Based on the early-year observations from Neil Gehrels Swift Observatory, Liang et al. (2007) performed a systematic analysis for the shallow decay component of gamma-ray bursts (GRBs) X-ray afterglow, in order to explore its physical origin. Here we revisit the analysis with an updated sample (with Swift/XRT GRBs between February 2004 and July 2017). We find that with a larger sample, 1) the distributions of the characteristic properties of the shallow decay phase (e.g. t b , S X , Γ X,1 , and α X,1 ) still accords with normal or lognormal distribution; 2) Γ X,1 and Γ γ still show no correlation, but the tentative correlations of durations, energy fluences, and isotropic energies between the gamma-ray and X-ray phases still exist; 3) for most GRBs, there is no significant spectral evolution between the shallow decay segment and its follow-up segment, and the latter is usually consistent with the external-shock models; 4) assuming that the central engine has a power-law luminosity release history as L(t) = L 0 ( t t0 ) −q , we find that the value q is mainly distributed between -0.5 and 0.5, with an average value of 0.16± 0.12; 5) the tentative correlation between E iso,X and t ′ b disappears, so that the global 3-parameter correlation (E iso,X − E ′ p − t ′ b ) becomes less significant; 6) the anti-correlation between L X and t ′ b and the three-parameter correlation (E iso,γ − L X − t b ) indeed exist with a high confidence level. Overall, our results are generally consistent with Liang et al. (2007), confirming their suggestion that the shallow decay segment in most bursts is consistent with an external forward shock origin, probably due to a continuous energy injection from a long-lived central engine.
Evidence for the central engine of gamma-ray bursts (GRBs) has been collected in the Neil Gehrels Swift data. For instance, some GRBs show an internal X-ray plateau followed by very steep decay, which is difficult interpret within the framework of a black hole (BH) central engine, but is consistent within a rapidly spinning magnetar engine picture. The very steep decay at the end of the plateau suggests a sudden cessation of the central engine, which is explained as the collapse of a supramassive magnetar into a BH when it spins down. Here we propose that some additional evidence, such as a second X-ray plateau feature, would show up if the fallback accretion could activate the newborn BH and sufficient energy could be transferred from the newborn BH to the GRB blast wave. With a systematic data analysis for all long GRBs, we find three candidates in the Swift sample, i.e., GRBs 070802, 090111, and 120213A, whose X-ray afterglow lightcurves contain two plateaus, with the first one being an internal plateau. We find that in a fairly loose and reasonable parameter space, the second X-ray plateau data for all 3 GRBs could be interpreted with our proposed model. Future observations are likely to discover similar events, which could offer more information on the properties of the magnetar, as well as the newborn BH.
The successful operation of dedicated detectors has brought us valuable information for understanding the central engine and the progenitor of gamma-ray bursts (GRBs). For instance, the giant X-ray and optical bumps found in some long-duration GRBs (e.g., GRBs 121027A and 111209A) imply that some extended central engine activities, such as the late X-ray flares, are likely due to the fall-back of progenitor envelope materials. Here we systemically search for long GRBs that consist of a giant X-ray or optical bump from the Swift GRB sample, and eventually we find 19 new possible candidates. The fall-back accretion model could interpret the X-ray and optical bump for all candidates within a reasonable parameter space. Six candidates showing simultaneous bump signatures in both X-ray and optical observations, which could be well fitted at the same time when scaling down the X-ray flux into optical by one order of magnitude, are consistent with the standard F ν ∝ ν 1/3 synchrotron spectrum. The typical fall-back radius is distributed around 1010–1012 cm, which is consistent with the typical radius of a Wolf–Rayet star. The peak fall-back accretion rate is in the range of ∼10−11–10−4 M ⊙ s−1 at time ∼102–105 s, which is relatively easy to fulfill as long as the progenitor’s metallicity is not too high. Combined with the sample we found, future studies of the mass supply rate for the progenitors with different mass, metallicity, and angular momentum distribution would help us to better constrain the progenitor properties of long GRBs.
Gamma-ray bursts (GRBs) at high redshifts are expected to be gravitationally lensed by objects of different mass scales. Other than a single recent claim, no lensed GRB has been detected so far by using gamma-ray data only. In this paper, we suggest that multiband afterglow data might be an efficient way to search for lensed GRB events. Using the standard afterglow model, we calculate the characteristics of the lensed afterglow lightcurves under the assumption of two popular analytic lens models: the point-mass and singular isothermal sphere models. In particular, when different lensed images cannot be resolved, their signals would be superimposed together with a given time delay. In this case, the X-ray afterglows are likely to contain several X-ray flares of similar width in linear scale and similar spectrum, and the optical afterglow lightcurve will show re-brightening signatures. Since the lightcurves from the image arriving later would be compressed and deformed in the logarithmic timescale, the larger time delay (i.e., the larger mass of the lens), the easier it is to identify the lensing effect. We analyzed the archival data of optical afterglows and found one potential candidate of the lensed GRB (130831A) with time delay ∼500 s; however, observations of this event in gamma-ray and X-ray bands seem not to support the lensing hypothesis. In the future, with the cooperation of the all-sky monitoring gamma-ray detectors and multiband sky survey projects, the method proposed in this paper would be more efficient in searching for strongly lensed GRBs.
The gravitational wave detection by LIGO-Virgo scientific collaboration show that the binary black hole (BBH) systems with BH mass of tens of solar masses widely exist in the universe. Two main types of scenarios have been invoked for the formation of BBH systems, including isolated binary evolution in galactic fields and dynamical interactions in dense environments. Here we propose that if the BBH systems are formed from isolated binary evolution, the supernova signal associated with the second core collapse would show some identifiable features, due to the accretion feedback from the companion BH. Depending on the binary properties, we show that the supernova lightcurve could present a sharp peak around ∼ 10 days, with luminosity even at the level of the super luminous supernovae ( e.g. ∼ 10 44 erg s −1 ) or present a plateau feature lasting for several tens of days with regular luminosity of core collapse supernovae. Comparing the event rate density of these special supernova signals with the event rate density of LIGO-Virgo detected BBH systems could help to distinguish the BBH formation channel.
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