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Register a new userA New Classification Method for Gamma-Ray Bursts
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Houjun Lv
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Recent Swift observations suggest that the traditional long vs. short GRB classification scheme does not always associate GRBs to the two physically motivated model types, i.e. Type II (massive star origin) vs. Type I (compact star origin). We propose a new phenomenological classification method of GRBs by introducing a new parameter epsilon=E_{gamma, iso,52}/E^{5/3}_{p,z,2}, where E_{gamma,iso} is the isotropic gamma-ray energy (in units of 10^{52} erg), and E_{p,z} is the cosmic rest frame spectral peak energy (in units of 100 keV). For those short GRBs with extended emission, both quantities are defined for the short/hard spike only. With the current complete sample of GRBs with redshift and E_p measurements, the epsilon parameter shows a clear bimodal distribution with a separation at epsilon ~ 0.03. The high-epsilon region encloses the typical long GRBs with high-luminosity, some high-z rest-frame-short GRBs (such as GRB 090423 and GRB 080913), as well as some high-z short GRBs (such as GRB 090426). All these GRBs have been claimed to be of the Type II origin based on other observational properties in the literature. All the GRBs that are argued to be of the Type I origin are found to be clustered in the low-epsilon region. They can be separated from some nearby low-luminosity long GRBs (in 3sigma) by an additional T_{90} criterion, i.e. T_{90,z}<~ 5 s in the Swift/BAT band. We suggest that this new classification scheme can better match the physically-motivated Type II/I classification scheme.
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The Fermi GBM Catalog has been recently published. Previous classification analyses of the BATSE, RHESSI, BeppoSAX, and Swift databases found three types of gamma-ray bursts. Now we analyzed the GBM catalog to classify the GRBs. PCA and Multiclustering analysis revealed three groups. Validation of these groups, in terms of the observed variables, shows that one of the groups coincides with the short GRBs. The other two groups split the long class into a bright and dim part, as defined by the peak flux. Additional analysis is needed to determine whether this splitting is only a mathematical byproduct of the analysis or has some real physical meaning.
The detection of six Fast Radio Bursts (FRBs) has recently been reported. FRBs are short duration ($sim$ 1 ms), highly dispersed radio pulses from astronomical sources. The physical interpretation for the FRBs remains unclear but is thought to involve highly compact objects at cosmological distance. It has been suggested that a fraction of FRBs could be physically associated with gamma-ray bursts (GRBs). Recent radio observations of GRBs have reported the detection of two highly dispersed short duration radio pulses using a 12 m radio telescope at 1.4 GHz. Motivated by this result, we have performed a systematic and sensitive search for FRBs associated with GRBs. We have observed five GRBs at 2.3 GHz using a 26 m radio telescope located at the Mount Pleasant Radio Observatory, Hobart. The radio telescope was automated to rapidly respond to Gamma-ray Coordination Network notifications from the Swift satellite and slew to the GRB position within $sim$ 140 s. The data were searched for pulses up to 5000 pc $rm cm^{-3}$ in dispersion measure and pulse widths ranging from 640 $rm mu$s to 25.60 ms. We did not detect any events $rm geq 6 sigma$. An in-depth statistical analysis of our data shows that events detected above $rm 5 sigma$ are consistent with thermal noise fluctuations only. A joint analysis of our data with previous experiments shows that previously claimed detections of FRBs from GRBs are unlikely to be astrophysical. Our results are in line with the lack of consistency noted between the recently presented FRB event rates and GRB event rates.
There exists an inevitable scatter in intrinsic luminosity of Gamma Ray Bursts(GRBs). If there is relativistic beaming in the source, viewing angle variation necessarily introduces variation in the intrinsic luminosity function(ILF). Scatter in the ILF can cause a selection bias where distant sources that are detected have a larger median luminosity than those detected close by. Median luminosity, as we know, divides any given population into equal halves. When the functional form of a distribution is unknown, it can be a more robust diagnostic than any that use trial functional forms. In this work we employ a statistical test based on median luminosity and apply it to test a class of models for GRBs. We assume that the GRB jet has a finite opening angle and that the orientation of the GRB jet is random relative to the observer. We parameterize the jet with constant Lorentz factor $Gamma$ and opening angle $theta_0$. We calculate $L_{median}$ as a function of redshift with an average of 17 grbs in each redshift bin($dz=0.01$) empirically, theoretically and use Fermi GBM data, noting that SWIFT data is problematic as it is biased, specially at high redshifts. We find that $L_{median}$ is close to $L_{max}$ for sufficiently extended GRB jet and does not fit the data. We find an acceptable fit with the data when $Gamma$ is between $100$ and $200$, $theta_0leq 0.1$, provided that the jet material along the line of sight to the on axis observer is optically thick, such that the shielded maximum luminosity is well below the bare $L_{max}$. If we associate an on-axis observer with a classically projected monotonically decreasing afterglow, we find that their ILF is similar to those of off-jet observer which we associate with flat phase afterglows.
It is now more than 40 years since the discovery of gamma-ray bursts (GRBs) and in the last two decades there has been major progress in the observations of bursts, the afterglows and their host galaxies. This recent progress has been fueled by the ability of gamma-ray telescopes to quickly localise GRBs and the rapid follow-up observations with multi-wavelength instruments in space and on the ground. A total of 674 GRBs have been localised to date using the coded aperture masks of the four gamma-ray missions, BeppoSAX, HETE II, INTEGRAL and Swift. As a result there are now high quality observations of more than 100 GRBs, including afterglows and host galaxies, revealing the richness and progress in this field. The observations of GRBs cover more than 20 orders of magnitude in energy, from 10^-5 eV to 10^15 eV and also in two non-electromagnetic channels, neutrinos and gravitational waves. However the continuation of progress relies on space based instruments to detect and rapidly localise GRBs and distribute the coordinates.
Time-resolved spectral analysis, though a very promising method to understand the emission mechanism of gamma-ray bursts (GRBs), is difficult to implement in practice because of poor statistics. We present a new method for pulse-wise time-resolved spectral study of the individual pulses of GRBs, using the fact that many spectral parameters are either constants or smooth functions of time. We use this method for the two pulses of GRB 081221, the brightest GRB with separable pulses. We choose, from the literature, a set of possible models which includes the Band model, blackbody with a power-law (BBPL), a collection of black bodies with a smoothly varying temperature profile, along with a power-law (mBBPL), and two blackbodies with a power-law (2BBPL). First, we perform time-resolved study to confirm the spectral parameter variations, and then construct the new model to perform a joint spectral fit. We find that any photospheric emission in terms of black bodies is required mainly in the rising parts of the pulses and the falling part can be adequately explained in terms of the Band model, with the low energy photon index within the regime of synchrotron model. Interestingly, we find that 2BBPL is comparable or sometimes even better, though marginally, than the Band model, in all episodes. Consistent results are also obtained for the brightest GRB of Fermi era --- GRB 090618. We point out that the method is generic enough to test any spectral model with well defined parameter variations.
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