arXiv:1104.3290v2 [astro-ph.HE] 30 Nov 2012 f[]wsaotdi 47.I hsmdlastar a model this In hole black of supermassive mass [4–7]. the model a with by in (SMBH) flare tidally adopted destructed disruption was was [3] tidal of the J164449.3+573451 phenomenon. different completely a be 144.+741fo h etro star-forming a of redshift center the at the galaxy Swift event from gamma-ray J164449.3+573451 super-long unusual the detected us-otnu.Tepa uioiyo hsburst this of luminosity and of level peak spikes, a light-curve The smaller reached variable numerous quasi-continuum. extremely peaks, a a first high an the keeps several On had it months. with event and few the a days, for days, few activity steady a but for softer activity huge sustained hseetcnb trbtdt h ls fsuperlong of class the to attributed duration a be (with can event this tutrleeeto aatcnce.Cmatsubclus- Compact nuclei. typical a galactic are of clusters and element star (NSs) Dense structural stars there. neutron compact (BHs) holes central of black a mergers multiple of and collapse cluster gravitational of model the in Earth the variable huge to small the activity. direction a producing jet for to accidental J164449.3+573451 similar Swift The is configuration to [8]. later precisely blazar almost The direction a Earth. supposed in the it produced and was jet SMBH, a the that onto accreted was debris the ∗ † -al [email protected] e-mail: [email protected] e-mail: sapsil nepeainteo Swift of the interpretation possible a As nMrh2,21,teSitsBrtAetTelescope Alert Burst Swift’s the 2011, 28, March On eew rps natraieepaain ae on based explanation, alternative an propose we Here wf 144.+741eet eeaini h collapsin the in generation event: J164449.3+573451 Swift ewrs aatcnce;nurnsas am-a burst gamma-ray stars; neutron nuclei; galactic Keywords: 98.70.Rz J1 98.54.Cm, Swift avai numbers: be the PACS can all nuclei exactly galactic reproduce of not collapses will such f observations, and model hole, this black supermassive if a the radiat Even be onto accreti quasi-steady gas also later and of may and accretion collisions) late days peaks the smaller two in Numerous first release hole. matter the to to during similar (due peaks (GRBs) supply Collisi bright bursts The mechanisms. gamma-ray several generate origin. th to can of due holes collapse black bands and different contraction cente in avalanche events galactic an the during in remnants subsystem st self-gravitating and stars compact Neutron a scenario. this Swi in event classification) gamma-ray early cosmic super-long unique the interpret edsustemliadeeg ees namdlo collap a of model a in release energy multiband the discuss We .INTRODUCTION I. > ∼ 0 )GB 2,isedi em to seems it instead [2], GRBs s) 500 5 × z 0 = 10 ∼ nttt o ula eerho h usa cdm fSci of Academy Russian the of Research Nuclear for Institute 48 10 . 0hOtbrAnvrayPopc a 132Mso,Russi Moscow, 117312 7a, Prospect Anniversary October 60th 3534 7 r s erg M ⊙ ± − nteglci centre, galactic the in 0 1 . ti obflthat doubtful is It . 02[] h burst The [1]. 0002 ..Dokuchaev V.I. Dtd ue2,2018) 27, June (Dated: ∗ n uN Eroshenko Yu.N. and iesaeo wf 144.+741is to variability J164449.3+573451 similar fastest Swift The proceed of [13]. can time-scale orbit, of system the close calculations this Before numerical a in the into formation GRBs. captured jet short are and of the remnants model two is ordinary as collision, the bursts, in gamma-ray generate case directly can holes and during collapse: bands core other the in after release energy and generation time relaxation by limited possibil- this is the [12]. collapse of and gravitational (core) stage, of part ity relativistic central a The halo approaches isothermal cluster core. relaxed central “evaporate” a the forming objects density around core, central fast central the the grow, from dispersion relaxation: dynam- velocity evolves two-body com- cluster and to of The subsystem due “cluster”. inner a ically this as to objects [9– refer pact supernovae hereafter as We explosions massive their evolu- most 11]. and of the center the course of to segregationsinking the stars mass in to form due can tion remnants stellar of ters u oahg a est nteclasn aatcnu- galactic collapsing the lasts in emission density prolonged gas The high cleus. with a compared to durations some as due having is GRBs broadening there short peaks usual if the only of light- J164449.3+573451 the mechanism in Swift peaks of high the curve explain could GRBs separate nsscnb epnil o ekgeneration. peak the for followingmech- responsible in the be located case can this not anisms in was directions, Earth jet were the collimated pulses because gamma direct observed Possibly, not [17]. of model jet two aeo SN n SB olsosteetne emis- extended for the sustained collisions be can NS-BH the sion In and NS-NS [16]. [15], of collisions case stars compacts and ordinary of s .Cliin fnurnsasadselrms black stellar-mass and stars neutron of Collisions 1. ray gamma of mechanisms possible several are There .Ec olso fcmatojcs(SB)provides (NS-BH) objects compact of collision Each 2. la-asbakhlscnfr evolutionary form can holes black ellar-mass al o eeto nohrevents. other in detection for lable o atceaclrto ysokwaves. shock by acceleration particle rom .Cliin n egso hs stellar these of merges and Collisions r. not h omn uemsieblack supermassive forming the onto on o a rs rmgaiainllensing, gravitational from arise can ion tJ6493535 GB102Aby 110328A (GRB J164449.3+573451 ft n fnurnsasadstellar-mass and stars neutron of ons 44.+741poete nfuture in properties 64449.3+573451 lse oecnpouepowerful produce can core cluster e h riaymdl fsotGRB short of models ordinary the igglci ulu,adw r to try we and nucleus, galactic sing osqec fmlil matter multiple of consequence † ences ∼ a 0 ,a twssoni the in shown was it as s, 100 trcluster? star g ∼ ≤ 10 .Ti a be can This s. 2 4 ∼ ntemodel the in s 0 .S the So s. 100 2 matter release in the form of debris, fireballs and jets. II. EVOLUTION OF COMPACT STAR This is an alternative source of gas accretion in addition CLUSTER to star tidal disruption [4–7], but the matter of a neu- tron star produces denser and more compact (initially) According to observations, about 50-80% of galaxies ejection. Collisional destructions of several neutron stars with low and intermediate luminosities contain the com- provide a larger power then a single ordinary star. Just pact central star clusters [25], as well as our Milky Way after the SMBH formation, several (from different col- [26]. Most massive stars of these star clusters inevitably lisions) mutually fighting accretion disks or one steady sank down to the dynamical centre and exploded as su- disk (along the angular momentum of SMBH) can form. pernova long ago at the initial epoch of the star clus- The bright peaks in the Swift J164449.3+573451 light ter evolution. Therefore, the formation of the compact curve during the first two days may therefore be a conse- subsystems of NSs and stellar mass BHs must be a com- quence of multiple episodes of violent gas accretion onto mon process in the Universe [9], [10], [11]. Let us con- the forming SMBH. Each NS collision and destruction sider the cluster of NSs and BHs, formed at the time is responsible for a separate peak in the light curve of ti t0.35, where t0.35 10 Gyr corresponds to the red Swift J164449.3+573451. Then the accretion disk forms, shift≪ z = 0.35. The subscript≃ “i” marks the quantities and different radiative processes develop in a similar way at the moment ti. During the dynamical evolution of to the ordinary scenario with tidal destruction of a star. the cluster’s core its mass Mc, number of compact ob- Therefore many properties of the tidal destruction model jects Nc Mc/m, velocity dispersion v, virial radius ≡ 2 could be relevant to our model with only minor modifi- Rc = GNcm/2v and the masses of separate objects m cations. are all evolving. We use the homological approximation 3. Numerous smaller peaks can arise due to gravita- for the mean properties of the core. In this approxima- tional lensing of the signals (from mergers and accretion) tion the behaviour of the core obeys the system of equa- of the forming SMBH and compact objects in the cluster. tion, obtained in [19]. The global parameters of the core Later lensings are similar to “mesolensings” [18] and can change due to objects dynamical evaporation from the produce numerous subpeaks in the light curve. core and due to energy loss for gravitational radiation, 4. Smooth flashes and quasisteady radiation can arise while the rate of the above processes and the inner struc- from gas accretion onto the SMBH at the time of its ture of the cluster are determined by the two-body relax- formation and rapid growth (for a few first days) and ation and mergers of the compact objects. We rewrite the later. system of equations from [19] through the new variables 2 2 5. The galactic nucleus as a whole will be source of x = v /c in the following form:

IR radiation because of multiple fireball formation in- −1 −1 side the gas-dust envelope. The debris of disrupted nor- x/x˙ = (α2 α1)tr +7tcap/5 ˙ − −1 mal stars form the envelope in which the NS/BH clus- M/M = α2tr (1)  −1 ter is submerged. Really, the X-ray spectrum of Swift  m/m˙ = tcap, J164449.3+573451 shows strong absorption which can witness a wide envelope around the central engine. Nev- where tcap takes into account the dissipative processes ertheless, to explain Swift J164449.3+573451 one must of mergers of compact objects and gravitational waves −1 √ suppose that this gas envelope is optically thin for gamma emission, tcap = σcapv/ 2, and tr is the two-body re- radiation. Acceleration of particles by multiple shock laxation time (characteristic time-scale for stellar cluster fronts of the fireballs is also possible. dynamical evolution) 6. This model also predicts huge multiple bursts of 1/2 2 v3 gravitational waves [19]. Similar events will be detectable t = , (2) r 3 4πG2m2nΛ in the near future by new gravitational wave telescopes.   The first of the effects listed above is similar to the where Λ = ln(0.4N) is the Coulomb logarithm and we GRB recurrence scenario which has been elaborated in put Λ 15 hereafter, i. e. Λ const with logarith- [20–22]. This model associates the recurrent and mul- mic accuracy,≃ n is the compact objects≃ number density. tiple GRBs with the final dissipative stage of central According to [27], the capture cross-section for the two star clusters in galactic nuclei, which precedes SMBH objects with masses m1 and m2 is formation. Accidental gravitational captures produce short-lived pairs of compact objects. According to the 2 10/7 2/7 2/7 6πG (m1 + m2) m1 m2 most popular scenarios, short GRBs (with a duration of σcap , (3) ≈ 210/7c10/7v18/7 2 s)are generated during coalescence of two NSs or a ≤ NS and a BH in binary systems [23, 24]. It has been and we put m1 = m2 = m in this Section. The coef- −4 −3 shown in recent calculations [13] that mergers really pro- ficients α1 = 8.72 10 and α2 = 1.24 10 were duce short GRBs. Similar but multiple GRBs can be obtained in the Fokker-Plank× model [19].× This formal- generated during the clusters core gravitational collapse ism describes the contraction of the clusters’ core, while just before the SMBH birth. This model could also ex- the evaporated objects form the isothermal halo around plain the unusual double burst GRB 110709B [14]. the core with density profile ρ r−2. The halo can ∝ 3

5/7 be involved into the avalanche contraction after the core of BH have time to grow up to mi(xf /xdis) 42mi, reaches a relativistic stage, as will be discussed in the according to (5), although the∼ validity of (5) is doubtful∼ next section. near the moment of the collapse. The homological model Most of the cluster lifetime is non-dissipative (one can gives a useful qualitative description, but numerical re- neglect the tcap terms). From the solution of (1) in this sults must be accepted with some caution. This picture case we have: of the dynamical evolution is somehow complicated in the presence of an angular momentum, but it is not impor- ν1 ν2 Mc = Mi[1 (t ti)/(κtri)] , Rc = Ri[1 (t ti)/(κtri)] , tant in the central relaxed part of the cluster. A serious − − − − (4) modification is required for a cluster with a pre-existing where ν1 = 2α2/(7α2 3α1), ν2 = 2(2α2 α1)/(7α2 massive BH at the center. In this case the central BH − − − 3α1), κ = 2/(7α2 3α1). The quantity te κtr,i grows slowly instead of a fast avalanche collapse of the − ≡ ≈ 330tr,i approximately equals to the duration of the clus- cluster. ter evolution up to the time of the full evaporation or In plenty of works, the dynamical evolution of globular gravitation collapse into a SMBH, because the final dis- clusters was considered in the “evaporation model” (see, sipative stage is rather short. e. g., [12]). This model supposes that an object leaves the The solution of the exact Equations (1) has the form core with zero kinetic energy, so that the total energy of 2 the evolving core is conserved Ec Mc /Rc const. 5/7 5/7 5/7 5/7 ∝ ≈ m(x)= mi(x + xdis )/(xi + xdis ), (5) The rate of evaporation was calculated from the high- velocity tail of Maxwellian distribution. Let us compare

α the evaporation model with the considered above model 7 2 α −α x5/7(x5/7 + x5/7) 5( 2 1) of [19]. In the evaporation model α1 = 0 and α2 = i dis −3 Mc(x)= Mi , (6) 7.4 10 , and the duration of core evolution till the x5/7(x5/7 + x5/7) × " dis i # collapse te 2/(7α2) 38.6tr,i is several times less in comparison≃ with the above≈ calculations. It means that where the core more easily reaches the relativistic stage and its 7/5 −4 initial mass Mi could be smaller. For this model xdis xdis 10Λ(α2 α1)/(7√3) 5.3 10 . (7) −2 −≃7 3.5 10 (v 0.19c), τdis 3.6 yr, xi 2 10 ≡ − ≃ × × −≃1 ≃ 6 ≃ × h i (vi 140 km s ) and Mmin 3 10 M⊙. The cluster of NS had formed long before the dissipative One≃ can roughly suppose that≃ × the epoch of collapses stage. Near the dissipative stage x xi. Putting m = lasts t0 10 Gyr, then the total rate of such collapses in ≫ ≃ 2mi in (5), we obtain x = xdis. Therefore, the dissipative the observable Universe is estimated to be stage begins at v/c 0.023, i. e. v 6.9 103 km s−1, ≃ ≃ × independently from the initial parameters of the cluster. 4π 3 −1 ng −1 N˙ h (ct0) ngt 0.1 yr , (11) Duration of the dissipative stage ∼ 3 0 ≈ 10−2Mpc−3   2 −1 Mc m where ng is the number density of structured galaxies τdis = tcap(xdis) 4 4 yr. (8) with central clusters. It seems natural that the collapse ≃ 10 M⊙ 2.8M⊙     predicted by this model could be seen for several years

Collapse occurs at x xf 0.1 [19]. If we require the of observations. Moreover, the clusters core just before ≃ ≃ 9 collapse at z =0.35, i.e. κtr,i = t0.35 9.8 10 yr, we the collapse can be a promising source of multiple gravi- get ≃ × tational wave bursts [19].

4/3 −2/3 −7 Mi mi x =9 10 , (9) III. AVALANCHE CONTRACTION AND i × 107M 1.4M  ⊙   ⊙  GRAVITATIONAL COLLAPSE OF THE −1 CENTRAL CORE and it corresponds to vi 280 km s . If we require additionally that at the beginning≃ of the collapse the core Now we consider the final highly dissipative stage of hasn’t evaporated, Mc(xf ) > 2mi, then we have from (6) and (9) the cluster evolution. We suppose that the central clus- ter consists of stellar-mass BHs with an admixture of 7 Mi >Mmin 3 10 M⊙, (10) non-coalesced NSs. The BH masses in the cluster grow ≃ × through mergers [19]. Suppose that their masses imme- 7 or N > 2 10 if m = 1.4M⊙. The boundary mass diately before the core’s collapse are mBH 10m , where i × i ∼ i Mmin sets the minimum for the initial mass of the clus- mi 1.4M⊙. In the central core mBH 42mi according ter in the scenario under consideration. Mass segregation to the≃ rough estimation of the previous∼ section, but we accelerates the evolution as compared with this homolog- put mBH 10m as mean over the cluster. We put also ∼ i ical model. Therefore, the real Mmin may be somehow that the cluster has the total mass M (mainly in the form smaller. We put in the following estimations Mmin of BHs) and the NSs constitute a fraction fNS 1 of M. 7 ∼ ≪ 10 M⊙. Note that till the moment x 0.1 the masses m The dynamical evolution of clusters is accompanied by f ≃ 4 the secular growth of the central velocity dispersion or, host galaxy of Swift J164449.3+573451 didn’t show activ- equivalently, by the growth of the central gravitational ity long before the event. The first activity, which could potential. Just near the collapse stage the cluster has a be associated with the beginning of the collapse and the dense core with mass Mc, radius Rc 3Rg,c (last stable fast collisions, was found in the Burst Alert Telescope’s 2 ≃ orbit), where Rg,c = 2GMc/c , and velocity dispersion data at 3 days before the main event, and the flux of the 0.3c. All objects with a small angular momentum will precursor was 7% of the first main peak [6]. ≃fall onto the core without returning to the outer part Just as the central∼ relativistic region had appeared it of the cluster. The avalanche occurs due to the pres- began to capture objects which flew through this region ence of quasielliptical orbits that connect different layers inside the “loss-cone”. For the object at the radial dis- of compact objects [28]. Objects from the loss-cone are tance r from the core’s centre the loss-cone is character- captured by the central core, and the central region col- ized by the angle φ b/r, where b2 R r (c/v)2 and ≃ ≃ g,c p lapses into the SMBH. Numerical calculations [29] shows the value of rp is the same as in the process of gravita- that the several percent of the total cluster mass will tional capture of two objects [27] with masses m1 = mBH collapse onto the forming SMBH during the several dy- and m2 = M mBH: c ≫ namical times tdyn = Rcl/v – duration of the avalanche 7/2 5/2 85πG mBHMc contraction, where Rcl is the radius of the whole cluster 7/2 rp . (15) (halo around the core). ≃ 12√2c5v2 In view of fNS 1, the NS-BH coalescences are For the calculations we use the isothermal density profile ≪ 2 more probable than the NS-NS ones. At the mass ra- of the relaxed cluster ρ(r) = M/(4πRclr ) till the core tio mBH 10mi, the NS does not fall into the BH as radius R and the isotropic velocity distribution. The ∼ c a whole but is disrupted by tidal forces and produces a number of BHs inside the “loss-cone” is estimated to be

fireball and GRB. The rate of NS-BH coalescences in the Rcl ˙ 2 2 5/7 2/7 core is Nc = σcapuNNSnBH, where the gravitational cap- 4πρ(r)r dr πb 5/7 Rg,c Rcl Ncone N 5.6 N . ture cross section for objects with different mass is given ≃ M 4πr2 ≃ × R Z g by (3), NNS = fNSMc/m is the number of NSs in the Rc core, and nBH is the BH number density in the core. For (16) 7 the particular parameters M = 10 M⊙ and v =0.05c we For the fiducial parameters, used above, we have numer- 5 −4 ically Ncone 3 10 . Therefore, the mass inside the have Rcl 10 pc and tdyn 2.3 days. The number of ≃ × NS-BH mergers≃ in the core during≃ this time is loss-cone is sufficient for our model. After the depletion of orbits inside the loss-cone dur- −1 ing the several dynamical times the objects fall onto ˙ fNS M Mc Nc tdyn 7 −5 7 5 . the SMBH only due to the slow diffusion of their or- ≃ × 2 10 10 M⊙ 5 10 M⊙  ×     ×  bits through the loss-cone boundary with the rate N˙ dif (12) ≃ N/(λt ), where t is given by (2) and λ ln(Rcl/3R ). The rate of NS-BH collisions in the entire cluster before r r ≃ g,c the core collapse: For the parameters in use the loss-cone is empty, be- cause N˙ dif tdyn

This value coincides with the average luminosity of Swift model with star tidal destruction, and we will not repeat −3 −2 J164449.3+573451. For the larger fNS 10 10 the corresponding calculations here. ≃ − the N˙ c is also greater, and one can achieve even the observed peak luminosity 5 1048 erg s−1 of Swift J164449.3+573451. Alternatively,∼ × the peak signal can C. Gravitational lensings come from the single NS-NS or NS-BH merge events with beamed radiation [13]. In this case the high luminosity Let us first consider the lensing of the point source is the result of collimation. But one must require some on the point lens in the case the both are inside the mechanism of the pulses broadening from the typical same cluster at the distances from the Earth Ds and 2 s duration of the short GRBs till the minimum time- D respectively, so that D D 1 Gpc (galaxy at ∼ l s ≈ l ≈ variability scale 100 s of the Swift J164449.3+573451. z = 0.35), and Ds Dl is of the order of the cluster ∼ −4 − The mechanisms for the prolonged or extended emission radius Rcl 10 pc. This configuration is similar to after the first short bursts were discussed in [15], [16], the “mesolensings”≃ in globular clusters [18]. When the [17]. But the light-curve of the Swift J164449.3+573451 Einstein radius of the lens is has no short 1 s pulses. A simple explanation of their 2 1/2 1/2 ∼ RE = 4GMl/c [Dl(1 Dl/Ds)] absence is that we are not at one of the jets directions, − and therefore another mechanism must be responsible for 1/
2 1/2 radiation with 100 s variability. 9 Ml Ds Dl ∼ 4 10 −14 cm, (18) ≃ × 10M⊙ 3 10 cm    ×  −2 where Ml is the lens’ mass. If one uses the r den- B. Radiation from the debris accretion sity profile of the cluster and suppose that the collision occurred in the core, then the integration gives the num- Collisions between NSs and BHs provide a release of ber of lensing BHs inside the Einstein radius at the level −4 matter which can be accreted as in the star tidal dis- Nl 5 10 . A similar small chance is in lensing on ruption models [4–7]. Ejections from NSs could be more the∼ central× supermassive BH because the large mass is dense and compact, so they could provide a larger power compensated by a small distance from the source to the as compared with a single ordinary star. Tidal disrup- lens (collisions are most probable in the core, near the tion of a non-ordinary star (a white dwarf) tidal disrup- central BH). This means that lensing of a point source 5 tion by the SMBH with mass 10 M⊙ as a model of or jet on BHs is unlikely. Swift J1644+57 was proposed in≤ [30] and the short time- The large fireball lenses easily, because the radius of −4 scales in the light curve were explained. Here we pro- the cluster Rcl 10 pc is of the order of the size Rsw pose an even more extreme model with the NSs multiple of shock waves∼ in GRBs where some part of the gamma- destructions. The difference is that NSs destructed not radiation are generated. First of all, the fireball scat- by the tidal forces of central SMBH, but in the collisions tered on the forming central SMBH. The hot medium with stellar mass BHs in the cluster. In this case the char- flew around the SMBH and was swallowed inside it. Let acteristic time-scales 100 s are related to the SMBH the observer and an NS-NS or NS-BH merging be on op- horizon light crossing∼ time 2GM/c3 as in the model with posite sides of the SMBH. The surface of the fireball is not the star tidal destruction. spherical from the observers side, it has a cavity in the Note that the orbits of NSs in the cluster are dis- SMBH direction. As a result, for every small BH in the 2 tributed almost isotropically. This means that the de- volume πRswRcl there is amoment of fireball evolution bris from the collisions fall onto the central SMBH from when the∼ rays, normal to the fireball surface, go through different directions. If each collision produces a separate the Einstein radius of the BH. Therefore multiple lens- short-lived accretion disk around the SMBH, the corre- ing events are possible, like “mesolensings” in globular sponding jets from different disks will also be pointed clusters [18]. But in contemporary models of GRBs the isotropically. We can reproduce the configuration of a main flux of gamma radiation comes from jets, therefore small blazar with an accidental jet direction to the Earth the fireball surface is a subdominant source. In the case in this case only by requiring that the actual number of A fireball scattered on the SMBH, different parts of of collisions was larger then the number of the observed its surface can be lensed similarly to [31]. These lens- peaks by a factor of 4π/Ωj, where Ωj is the solid angle ings can explain the observed fast variability of the light of the jet collimation. curve of Swift J164449.3+573451during the first days. A Another possibility states the existence of a steady ac- characteristic feature of lensing is synchronous radiation cretion disk around the SMBH that supports a single jet in different bands. in the blazar configuration with a matter feed from the NS-BH collisions. In this model the jet direction is not related to the orientation of the debris orbits, but can D. Quasi-steady radiation be determined, for example, by the SMBHspin that had originated from the common rotation of the parent clus- Multiple NS and BH collisions first produce dense flows ter. Radio and other signals can be generated as in the of debris and then an extended hot gas cloud in the cen- 6 tral region of the cluster. Hot gas from the surrounding V. CONCLUSION cloud falls onto the SMBH and radiates with the Ed- dington luminosity LE =4πGMSMBHmpc/σT, where mp We propose a scenario of multiband powerful signal is the proton mass and σT is the Thompson cross-section. generation in an evolved star cluster in a galactic nu- 45 −1 7 This luminosity is 1.5 10 erg s in the case of 10 M⊙ × cleus during the gravitational collapse of its dense cen- SMBH which is the minimum inter-peak luminosity dur- tral core. This model naturally provides necessary condi- ing the first several days of Swift J164449.3+573451. Ra- tions for the event Swift J164449.3+573451 because the diation from slow gas accretion onto the newly formed huge power, duration, and variabilitymay indicate a more SMBH requires the gas envelope to be thin for radiation. catastrophic event (or sequence of events) than tidal de- struction of a single star. In the typical case, the dynamical evolution of a central star cluster results in collisions and destruction of a sub- E. Multiple fireballs in the dense cloud stantial fraction of stars and formation of a gas envelope [12]. Simultaneously, a compact cluster of NSs and BHs The cluster of NSs and BHs is enveloped by a gas can form in its evolution, due to sinking of massive stars cloud originating from destruction of ordinary stars and to the center of the cluster and their explosions as super- partly from primordial gas in the galactic center. Mul- novae [9–11]. The evolution of such a cluster leads to an tiple GRBs are generated in the medium “prepared” by avalanche contraction and gravitational collapse of the the preceding GRBs, so the evolution of multiple fireballs central region [19]. This mechanism cannot explain the must be considered as in [32], [33]. appearance of all SMBHs, especially in quasars at high Collisions of compact objects produce the relativis- redshifts, because of the lack of dynamical time. But tic shock with Lorentz factor Γ 1, which propagates star cluster collapses are nevertheless inevitable in many ahead of the fireball pushing the≫ gas in the nucleus. galaxies at low redshifts. Recently, the sub-dominant role The shock becomes non-relativistic at the Sedov length of galaxies mergers in triggering the AGN activity was re- 2 1/3 ls = [3E0/(4πρec )] , where ρe is the density of the gas vealed [34]. Avalanche contractions and collapses of star in the envelope. The length ls gives an estimate for cav- clusters could be an alternative mechanism for AGN ig- ity radius, produced by the single fireball in the absence nition at small redshifts along with tidal disruptions of of the central SMBH. The multiple expanding fireballs in stars or disk instabilities. the cavity have the shape of thin shells and are separated Multiple dense gas releases and fireballs in NS-BH col- by the distance Rc = ctcap, where tcap corresponds only lisions can produce powerful peaks in the light curve of to NS-NS and NS-BH collisions in this case. The gas be- Swift J164449.3+573451. The additional inter-peak radi- tween each two fireballs is swept up and compressed by ation comes from gas accretion onto the forming SMBH. the preceding one. Multiple fireballs produce the station- We can expect that the properties of fireballs and gamma ˙ 3 1/2 ary cavity [32] with radius Rcav = [NcE0/(4πρecs)] , radiation in this model are distorted by their scattering where cs is the sound speed in the gas. For the relativistic on the growing central SMBH (the characteristic time fireballs the gravitational attraction of the central SMBH scale is 2GM/c3 100 s), and by gravitational lensing ∼ is not important except for the later non-relativistic stage on compact objects in the cluster. with Γ 1. The SMBH causes a deformation of the fireball,≤ opening the conditions for gravitational lensing. One can assume an equipartition of the magnetic field in- ACKNOWLEDGMENTS duced by turbulence and by the dynamo mechanism. In this situation the Fermi II acceleration mechanism oper- The authors thank S.B. Popov for fruitful discussions ates, and high energy particle creation is possible. The and the anonymous referee for useful comments and sug- signatures of these processes are expected from the di- gestions. This study was supported in part by the grants rection of Swift J164449.3+573451. OFN-17, RFBR 10-02-00635, and NSh- 871.2012.2.

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