Collapsar Model Research Articles

GRBs from Collapse of Thorne–Żytkow–like Objects as the Aftermath of WD-NS Coalescence

The Type II gamma-ray burst (GRB) 200826A challenges collapsar models by questioning how they can generate a genuinely short-duration event. The other Type I GRB 211211A confused us with a kilonova signature observed in the afterglow of a long burst. Here, we propose a comprehensive model in which both bursts are the results of the collapse of Thorne–Żytkow–like objects (TZlOs). The TZlO consists of a central neutron star (NS), with a dense white dwarf (WD) material envelope, which is formed as the aftermath of a WD-NS coalescence. We find that the characteristics of the resultant GRBs depend on whether the TZlO collapses immediately following the WD-NS merger or not. Additionally, the observational properties of the consequent GRBs manifest variations contingent upon whether the collapse of the TZlO results in a magnetar or a black hole. We also show that our model is consistent with the observations of GRB 211211A and GRB 200826A. Specifically, the optical excess in GRB 211211A can be attributed to an engine-fed kilonova, while the supernova bump in GRB 200826A is likely due to the collision between the ejecta and the disk wind shell.

Self-gravitating collapsing star and black hole spin-up in long gamma ray bursts

Context. Long gamma ray bursts (GRBs) originate from the collapse of massive, rotating stars. Some of the GRBs exhibit much stronger variability patterns in the prompt GRB emission than the usual stochastic variations. We discuss the mechanisms able to account for this effect. Aims We aim to model the process of stellar collapse in the scenario of a self-gravitating collapsing star. We account for the changes in Kerr metric induced by the growth of the black hole; accretion of angular momentum; and the self-gravity effect due to a large mass of the collapsing stellar core falling onto black hole in a very short time. We also investigate the existence of accretion shocks in the collapsar, and the role of magnetic field in their propagation. Methods. We compute the time-dependent axially symmetric general relativistic magnetohydrodynamic model of a collapsing stellar core in the dynamical Kerr metric. We explore the influence of self-gravity in such a star, where the newly formed black hole is increasing the mass and changing its spin. The Kerr metric evolves according to the mass and angular momentum changes during the collapse. We parameterize the rotation inside the star, and account for the presence of large-scale poloidal magnetic field. For the set of the global parameters, such as the initial black hole spin and the initial content of specific angular momentum in the stellar envelope, we determine the evolution of black hole parameters (mass and spin) and quantify the strength of the gravitational instability. We then estimate the variability timescales and amplitudes. Results. We find that the role of the gravitational instability measured by the value of the Toomre parameter is relatively important in the innermost regions of the collapsing star. The character of accretion rate variability strongly depends on the assumption of self-gravity in the model, and is also affected by the magnetic field. Additional factors are initial spin and rotation of the stellar core. We find that for subcritical rotation of the precollapsed star, a centrifugally supported mini-disc is present at the equatorial plane, and it may be subject to fragmentation due to self-gravitating instability. We also find that self-gravity may play a role in the angular momentum transport and that it generally lowers the final mass and spin of the black hole, while the accretion-rate variability amplitude is much larger in self-gravitating objects. The effect of magnetic field is rather weak, while it seems to decrease the strength of accretion shocks. The magnetisation affects the global properties of the flow in a non-linear way, and is manifested mostly in models with moderate initial black hole spins but supercritical rotation of the collapsing star. Conclusions. Our computations confirm that gravitational instability can account for flaring activity in GRBs and the variations in their prompt emission. Rapid variability detected in the brightest GRBs (most likely powered by rapidly spinning black holes) is consistent with the self-gravitating collapsar model, where the transonic shocks are formed. The effect should be weakened by magnetic field.

A Collapsar Origin for GRB 211211A Is (Just Barely) Possible

Gamma-ray bursts (GRBs) have historically been divided into two classes. Short-duration GRBs are associated with binary neutron star mergers (NSMs), while long-duration bursts are connected to a subset of core-collapse supernovae (SNe). GRB 211211A recently made headlines as the first long-duration burst purportedly generated by an NSM. The evidence for an NSM origin was excess optical and near-infrared emission consistent with the kilonova observed after the gravitational-wave-detected NSM GW170817. Kilonovae derive their unique electromagnetic signatures from the properties of the heavy elements synthesized by rapid neutron capture (the r-process) following the merger. Recent simulations suggest that the “collapsar” SNe that trigger long GRBs may also produce r-process elements. While observations of GRB 211211A and its afterglow rule out an SN typical of those that follow long GRBs, an unusual collapsar could explain both the duration of GRB 211211A and the r-process-powered excess in its afterglow. We use semianalytic radiation transport modeling to evaluate low-mass collapsars as the progenitors of GRB 211211A–like events. We compare a suite of collapsar models to the afterglow-subtracted emission that followed GRB 211211A, and find the best agreement for models with high kinetic energies and an unexpected pattern of 56Ni enrichment. We discuss how core-collapse explosions could produce such ejecta, and how distinct our predictions are from those generated by more straightforward kilonova models. We also show that radio observations can distinguish between kilonovae and the more massive collapsar ejecta we consider here.

Black Hole Hyperaccretion in Collapsars: A Review

The collapsar model is widely accepted as one of the standard scenarios for gamma-ray bursts (GRBs). In the massive collapsar scenario, the core will collapse to a black hole (BH) surrounded by a temporary hyperaccretion disk with a very high accretion rate. The newborn BH hyperaccretion system would launch the relativistic jets via neutrino annihilation and Blandford-Znajek (BZ) mechanism. At the initial accretion stage, the accretion disk should be a neutrino-dominated accretion flow (NDAF). If the jets can break out from the envelope and circumstellar medium, then a GRB will be triggered. In this review, we summarize the theoretical progress on the multimessenger astronomy of the BH hyperaccretion in the center of collapsars. The main topics include: jet propagation in collapsar, MeV neutrinos from NDAFs and proto-neutron stars, gravitational waves from collapsars.

R-process Viable Outflows are Suppressed in Global Alpha-viscosity Models of Collapsar Disks

Collapsar disks have been proposed to be rich factories of heavy elements, but the major question of whether their outflows are neutron rich and could therefore represent significant sites of the rapid neutron-capture (r-) process or dominated by iron-group elements remains unresolved. We present the first global models of collapsars that start from a stellar progenitor and self-consistently describe the evolution of the disk, its composition, and its outflows in response to the imploding stellar mantle, using energy-dependent M1 neutrino transport and an α-viscosity to approximate turbulent angular-momentum transport. We find that a neutron-rich, neutrino-dominated accretion flow (NDAF) is established only marginally—either for short times or relatively low viscosities—because the disk tends to disintegrate into an advective disk already at relatively high mass-accretion rates, launching powerful outflows but preventing it from developing a hot, dense, and therefore neutron-rich core. Viscous outflows disrupt the star within ∼100 s with explosion energies close to that of hypernovae. If viscosity is ignored, a stable NDAF with disk mass of about 1 M ☉ is formed but is unable to release neutron-rich ejecta, while it produces a relatively mild explosion powered by a neutrino-driven wind blown off its surface. With ejecta electron fractions close to 0.5, all models presumably produce large amounts of 56Ni. Our results suggest that collapsar models based on the α-viscosity are inefficient r-process sites and that genuinely magnetohydrodynamic effects may be required to generate neutron-rich outflows. A relatively weak effective viscosity generated by magnetohydrodynamic turbulence would improve the prospects for obtaining neutron-rich ejecta.

GRB 200826A: A Precursor of a Long Gamma-Ray Burst with Missing Main Emission

The recently discovered peculiar gamma-ray burst GRB 200826A poses a dilemma for the collapsar model. Although all other characteristics of the burst are consistent with it being a Type II (i.e., collapse of a massive star) event, the observed duration of the event is only approximately 1 s, which is at odds with the predicted allowable timescale range for a collapsar event. To resolve this dilemma, this Letter proposes that the original burst could be an intrinsically long GRB comprising a precursor and a main emission phase. However, the main emission phase is missed due to either precession of the jet or the obstruction by a companion star, leaving only the precursor observed as a short-duration GRB 200826A. Interestingly, we found that the temporal and spectral properties of GRB 200826A broadly resembled those of the bright precursor observed in GRB 160625B. Furthermore, assuming the prototype burst of GRB 200826A is similar to that of GRB 160625B, we found that the observer may indeed miss its main emission because of geometric effects caused either by jet precession or companion-obstruction models. Our approach provides a natural explanation for the GRB 200826A–like bursts and agrees with the rarity of those events.

Thermonuclear 17O(n, γ)18O Reaction Rate and Its Astrophysical Implications

Abstract A new thermonuclear 17O(n,γ)18O rate is derived based on a complete calculation of the direct-capture (DC) and resonant-capture contributions, for a temperature region up to 2 GK of astrophysical interest. We have first calculated the DC and subthreshold contributions in the energy region up to 1 MeV, and estimated the associated uncertainties by a Monte Carlo approach. It shows that the present rate is remarkably larger than that adopted in the JINA REACLIB in the temperature region of 0.01 ∼ 2 GK, by up to a factor of ∼80. The astrophysical impacts of our rate have been examined in both s-process and r-process models. In our main s-process model, which simulates flash-driven convective mixing in metal-deficient asymptotic giant branch stars, both 18O and 19F abundances in interpulse phases are enhanced dramatically by factors of ∼20–40 due to the new larger 17O(n,γ)18O rate. It shows, however, that this reaction hardly affects the weak s-process in massive stars since the 17O abundance never becomes significantly large in the massive stars. For the r-process nucleosynthesis, we have studied impacts of our rate in both the collapsar and neutron burst models, and found that the effect can be neglected, although an interesting loophole effect is found owing to the enhanced new rate, which significantly changes the final nuclear abundances if fission recycling is not involved in the model; however, these significant differences are almost completely eliminated if the fission recycling is considered.

One star, two stars, or both? Investigating metallicity-dependent models for gamma-ray burst progenitors with the IllustrisTNG simulation

ABSTRACT The rate of long-duration gamma-ray bursts (GRBs) has been identified as a potential proxy for the star formation rate (SFR) across redshift, but the exact relationship depends on GRB progenitor models (single versus binary). The single-progenitor collapsar model accounts for the preference towards low-metallicity GRB progenitors, but is in apparent tension with some high-metallicity GRB host galaxy measurements. As a possible solution, we consider the scenario where high-metallicity GRB hosts harbour low-metallicity regions in which GRB progenitors form. For this, we use the IllustrisTNG cosmological hydrodynamical simulation to investigate the internal metallicity distribution of GRB hosts, implementing in post-processing different GRB formation models. Predictions (GRB rate, host metallicities, and stellar masses) are compared to the high-completeness GRB legacy surveys BAT6 and SHOALS and a sample of high-redshift GRB-DLA metallicities, allowing us to compute their relative likelihoods. When the internal metallicity distribution of galaxies is ignored, the best-fitting model requires a metallicity-independent channel, as previously proposed by Trenti, Perna, & Jimenez. However, when the internal metallicity distribution is considered, a basic metallicity bias model with a cutoff at $Z_{\rm max}=0.35\, \mathrm{\it Z}_\odot$ is the best-fitting one. Current data are insufficient to discriminate among more realistic metallicity bias models, such as weak metallicity dependence of massive binaries versus stronger metallicity bias of collapsars. An increased sample of objects, and direct measurements of host stellar masses at redshift z > 2 would allow to further constrain the origin of long GRBs.

Constraining Collapsar r-process Models through Stellar Abundances

Abstract We use observations of heavy elements in very metal-poor stars ([Fe/H] < −2.5) in order to place constraints on the viability of collapsar models as a significant source of the r-process. We combine bipolar explosion nucleosynthesis calculations with recent disk calculations to make predictions of the observational imprints that these explosions would leave on very metal-poor stars. We find that a source of low (≈0.1–0.5 ) Fe mass, which also yields a relatively high (>0.08 ) r-process mass, would, after subsequently mixing and forming new stars, result in [r/Fe] abundances up to three orders of magnitude higher than those seen in stars. In order to match inferred abundances, 10–103 of Fe would need be efficiently incorporated into the r-process ejecta. We show that Fe enhancement, and hence [r/Fe] dilution from other nearby supernovae, is not able to explain the observations unless significant inflow of pristine gas occurs before the ejecta are able to form new stars. Finally, we show that the inferred [Eu/Fe] abundances require levels of gas mixing that are in conflict with other properties of r-process enhanced metal-poor stars. Our results suggest that early r-process production is likely to be spatially uncorrelated with Fe production, a condition that can be satisfied by neutron star mergers due to their large kick velocities and purely r-process yields.

Can we quickly flag ultra-long gamma-ray bursts?

Abstract Ultra-long gamma-ray bursts are a class of high-energy transients lasting several hours. Their exact nature is still elusive, and several models have been proposed to explain them. Because of the limited coverage of wide-field gamma-ray detectors, the study of their prompt phase with sensitive narrow-field X-ray instruments could help in understanding the origin of ultra-long GRBs. However, the observers face a true problem in rapidly activating follow-up observations, due to the challenging identification of an ultra-long GRB before the end of the prompt phase. We present here a comparison of the prompt properties available after a few tens of minutes of a sample of ultra-long GRBs and normal long GRBs, looking for prior indicators of the long duration. We find that there is no such clear prior indicator of the duration of the burst. We also found that statistically, a burst lasting at least 10 and 20 minutes has respectively $28{{\ \rm per\ cent}}$ and $50{{\ \rm per\ cent}}$ probability to be an ultralong event. These findings point towards a common central engine for normal long and ultra-long GRBs, with the collapsar model privileged.

The environment of the SN-less GRB 111005A at z = 0.0133

The collapsar model has proved highly successful in explaining the properties of long γ-ray bursts (GRBs), with the most direct confirmation being the detection of a supernova (SN) coincident with the majority of nearby long GRBs. Within this model, a long GRB is produced by the core-collapse of a metal-poor, rapidly rotating, massive star. The detection of some long GRBs in metal-rich environments, and more fundamentally the three examples of long GRBs (GRB 060505, GRB 060614 and GRB 111005A) with no coincident SN detection down to very deep limits is in strong contention with theoretical expectations. In this paper we present MUSE observations of the host galaxy of GRB 111005A, which is the most recent and compelling example yet of a SN-less, long GRB. At z = 0.01326, GRB 111005A is the third closest GRB ever detected, and second closest long duration GRB, enabling the nearby environment to be studied at a resolution of 270 pc. From the analysis of the MUSE data cube, we find GRB 111005A to have occurred within a metal-rich environment with little signs of ongoing star formation. Spectral analysis at the position of the GRB indicates the presence of an old stellar population (τ ≥ 10 Myr), which limits the mass of the GRB progenitor to MZAMS < 15 M⊙, in direct conflict with the collapsar model. Our deep limits on the presence of any SN emission combined with the environmental conditions at the position of GRB 111005A necessitate the exploration of a novel long GRB formation mechanism that is unrelated to massive stars.

The collapsar model of gamma ray burst central engine

Gamma ray bursts (GRBs) are the most explosive and brightest known photon sources in the universe. The initial bursts of GRBs found so far typically have a spectral peak between [Formula: see text][Formula: see text]keV and 1[Formula: see text]MeV. Most of their gamma ray prompt emissions last for [Formula: see text][Formula: see text]s or less. The jets ejected from ‘the central engine’ are thought to be responsible for these bursts. According to the currently accepted scenario, a black hole produced by the merger of compact objects (such as black holes and/or neutron stars) is responsible for shorter bursts while collapse of a massive star directly to a black hole produces longer bursts, although clear-cut distinction may not exist. In this paper, we consider the latter case, a collapsar model responsible for longer GRBs. They are more interesting in the sense that they are more energetic and hence their studies can be extended to first Population III GRBs, which may give some constraints on the understanding of the early universe. In this paper, we are interested mainly in the “central engine”, the region created by the collapse of a rotating massive star to a spinning black hole and its surrounding area created by the stellar remnant because the conditions of the jet propagation change once they are outside in the interstellar medium. We first follow the process of how a progenitor massive star collapses and forms a central disk-black hole system, how powerful jets are produced and ejected from that system, and how the jets propagate and penetrate the surrounding stellar medium. The continued journey of the jets outside the star and the mechanisms for GRB emissions are outside the scope of this short paper.

A Collapsar Model with Disk Wind: Implications for Supernovae Associated with Gamma-Ray Bursts

We construct a simple but self-consistent collapsar model for gamma-ray bursts (GRBs) and SNe associated with GRBs (GRB-SNe).Our model includes a black hole, an accretion disk, and the envelope surrounding the central system.The evolutions of the different components are connected by the transfer of the mass and angular momentum.To address properties of the jet and the wind-driven SNe, we consider competition of the ram pressure from the infalling envelope and those from the jet and wind.The expected properties of the GRB jet and the wind-driven SN are investigated as a function of the progenitor mass and angular momentum.We find two conditions which should be satisfied if the wind-driven explosion is to explain the properties of the observed GRB-SNe. (1) The wind should be collimated at its base, and (2) it should not prevent further accretion even after the launch of the SN explosion. Under these conditions, some relations seen in the properties of the GRB-SNe could be reproduced by a sequence of different angular momentum in the progenitors.Only the model with the largest angular momentum could explain the observed (energetic) GRB-SNe, and we expect that the collapsar model can result in a wide variety of observational counterparts mainly depending on the angular momentum of the progenitor star.

Weibel Instability in Hot Plasma Flows with the Production of Gamma-Rays and Electron–Positron Pairs

Abstract We present the results of the theoretical analysis and numerical simulations of the Weibel instability in two counterstreaming hot relativistic plasma flows, for instance the flows of electron–proton plasmas with rest-mass densities , Lorentz factors , and proper temperatures . The instability growth rate and the filament size at the linear stage are found analytically and are in qualitative agreement with the results of three-dimensional particle-in-cell simulations. In the simulations, incoherent synchrotron emission and pair photoproduction in electromagnetic fields are taken into account. If the plasma flows are dense, fast, and hot enough, the overall energy of the synchrotron photons can be much higher than the energy of the generated electromagnetic fields. Furthermore, a sizable number of positrons can be produced due to the pair photoproduction in the generated magnetic field. We propose a rough criterion to judge copious pair production and considerable synchrotron losses. By means of this criterion, we conclude that the incoherent synchrotron emission and the pair production during the Weibel instability can have implications for the collapsar model of gamma-ray bursts.

Observational evidence for mass ejection accompanying short gamma-ray bursts

Abstract The plateau in the duration distribution of long gamma-ray bursts (LGRBs) provides a direct observational evidence for the Collapsar model. The plateau reflects the fact that the observed duration satisfies: T90 = te − tb where te is the time that the central engine operates and tb is the threshold time, interpreted within the Collapsar model as the time it takes for the relativistic jet to penetrate the stellar envelope. Numerical simulation and macronova observations suggest that compact binary mergers involve mass ejection. If short-gamma ray bursts (sGRBs) arise from such mergers, their jets should cross this surrounding ejecta before producing the prompt emission. Like in LGRBs, this should result in a distinct short plateau in the GRBs’ duration distribution. We present a new analysis of the duration distribution for the three GRB satellites: BATSE, Swift and Fermi. We find a clear evidence for a short (∼0.4 s) plateau in the duration distribution. This plateau is consistent with the expected jet crossing time, provided that the ejecta is of order of a few percent of solar masses.

A common central engine for long gamma-ray bursts and Type Ib/c supernovae

Long-duration, spectrally-soft Gamma-Ray Bursts (GRBs) are associated with Type Ic Core Collapse (CC) Supernovae (SNe), and thus arise from the death of massive stars. In the collapsar model, the jet launched by the central engine must bore its way out of the progenitor star before it can produce a GRB. Most of these jets do not break out, and are instead "choked" inside the star, as the central-engine activity time, $t_{\rm e}$, is not long enough. Modelling the long-soft GRB duration distribution assuming a power-law distribution for their central-engine activity times, $\propto t_{\rm e}^{-\alpha}$ for $t_{\rm e}>t_{\rm b}$, we find a steep distribution ($\alpha\sim4$) and a typical GRB jet breakout time of $t_{\rm b}\sim 60\text{ s}$ in the star's frame. The latter suggests the presence of a low-density, extended envelope surrounding the progenitor star, similar to that previously inferred for low-luminosity GRBs. Extrapolating the range of validity of this power law below what is directly observable, to $t_{\rm e}<t_{\rm b}$, by only a factor of $\sim$4-5 produces enough events to account for all Type Ib/c SNe. Such extrapolation is necessary to avoid fine-tuning the distribution of central engine activity times with the breakout time, which are presumably unrelated. We speculate that central engines launching relativistic jets may operate in all Type Ib/c SNe. In this case, the existence of a common central engine would imply that (i) the jet may significantly contribute to the energy of the SN; (ii) various observational signatures, like the asphericity of the explosion, could be directly related to jet's interaction with the star.

THE BLACK HOLE CENTRAL ENGINE FOR ULTRA-LONG GAMMA-RAY BURST 111209A AND ITS ASSOCIATED SUPERNOVA 2011KL

ABSTRACT Recently, the first association between an ultra-long gamma-ray burst (GRB) and a supernova was reported, i.e., GRB 111209A/SN 2011kl, enabling us to investigate the physics of central engines or even progenitors for ultra-long GRBs. In this paper, we inspect the broadband data of GRB 111209A/SN 2011kl. The late-time X-ray light curve exhibits a GRB 121027A-like fallback bump, suggesting a black hole (BH) central engine. We thus propose a collapsar model with fallback accretion for GRB 111209A/SN 2011kl. The required model parameters, such as the total mass and radius of the progenitor star, suggest that the progenitor of GRB 111209A is more likely a Wolf–Rayet star instead of a blue supergiant, and the central engine of this ultra-long burst is a BH. The implications of our results are discussed.

Long-term evolution of a collapsar

During the collapse of a massive star greater than 35- 40 M ⊙ , stellar core is considered to promptly collapse to a black hole. and stellar material may fall onto the hole greater than several solar masses with extremely high accretion rates (> 1 M ⊙ s -1 ). If the star has a sufficiently high angular momentum before the collapse, an accretion disk is likely to form around the black hole. Jets are suggested to be launched from the inner region of the accretion disk near the hole through magnetic and/or neutrino processes. Gamma-Ray Bursts (GRBs) are expected to be driven by the jets. This scenario of GRBs is called the collapsar model. Recently, magnetohydrodynamic (MHD) simulations were performed of the collapse of rapidly rotating, magnetized 25 M ⊙ and 40 M ⊙ stars in light of the collapsar model of GRBs (see papers listed at the end of the paper). For assumed angular velocity distributions and the uniform magnetic fields, the collapsars are shown to eject jets driven through the magnetic pressure. The simulations are however performed up to 4 s, which are shorter than durations of long GRBs ∼ 10 s. In the present work. we have performed long-tern (∼ 10 s), two-dimensional, MHD simulations of the collapse of a rapidly totaling, magnetized 40 M ⊙ star.

FROM ENGINE TO AFTERGLOW: COLLAPSARS NATURALLY PRODUCE TOP-HEAVY JETS AND EARLY-TIME PLATEAUS IN GAMMA-RAY BURST AFTERGLOWS

We demonstrate that the steep decay and long plateau in the early phases of gamma-ray burst X-ray afterglows are naturally produced in the collapsar model, by a means ultimately related to the dynamics of relativistic jet propagation through a massive star. We present two-dimensional axisymmetric hydrodynamical simulations that start from a collapsar engine and evolve all the way through the late afterglow phase. The resultant outflow includes a jet core that is highly relativistic after breaking out of the star, but becomes baryon loaded after colliding with a massive outer shell, corresponding to mass from the stellar atmosphere of the progenitor star which became trapped in front of the jet core at breakout. The prompt emission produced before or during this collision would then have the signature of a high Lorentz factor jet, but the afterglow is produced by the amalgamated post-collision ejecta that has more inertia than the original highly relativistic jet core and thus has a delayed deceleration. This naturally explains the early light curve behavior discovered by Swift, including a steep decay and a long plateau, without invoking late-time energy injection from the central engine. The numerical simulation is performed continuously from engine to afterglow, covering a dynamic range of over 10 orders of magnitude in radius. Light curves calculated from the numerical output demonstrate that this mechanism reproduces basic features seen in early afterglow data. Initial steep decays are produced by internal shocks, and the plateau corresponds to the coasting phase of the outflow.

Three-peak GRBs and their implications for central engines

GRB 110709B presented a peculiar three-peak lightcurve; this burst twice triggered the BAT detector onboard Swift. The two triggers were separated by ∼10 min. In order to explain such an event, we unify into a single description the millisecond (ms) protomagnetar and the collapsar central-engine models. We find that such a scenario could produce GRBs with three peaks. One for the ms-protomagnetar stage, a second one for the BH-formation event and a third one for the collapsar phase. We show that the three peaks for GRB 110709B originate from different phases of the same collapsing object. We estimate the energies and timescales of the different episodes of this burst using our model and compare with previous results as well as with a reanalysis we perform on the data. We show that not only the light curve, but also the photon index evolution and the delay between the prompt emission and the afterglow of the second central-engine activity phase point toward a model like the one proposed here. We find that, with reasonable assumptions, our model correctly describes the activity in GRB 110709B. We further suggest careful study of future GRBs lightcurves which may help show the validity of our model. If our model is correct, this would be the first time that the formation of a BH from a core-collapse event is observed unimpededly.