The updated E_peak - E_gamma correlation in GRBs

The recently discovered correlation between the rest frame GRB peak spectral energy $E_{\rm peak}$ and the collimation corrected energy $E_\gamma$ in long GRBs is potentially very important, yet awaits confirmation from an independent sample. It may help to shed light on the radiation mechanism of the prompt GRB phase and on the way -- and in which form -- the energy is released from the central engine. We here present some additional evidence for the correlation (two new bursts) and re-derive the best-fit parameters. The tightness of the correlation is confirmed (sigma=0.1 dex). We show that this correlation allows us, for the first time, to use GRBs as cosmological probes to constrain the expansion history of the universe.Comment: 4 pages, 1 figure, submitted to Il Nuovo Cimento (4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004). Additional material at http://www.merate.mi.astro.it/~ghirla/deep/blink.ht

Gamma Ray Bursts: new rulers to measure the Universe

The best measure of the Universe should be done using a standard "ruler" at any redshift. Type Ia Supernovae (SN Ia) probe the universe up to z$\sim$1.5, while the Cosmic Microwave Background (CMB) primary anisotropies concern basically $z\sim$1000. Apparently, Gamma--Ray Bursts (GRBs) are all but standard candles. However, their emission is collimated and the collimation--corrected energy correlates tightly with the frequency at which most of the radiation of the prompt is emitted, as found by Ghirlanda et al. (2004). Through this correlation we can infer the burst energy accurately enough to probe the intermediate redshift ($z<10$) Universe. Using the best known 15 GRBs we find very encouraging results that emphasize the cosmological GRB role. A combined fit with SN Ia yields $\Omega_{\rm M}=0.37\pm0.10$ and $\Omega_{\Lambda}=0.87\pm 0.23$. Assuming in addition a flat Universe, the parameters are constrained to be $\Omega_{\rm M}=0.29\pm0.04$ and $\Omega_{\Lambda}=0.71\pm 0.05$. GRBs accomplish the role of "missing link" between SN Ia and CMB primary anisotropies. They can provide a new insight on the cosmic effects of dark energy, complementary to the one supplied by CMB secondary anisotropies through the Integrated Sachs Wolfe effect. The unexpected Standard Candle cosmological role of GRBs motivates the most optimistic hopes for what can be obtained when the GRB-dedicated satellite, Swift, will be launched.Comment: 11 pages, 4 color figures, ApJ Letters (vol. 613) in pres

Cosmological constraints with GRBs: homogeneous medium vs wind density profile

We present the constraints on the cosmological parameters obtained with the $E_{\rm peak}$--$E_{\gamma}$ correlation found with the most recent sample of 19 GRBs with spectroscopically measured redshift and well determined prompt emission spectral and afterglow parameters. We compare our results obtained in the two possible uniform jet scenarios, i.e. assuming a homogeneous density profile (HM) or a wind density profile (WM) for the circumburst medium. Better constraints on $\Omega_{M}$ and $\Omega_{\Lambda}$ are obtained with the (tighter) $E_{\rm peak}$--$E_{\gamma}$ correlation derived in the wind density scenario. We explore the improvements to the constraints of the cosmological parameters that could be reached with a large sample, $\sim$ 150 GRBs, in the future. We study the possibility to calibrate the slope of these correlations. Our optimization analysis suggests that $\sim 12$ GRBs with redshift $z\in(0.9,1.1)$ can be used to calibrate the $E_{\rm peak}$--$E_{\gamma}$ with a precision better than 1%. The same precision is expected for the same number of bursts with $z\in(0.45,0.75)$. This result suggests that we do not necessarily need a large sample of low z GRBs for calibrating the slope of these correlations.Comment: 7 pages, 7 figures, submitted to A&

The effects of Non-Gaussian initial conditions on the structure and substructure of Cold Dark Matter halos

We study the structure and substructure of halos obtained in N-body simulations for a Lambda Cold Dark Matter (LCDM) cosmology with non-Gaussian initial conditions (NGICs). The initial statistics are lognormal in the gravitational potential field with positive (LNp) and negative (LNn) skewness; the sign of the skewness is conserved by the density field, and the power spectrum is the same for all the simulations. Our aim is not to test a given non-Gaussian statistics, but to explore the generic effect of positive- and negative-skew statistics on halo properties. From our low-resolution simulations, we find that LNp (LNn) halos are systematically more (less) concentrated than their Gaussian counterparts. This result is confirmed by our Milky Way- and cluster-sized halos resimulated with high-resolution. In addition, they show inner density profiles that depend on the statistics: the innermost slopes of LNp (LNn) halos are steeper (shallower) than those obtained from the corresponding Gaussian halos. A subhalo population embedded in LNp halos is more susceptible to destruction than its counterpart inside Gaussian halos. On the other hand, subhalos in LNn halos tend to survive longer than subhalos in Gaussian halos. The spin parameter probability distribution of LNp (LNn) halos is skewed to smaller (larger) values with respect to the Gaussian case. Our results show how the statistics of the primordial density field can influence some halo properties, opening this the possibility to constrain, although indirectly, the primordial statistics at small scale.Comment: 15 pages, 8 figures. Slight corrections after referee report. To appear in ApJ, v598, November 20, 200

Turbulent dissipation in the ISM: the coexistence of forced and decaying regimes and implications for galaxy formation and evolution

We discuss the dissipation of turbulent kinetic energy Ek in the global ISM by means of 2-D, MHD, non-isothermal simulations in the presence of model radiative heating and cooling. We argue that dissipation in 2D is representative of that in three dimensions as long as it is dominated by shocks rather than by a turbulent cascade. Energy is injected at a few isolated sites in space, over relatively small scales, and over short time periods. This leads to the coexistence of forced and decaying regimes in the same flow. We find that the ISM-like flow dissipates its turbulent energy rapidly. In simulations with forcing, the input parameters are the radius l_f of the forcing region, the total kinetic energy e_k each source deposits into the flow, and the rate of formation of those regions, sfr_OB. The global dissipation time t_d depends mainly on l_f. In terms of measurable properties of the ISM, t_d >= Sigma_g u_rms^2/(e_k sfr_OB), where Sigma_g is the average gas surface density and u_rms is the rms velocity dispersion. For the solar neighborhood, t_d >= 1.5x10^7 yr. The global dissipation time is consistently smaller than the crossing time of the largest energy-containing scales. In decaying simulations, Ek decreases with time as t^-n, where n~0.8-0.9. This suggests a decay with distance d as Ek\propto d^{-2n/(2-n)} in the mixed forced+decaying case. If applicable to the vertical direction, our results support models of galaxy evolution in which stellar energy injection provides significant support for the gas disk thickness, but not models of galaxy formation in which this energy injection is supposed to reheat an intra-halo medium at distances of up to 10-20 times the optical galaxy size, as the dissipation occurs on distances comparable to the disk height.Comment: 23 pages, including figures. To appear in ApJ. Abstract abridge

Long Gamma-Ray Bursts as standard candles

As soon as it was realized that long GRBs lie at cosmological distances, attempts have been made to use them as cosmological probes. Besides their use as lighthouses, a task that presents mainly the technological challenge of a rapid deep high resolution follow-up, researchers attempted to find the Holy Grail: a way to create a standard candle from GRB observables. We discuss here the attempts and the discovery of the Ghirlanda correlation, to date the best method to standardize the GRB candle. Together with discussing the promises of this method, we will underline the open issues, the required calibrations and how to understand them and keep them under control. Even though GRB cosmology is a field in its infancy, ongoing work and studies will clarify soon if and how GRBs will be able to keep up to the promises.Comment: To appear in the proceedings of the 16th Annual October Astrophysics Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N. Gehrels & J. Nouse

On the future of Gamma-Ray Burst Cosmology

With the understanding that the enigmatic Gamma-Ray Burts (GRBs) are beamed explosions, and with the recently discovered ``Ghirlanda-relation'', the dream of using GRBs as cosmological yardsticks may have come a few steps closer to reality. Assuming the Ghirlanda-relation is real, we have investigated possible constraints on cosmological parameters using a simulated future sample of a large number of GRBs inspired by the ongoing SWIFT mission. Comparing with constraints from a future sample of Type Ia supernovae, we find that GRBs are not efficient in constraining the amount of dark energy or its equation of state. The main reason for this is that very few bursts are available at low redshifts.Comment: 5 pages, 2 figures, matches version accepted for publication in JCA

Structure and Subhalo Population of Halos in a Self-Interacting Dark Matter Cosmology

We study the structure of Milky Way (MW)- and cluster-sized halos in a Lambda Cold Dark Matter (CDM) cosmology with self-interacting (SI) dark particles. The cross section per unit of particle mass has the form sigma = sig_0(1/v_100)^alpha, where sig_0 is a constant in units of cm^2/gr and v_100 is the relative velocity in units of 100 km/s. Different values for sigma with alpha= 0 or 1 were used. For small values of sigma = const. (sig_0<0.5), the core density of the halos at z=0 is typically higher at a given mass for lower values of sig_0 or, at a given sig_0, for lower masses. For values of sig_0 as high as 3.0, the halos may undergo the gravothermal catastrophe before z=0. When alpha = 1, the core density of cluster- and MW-sized halos is similar. Using sigma = 0.5-1.0x(1/v_100), our predictions agree with the central densities and the core scaling laws of halos both inferred from the observations of dwarf and LSB galaxies and clusters of galaxies. The cumulative Vmax-functions of subhalos in MW-sized halos with (sig_0,alpha) = (0.1,0.0), (0.5,0.0) and (0.5,1.0) agree roughly with observations (luminous satellites) for Vmax > 30 km/s, while at Vmax = 20 km/s the functions are a factor 5-8 higher, similar to the CDM predictions. The halos with SI have slightly more specific angular momentum at a given mass shell and are rounder than their CDM counterparts. We conclude that the introduction of SI particles with sigma \propto 1/v_100 may remedy the cuspy core problem of the CDM cosmogony, while the subhalo population number remains similar to that of the CDM halos.Comment: To appear in ApJ, December 20, 2002. We added plots showing the evolution of the heat capacity profile for halos in the core expansion and gravothermal catastrophe phases. Minor changes in the text were introduce