Hai-Nan Lin

Testing the isotropy of the Universe by using the JLA compilation of Type Ia supernovae

We probe the possible anisotropy of the Universe by using the joint light-curve analysis (JLA) compilation of Type Ia supernovae. We apply the Markov Chain Monte Carlo method to constrain the amplitude and direction of anisotropy in three cosmological models. For the dipole-modulated Lambda cold dark matter (Lambda CDM) model, the anisotropic amplitude is consistent with zero at 68 per cent cl, and has an upper bound AD < 1.98 x 10(-3) at 95 per cent cl. Regardless of much larger uncertainty, we amazingly find that the dipole direction of JLA is almost opposite to that of Union2. Similar results are found for the dipole-modulated wCDM and Chevallier-Polarski-Linder (CPL) models. Thus, the Universe is still well consistent with the isotropy according to the JLA compilation.

The significance of anisotropic signals hiding in the type Ia supernovae

We use two different methods, i.e. dipole-fitting (DF) and hemisphere comparison (HC), to search for the anisotropic signals hiding in the Union2.1 data set. We find that the directions of maximum matter density derived using these two methods are about 114A degrees away from each other. We construct four Union2.1-like mock samples to test the statistical significance of these two methods. It is shown that DF method is statistically significant, while HC method is strongly biased by the distribution of data points in the sky. Then we assume that the anisotropy of distance modulus is mainly induced by the anisotropy of matter density, which is modelled to be the dipole form Omega(M) = Omega(M0)(1 - cos theta). We fit our model to Union2.1, and find that the direction of maximum matter density is well consistent with the direction derived using DF method, but it is very different from the direction previously claimed. Monte Carlo simulations show that our method is statistically more significant than HC method, although it is not as significant as DF method. The statistical significance can be further improved if the data points are homogeneously distributed in the sky. Due to the low quality of present supernovae data, however, it is still premature to claim that the Universe has any preferred direction.

Are long gamma-ray bursts standard candles?

Gamma-ray bursts (GRBs) are widely proposed as an effective probe to trace the Hubble diagram of the Universe in high-redshift range. However, the calibration of GRBs is not as easy as that of Type-Ia supernovae (SNe Ia). Most calibrating methods at present make use one or some of the empirical luminosity correlations, e.g. Amati relation. One of the underlying assumptions of these calibrating methods is that the empirical correlation is universal over all redshifts. In this paper, we check to what extent this assumption holds. Assuming that SNe Ia exactly trace the Hubble diagram of the Universe, we re-investigate the Amati relation for low-redshift (z 1.4) GRBs, respectively. It is found that the Amati relation of low-z GRBs differs from that of high-z GRBs at more than 3 sigma confidence level. This result is insensitive to cosmological models. We should be cautious when using Amati relation to reconstruct the Hubble diagram of the Universe.

Testing Einstein's Equivalence Principle with Short Gamma-ray Bursts

Einsteins equivalence principle (EEP) can be tested by the time delay between photons with different energies passing through a gravitational field. As one of the most energetic explosions in the Universe, gamma-ray bursts (GRBs) provide an effective tool to test the accuracy of EEP. In this paper, we use the continuous spectra of 20 short GRBs detected by the Swift/Burst Alert Telescope to test the validity of EEP. Taking the duration of GRBs as the upper limit of the time delay induced by EEP violation (assuming that the high energy photons arrive later than the low energy photons), the difference of the parametrized post-Newtonian parameter is constrained with high accuracy. The strictest constraint, |gamma(150 keV) - gamma(15 keV)| < 5.59 x 10(-10) from GRB 150101B, is about 1 similar to 2 orders of magnitude tighter than previous constraints. Moreover, our result is more statistically significant than previous results because we use the continuous spectra instead of isolated photons.

Constraining Lorentz invariance violation from the continuous spectra of short gamma-ray bursts

In some quantum gravity theories, a foamy structure of space-time may lead to Lorentz invariance violation（LIV）. As the most energetic explosions in the Universe, gamma-ray bursts（GRBs） provide an effect way to probe quantum gravity effects. In this paper, we use the continuous spectra of 20 short GRBs detected by the Swift satellite to give a conservative lower limit of quantum gravity energy scale MQG. Due to the LIV effect, photons with different energy have different velocities. This will lead to the delayed arrival of high energy photons relative to low energy ones. Based on the fact that the LIV-induced time delay cannot be longer than the duration of a GRB,we present the most conservative estimate of the quantum gravity energy scales from 20 short GRBs. The strictest constraint, MQG > 5.05 × 1014 GeV in the linearly corrected case, is from GRB 140622 A. Our constraint on MQG,although not as tight as previous results, is the safest and most reliable so far.

Effect of gamma-ray burst (GRB) spectra on the empirical luminosity correlations and the GRB Hubble diagram

The spectra of gamma-ray bursts (GRBs) in a wide energy range can usually be well described by the Band function, which is a two smoothly jointed power laws cutting at a breaking energy. Below the breaking energy, the Band function reduces to a cut-off power law, while above the breaking energy it is a simple power law. However, for some detectors (such as the Swift-BAT) whose working energy is well below or just near the breaking energy, the observed spectra can be fitted to cut-off power law with enough precision. Besides, since the energy band of Swift-BAT is very narrow, the spectra of most GRBs can be fitted well even using a simple power law. In this paper, with the most up-to-date sample of Swift-BAT GRBs, we study the effect of different spectral models on the empirical luminosity correlations, and further investigate the effect on the reconstruction of GRB Hubble diagram. We mainly focus on two luminosity correlations, i.e., the Amati relation and Yonetoku relation. We calculate these two luminosity correlations on both the case that the GRB spectra are modeled by Band function and cut-off power law. It is found that both luminosity correlations only moderately depend on the choice of GRB spectra. Monte Carlo simulations show that Amati relation is insensitive to the high-energy power-law index of the Band function. As a result, the GRB Hubble diagram calibrated using luminosity correlations is almost independent on the GRB spectra.

Model-independent distance calibration of high-redshift gamma-ray bursts and constrain on the Lambda CDM model

Gamma-ray bursts (GRBs) are luminous enough to be detectable up to redshift z similar to 10. They are often proposed as complementary tools to Type Ia supernovae (SNe Ia) in tracing the Hubble diagram of the Universe. The distance calibrations of GRBs usually make use of one or some of the empirical luminosity correlations, such as iota(lag)-L, V-L, E-p-L, E-p-E-gamma, tau(RT)-L and E-p-E-iso relations. These calibrating methods are based on the underlying assumption that the empirical luminosity correlations are universal over all redshift range. In this paper, we test the possible redshift dependence of six luminosity correlations by dividing GRBs into low-z and high-z classes according to their redshift smaller or larger than 1.4. It is shown that the E-p-E-gamma relation for low-z GRBs is consistent with that for high-z GRBs within 1 sigma uncertainty. The intrinsic scatter of V-L relation is too large to make a convincing conclusion. For the rest four correlations, however, low-z GRBs differ from high-z GRBs at more than 3 sigma confidence level. As such, we calibrate GRBs using the E-p-E-gamma relation in a model-independent way. The constraint of high-z GRBs on the Lambda cold dark matter (Lambda CDM) model gives Omega(M) = 0.302 +/- 0.142(1 sigma), well consistent with the Planck 2015 results.

Gamma-ray burst polarization reduction induced by the Lorentz invariance violation

It has been observed that photons in the prompt emission of some gamma-ray bursts (GRBs) are highly polarized. The high polarization is used by some authors to give a strict constraint on the Lorentz invariance violation (LIV). If the Lorentz invariance is broken, the polarization vector of a photon may rotate during its propagation. The rotation angle of polarization vector depends on both the photon energy and the distance of source. It is believed that if high polarization is observed, then the relative rotation angle (denoted by alpha) of polarization vector of the highest energy photon with respect to that of the lowest energy photon should be no more than pi/2. Otherwise, the net polarization will be severely suppressed, thus could not be as high as what was actually observed. In this paper, we will give a detailed calculation on the evolution of GRB polarization arising from LIV effect duration the propagation. It is shown that the polarization degree rapidly decrease as alpha increases, and reaches a local minimum at alpha approximate to pi, then increases until alpha approximate to 3 pi/2, after that decreases again until alpha approximate to 2 pi, etc. The polarization degree as a function of alpha oscillates with a quasi-period T a approximate to pi, while the oscillating amplitude gradually decreases to zero. Moreover, we find that a considerable amount (more than 60 per cent of the initial polarization) of polarization degree can be conserved when alpha approximate to pi/2. The polarization observation in a higher and wider energy band, a softer photon spectrum, and a higher redshift GRB is favourable in order to tightly constrain LIV effect.

A tomographic test of cosmological principle using the JLA compilation of type Ia supernovae

We test the cosmological principle by fitting a dipolar modulation of distance modulus and searching for an evolution of this modulation with respect to cosmological redshift. Based on a redshift tomographic method, we divide the Joint Light-curve Analysis compilation of supernovae of type Ia into different redshift bins, and employ a Markov-Chain Monte-Carlo method to infer the anisotropic amplitude and direction in each redshift bin. However, we do not find any significant deviations from the cosmological principle, and the anisotropic amplitude is stringently constrained to be less than a few thousandths at

New constraints on the distance duality relation from the local data

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