30 arcmin2) pointed ∼ 8 hours after the deviation from the Euclidean brightness distribu- events. Of the remaining four GRBs, two error tion [44] ? Significant luminosity and spectral boxes could not be rapidly pointed (GRB 960720 cosmological evolution of GRBs at z ∼> 1 may be and 980109), and the others pointed within ∼ required [44]. 14 − 16 hours show faint sources with ambiguous About half of the GRBs promptly studied by associations (GRB 970111 and 971227). Another optical searches do not show any OT below R ∼ GRB error box pointed by ASCA within 1 day 22 − 23 (GRB 970111, 970402, 970828, 970828, after the event (GRB 970828) also shows an X- 971227, 980109). Is the lack of OTs in more than ray fading afterglow source [27,51]. Usually the half of GRBs due to absorption within the host ? X-ray flux decay is well represented by a power- Is the lack of detectable absorption in many of the −α law of the type t . Why are the time exponents X-ray afterglow spectra obtained by SAX consis- α’s of X-ray afterglows different from burst to tent with optical absorption [taking into account burst ? Is there any correlation between the after- redshifting by (1+z)3 of the X-ray cutoff energy] ? glow strength and the initial GRB peak intensity GRB 970508, the event associated with an or spectrum ? extragalactic OT, is ‘anomalous’ in many ways At present, three optical transients were identi- when compared with other GRBs. (i) It is the fied within the WFC error boxes of GRB 970228 only GRB with a non-monotonic X-ray afterglow [46], GRB 970508 [3] and GRB 971214 [17]. All decay ∼ 1 − 3 days after the event [37]. Four are within the error boxes of the fading X-ray BSAX TOO observations were necessary to fi- sources, reinforcing their associations with their nally observe its decay in the 2–10 keV band (but respective GRBs. However, they all show differ- apparently not in the softer LECS band [37]). (ii) ent characteristics. The delayed (∼ 2 days) opti- Its associated OT was detected to rise ∼ 2 days cal transient (OT) associated with GRB 970508 after the event after a stable plateau state [3], resulted in the spectral identification of absorp- contrary to the other two GRBs showing mono- tion lines of an object at z ≃ 0.83 (either the 3

980109 970402

960720

971227

970111

Figure 1. GRBs detected by BSAX marked on the cumulative brightness distribution of the 4th BATSE catalogue [29]. Peak intensities in the BSAX-GRBM detector have been rescaled to fit BATSE’s energy range (50-300 keV) assuming a power-law photon index of 2. BSAX WFCs are clearly capable of detecting faint GRBs near the detection threshold for BATSE. Three GRBs associated with optical transients (OT) are marked with their respective peak magnitudes (GRB 970228 [46], GRB 970508 [6], GRB 971214 [17]). No OT was discovered in other GRB error boxes. The only GRB associated with a (possibly scintillating) radio source is GRB 970508 [11] of low gamma-ray flux [37]. Radio searches in other GRB error boxes were rapidly performed with null results [13]. 4 tonically decreasing OTs (GRB 970228 [46], and ing that possible deviations [38] from the phe- GRB 971214 [17]). (iii) GRB 970508 is also the nomenological Band’s model [1] affect a minority only GRB so far associated with a (scintillat- of GRBs. ing ?) radio source [11]. Are the peculiar proper- (2) Discovery of substantial spectral re- ties of GRB 970508 caused by the ‘environment’ hardening (e.g., GRB 970228) for late GRB surrounding the GRB source, or can they be at- pulses. Usually hard-to-soft spectral evolution tributed to a background AGN ? Why is only the dominate across individual GRB pulses [28] or very weak GRB 970508 associated with an appar- among different pulses of complex GRBs [33]. ently persistent [12] radio source ? The broad-band detection by BSAX of the com- If delayed radio emission is common in bright plex GRB 970228 [14] clearly shows how a sec- GRBs produced by relativistic fireballs (e.g., ond train of pulses separated from the first part [31]), more radio detections would have been ex- of the GRB by several tens of seconds can be pected (see Fig. 1). much harder than expected from simple extrap- Finding satisfactory explanations to all these olation. Particle re-energization clearly occurs questions will be challenging for any theory of tens of seconds after the main event, giving sup- GRBs. The issues facing cosmological models port to the idea that the second train of pulses in [20,43,47–49] in the ‘post–BSAX era’ of GRB re- GRB 970228 may be related to its strong X-ray search include: an energy crisis, burst number afterglow [8]. density evolution vs. star forming galaxies, lu- (3) Confirming the energy dependence of GRB minosity and spectral evolution, absorption and lightcurves. Many initial GRB pulses detected reprocessing properties of GRB environments in by BSAX show a characteristic apparent ‘time distant galaxies, diversity of X-ray and optical de- delay’ between the hard (50-600 keV) and soft (2- cays, persistent optical emission ∼ 6 months after 30 keV) lightcurves. This phenomenon has also GRB 970228, and lack of radio emission for the been clearly observed in different energy ranges majority of GRBs. by CGRO instruments (e.g., [28]). Rapid spectral Other models should be considered, and a su- evolution with the ‘sweeping’ of a fixed energy perposition of GRB populations of different ori- band by the peak energy Ep of the ν Fν spectrum gins may still be viable. More detections of opti- can satisfactorily explain the observations. cal/radio transients in GRB error boxes are defi- (4) Pulse width vs. photon energy relation: nitely needed. ∆ τ ∝ E−1/2. BSAX clearly confirms the pulse broadening as a function of decreasing photon energy detected also in the BATSE range (e.g., [28]). 2. Contributions by BSAX This feature of GRB pulses strongly support syn- BSAX observations contribute to address im- chrotron models of emission [41]. portant aspects of the GRB emission mechanism (5) Discovery of long-duration X-ray afterglows for both the prompt impulsive emission and the (XRA) occasionally lasting ∼ days/weeks after delayed afterglows. We indicate here ten impor- the events (as in the case of GRB 970228 [8]). tant contributions by BSAX that can be used for Lower limits to the X-ray afterglow fluences can detailed theoretical modelling. be deduced in the range of several tens of a per- (1) Extending the GRB spectrum to X-ray cent of the main pulse fluences. energies: time resolved spectroscopy. BSAX (6) Discovery of ‘average’ XRA power-law de- −α data complement and improve those obtained by cays: Fx ∼ t , with α = 1.1 − 1.6. Inter- GINGA [50,40]. Combining WFC and GRBM preted as manifestations of persistently acceler- data for sufficiently intense GRBs can provide a ated particles, XRAs can be used to derive con- unique database for studying time-resolved spec- straints on the initial and maximum energies of troscopy from ∼ 2 keV to hundreds of keV. No radiating leptons in relativistic shock waves [43]. systematic low-energy ‘suppression’ or spectral Only forward shocks agree with BSAX observa- ‘up-turns’ is detected by BSAX, clearly confirm- tions [43,49], predicting an X-ray flux decay of the 5

−3/2 type fx ∝ ξx(t) t with ξx(t) an X-ray ‘win- (SSM) is of special relevance to relativistic ﬁreball dow’ function depending on spectral evolution. models [41,42]. BSAX results conﬁrm SSM pre- The requirements on the initial (γ∗) and maxi- dictions to ∼ keV energies for the majority of de- mum (γm) energies of particles accelerated by an tected GRB pulses (exceptions will be discussed impulsive relativistic shock are stringent [43] below). The SSM is based on optically-thin syn-

2 1 2 15 −1 2 −3 2 −1 2 3 2 chrotron emission of relativistic particles (elec- / ≃ / / / / ± γm γ∗ 10 no Γ0,2 λB (t/8 hr) (1) trons and/or e -pairs) radiating in the presence of a weak to moderate magnetic field (to avoid where no is the average number density of the magnetic absorption processes) [41,42]. A model surrounding medium, Γ0,2 the initial value of the bulk Lorentz factor of the shock wave in units of that successfully describes broad-band GRB spec- 2 tra is based on a particle energy distribution con- 10 , λB a parameter set to unity for equipartition between kinetic and magnetic field energy densi- sisting of a relativistic Maxwellian below a crit- ties, and t the time after the burst in the observer ical energy. This reflects the physical conditions frame. Fluctuations are occasionally detected in of a thermalized electromagnetic/particle outflow BSAX X-ray afterglows possibly indicating re- produced by an initial impulsive burst. Depend- acceleration episodes (see also the GRB 970828 ing on the characteristics of external media and afterglow detected by ASCA [51]). their radiative environments, an MHD wind is (7) Discovery of strong violations of power-law assumed to interact in an optically-thin environ- XRA decays: the case of GRB 970508 [37]. ment with magnetic turbulence or hydromagnetic (8) First observation of a non-thermal XRA shocks leading to rapid particle acceleration and spectrum (GRB 970228) of photon index near to the formation of a prominent supra-thermal 2 [14]. This important discovery is also sup- component of power-law index δ. Depending ported by a similar spectral determination of the on the efficiency of the acceleration mechanism, GRB 970828 afterglow by ASCA [51]. It can be the particle energy distribution can have different shown that the combination of flux decay and shapes [42]. The SSM results in the dimension- spectral properties of GRB afterglows is inconsis- less spectral function of refs. [41,42]. Fig. 2 shows tent with simple cooling models of neutron star SSM calculations of GRB spectra that reproduce surfaces [43]. with good accuracy the broad-band spectra ob- (9) Lack of prominent X-ray ‘precursors’. tained by CGRO. BSAX spectral data allow to ∼ WFC data can be used to test for the first time extend the energy range of these fits to keV en- the possible existence of precursor X-ray emis- ergies with good agreement between theory and sion as predicted in several fireball models. No observations. We note that the phenomenological evidence for fireball thinning is found, implying spectral model by Band [1] is in excellent agree- values of the bulk Lorentz factor of the expand- ment with SSM spectra, that therefore provide a > natural theoretical foundation for it. ing shell Γ ∼ a few. It can be shown that BSAX and CGRO spec- (10) Delayed GRB emission allows optical tral data imply a maximally efficient acceleration identification. Probably the most important con- mechanism [41,42] not dissimilar from what cal- sequence of the observations originating from the culated [18] and observed (e.g., [9]) in synchrotron BSAX detections. nebulae. The SSM spectral shape turns out to be a universal 2-parameter function in the energy 3. The Synchrotron Shock Emission Model range ∼ 1 keV–1 GeV [the critical energy εc (pro- Among the many possible spectral models of portional to the local magnetic field strength Bs 2 GRB emission (Compton attenuation due to and to γ∗ ), and the index δ]. In the absence Klein-Nishina scattering effects [4], absorption ef- of a supra-thermal component, the low-energy fects by thick material near compact objects [22], (E ≪ εc) emission is dictated by synchrotron or processes leading to electron/positron creation emissivity of energy index 1/3. For intermediate and beaming [2]), the shock synchrotron model 6

GRB 910503

GRB 910814

GRB 910601 GRB 940217

GRB 920622

Figure 2. Calculated SSM ν Fν spectra reproducing the burst-averaged spectra of the GRBs simulta- neously detected by BATSE, COMPTEL and EGRET on board of CGRO (adapted from ref. [41]). Also time-resolved BSAX spectral data are in agreement with these SSM calculations assuming optically thin radiation environments. BSAX data are consistent with SSM spectra of initial peak energies in the range < < 10 keV ∼ Ep ∼ 500 keV and subsequent hard-to-soft spectral evolution. An interesting exception is indi- cated by the initial part of the GRB 960720 pulse showing an apparent absorption below Ep ≃ 100 keV compared to the SSM prediction. Most likely, a modiﬁcation of optically thin SSM by relativistic plasma eﬀects is implied for this type of bursts [45]. 7

< photon energies E ∼ εc the spectrum steepens, to a thinner medium as expected in plasma accel- and SSM predicts a very distinctive ‘curvature’ of eration models [45]. the continuum for specific combinations of the rel- More surprises may enrich our study of GRBs ativistic Maxwellian and power-law components in the near future. We need more optical/radio [41,42]. The observed ‘peak’ photon energy Ep identifications of reliable GRB counterparts to is proportional to the relativistic synchrotron en- settle the issue of the GRB origin. A system- ergy εc (modulo possible factors due to Doppler atic time-resolved spectral analysis is necessary blueshift ∼ Γ, cosmological redshift ∼ (1 + z)−1, for both BSAX and BATSE data to test emission and overall IC upscattering, if applicable). No models. Joint BSAX and BATSE spectral analy- substantial modification of GRB spectral shapes sis will be of great importance for GRBs detected by inverse Compton (IC) processes is evident in by both instruments. BSAX and CGRO data. This indicates that IC The author thanks all members of the BSAX cooling, if it occurs at all, ‘gently’ shifts the over- team for many lively discussions and exchange of all spectrum dictated by synchrotron emission. information that stimulated this work. This is a crucial constraint for many cosmological models characterized by an overproduction of IC emission compared to pure synchrotron (e.g., REFERENCES [24]). SSM physical parameters can be derived by by the combined set of CGRO and BSAX spectral 1. Band, D., et al., 1993, ApJ, 413, 281. data [45] 2. Baring, M.G., 1993, ApJ, 418, 391. 3 < < 6 < < 3 3. Bond, H.E., 1997, IAUC no. 6654. 10 ∼ γ∗ ∼ 10 1 G ∼ B ∼ 10 G (2) s 4. Brainerd, J.J., 1994, ApJ, 428, 21. modulo redshift and IC upscattering factors. 5. Caraveo, P. et al., 1998, Proc. 4th Huntsville These values are quite natural for optically thin Symp. on GRBs, eds. C.A. Meegan & P. synchrotron nebulae powered by MHD outflows. Cushman (AIP Conf. Proc. Series), in press. Interesting deviations from the average low- 6. Castro-Tirado, A.J. et al., 1997, IAUC no. energy spectrum are observed in a few cases. An 6657. apparent suppression of low-energy photons dur- 7. Crider, A., et al., 1997, ApJ, 479, L39. ing the initial part of some GRB pulses is detected 8. Costa, E. ,et al., 1997, Nature, 387, 783. in a small fraction (∼< 15%) of events by BATSE 9. De Jager, O.C. et al., 1996, ApJ, 457, 253. [7], and GINGA [40]. However, difficulties of the 10. Fenimore, E.E. & Bloom, J.S., 1995, ApJ, BATSE spectral analysis at low energies and in- 453, 25. tensities, and uncertainties on the incidence angle 11. Frail, D.A. et al., 1997, Nature, 389, 261. of GRBs detected by GINGA make these mea- 12. Frail, D.A., 1997, BAAS, 29(5), 1303. surements somewhat uncertain. BSAX data can 13. Frail, D.A. et al., 1998, Proc. 4th Huntsville resolve the issue. Among the GRBs detected by Symp. on GRBs, eds. C.A. Meegan & P. the WFCs, only GRB 960720 [36] (during the Cushman (AIP Conf. Proc. Series), in press. first second of the main pulse lasting ∼ 10 sec 14. Frontera, F. et al., 1998, ApJL, in press. in the GRBM energy band) shows a clear sign 15. Frontera, F. et al., 1997, IAUC no. 6637. of low-energy suppression. This time-dependent 16. Fruchter, A. et al., 1998, Proc. 4th Huntsville suppression suggests a modification from the ide- Symp. on GRBs, eds. C.A. Meegan & P. alized optically thin conditions assumed in the Cushman (AIP Conf. Proc. Series), in press. simplest version of the SSM. Low-energy SSM 17. Halpern, J. et al., 1997, IAUC no. 6788. emission below ∼ 50 keV can be temporarily sup- 18. Hoshino, M., et al., 1992, ApJ, 390, 454. pressed by relativistic plasma effects [45]. The 19. Hurley, K., et al., 1994, Nature, 372, 652. fact that low-energy suppression occurs at the be- 20. Katz, J.I. & Piran, T., 1998, Proc. 4th ginning of GRB pulses is a clear indication that Huntsville Symp. on GRBs, eds. C.A. Mee- the radiative environment relaxes from a complex gan & P. Cushman (AIP Conf. Proc. Series), 8

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