Joel R. Primack

IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array

Gamma rays at rest frame energies as high as 90 GeV have been reported from gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is considerable hope that a confirmed GRB detection will be possible with the upcoming Cherenkov Telescope Array (CTA), which will have a larger effective area and better low-energy sensitivity than current-generation imaging atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a detection, we have developed a phenomenological model for GRB emission between 1 GeV and 1 TeV that is motivated by the high-energy GRB detections of Fermi-LAT, and allows us to extrapolate the statistics of GRBs seen by lower energy instruments such as the Swift-BAT and BATSE on the Compton Gamma-ray Observatory. We show a number of statistics for detected GRBs, and describe how the detectability of GRBs with CTA could vary based on a number of parameters, such as the typical observation delay between the burst onset and the start of ground observations. We also consider the possibility of using GBM on Fermi as a finder of GRBs for rapid ground follow-up. While the uncertainty of GBM localization is problematic, the small field-of-view for IACTs can potentially be overcome by scanning over the GBM error region. Overall, our results indicate that CTA should be able to detect one GRB every 20–30 months with our baseline instrument model, assuming consistently rapid pursuit of GRB alerts, and provided that spectral breaks below ~100 GeV are not a common feature of the bright GRB population. With a more optimistic instrument model, the detection rate can be as high as 1 to 2 GRBs per year.

WHAT IS THE DARK MATTER? IMPLICATIONS FOR GALAXY FORMATION AND PARTICLE PHYSICS

We discuss three arguments that the dark matter which dominates the present universe is not baryonic — based on excluding specific baryonic models, deuterium abundance, and the absence of small-angle fluctuations in the microwave background radiation. If the dark matter consists of elementary particles, it may be classified as hot (free streaming erases all but superclusterscale fluctuations), warm (free streaming erases fluctuations smaller than galaxies), or cold (free streaming is unimportant). We consider scenarios for galaxy formation in all three cases. We discuss several potential problems with the hot (neutrino) case: making galaxies early enough, with enough baryons, and without too much increase in Mtot/Mlum from galaxy to rich cluster scales. The reported existence of dwarf spheroidal galaxies with relatively heavy halos is a serious problem for both hot and warm scenarios. Zeldovich (n = 1) adiabatic initial fluctuations in cold dark matter (axions, or a heavy stable “ino”) appear to be lead to observed sizes and other properties of galaxies, and may also yield large scale structure such as voids and filaments.

Growth of Perturbations between Horizon Crossing and Matter Dominance: Implications for Galaxy Formation

The dark matter (DM) that appears to be gravitationally dominant on all astronomical scales larger than the cores of galaxies can be classified, on the basis of its characteristic free-streaming damping mass MD), as hot (MD) ~ 1015M⊙), warm (MD ~ 1011M⊙), or cold (MD < 108M⊙) (Bond and Szalay, 1983; Primack and Blumenthal, 1983). For the case of cold DM, the shape of the DM fluctuation spectrum is determined by (a) the primordial spectrum (on scales larger than the horizon), which is usually assumed to have a power spectrum of the form |δk|2 ∞ kn (inflationary models predict the “Zeldovich spectrum” n = 1); and (b) “stagspansion”, the stagnation of growth of DM fluctuations which enter the horizon while the universe is still radiation-dominated. Stagspansion flattens the fluctuation spectrum for M ≲ 1015M⊙. We report here the results of a numerical evaluation of the fluctuation spectrum in which all relevant physical effects have been included (Blumenthal and Primack, in preparation), together with implications for galaxy formation (Blumenthal, Faber, Primack, and Rees, in preparation; hereafter BFPR).

Dark Matter, Galaxies, Superclusters and Voids

Two surprising facts of fundamental importance for understanding the large scale structure of the universe have become apparent in the last few years. The first is that most of the mass in the universe is invisible, and the matter it represents may not even be composed of baryons and leptons. The second fact is that on large scales, galaxies are distributed in a network of thick, flattened or filamentary structures, called superclusters, separated by vast voids.

Status of Cosmological Parameters

The cosmological parameters that I will discuss are the Hubble parameter H 0 ≡100h km s-1 Mpc-1, the age of the universe t 0, the average density Ω0, and the cosmological constant Λ. The most important recent development is the new analyses based on Hipparchos data indicating that the oldest Globular Clusters in our Galaxy have ages of ∼ 11 Gyr. The evidence would favor small Ω0≈ 0.3 if (1) the Hubble parameter actually has the high value h≈ 0.7 still favored by some observers, and the age of the universe t 0 ≥ 13 Gy, despite the new Hipparchos results; (2) the baryonic/total mass ratio in clusters of galaxies is actually ∼ 20%, about twice as large as expected for standard BBN in an Ω=1 universe with the Tytler et al. value D/H ≈ 2 × 10-5; or (3) the comoving number density of clusters does not decrease much with increasing redshift. The evidence would favor ∼ ~ 1 if (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the present-epoch non-Gaussian density distribution from Gaussian initial fluctuations in a low-a universe; or (2) the preliminary results from high-redshift Type Ia supernovae, which suggest that Ω0 ∼ 1 and ΩΛ is small, are confirmed. The latest small-angle CMB anisotropy data favor cosmologies in which Ω0 + ΩΛ ≈ 1. but statistics on gravitational lensing of quasars provide an independent upper limit on Λ,

Non-Zel’dovich Fluctuations from Inflation

I review the recent work of the Santa Cruz group on the generation of non-Zel’dovich (including non-Gaussian) fluctuations in chaotic inflation. With a single inflaton having the most general quartic polynomial potential, most of the space of the parameters of the potential corresponds to fluctuations with an approximately Zel’dovich spectrum. To the extent that significant deviations from the Zel’dovich spectrum arise, the spectrum characteristically has a dip at a particular scale; in this case the usual upper limit on the quartic coefficient is relaxed and the reheat temperature is correspondingly increased. In the context of the cold dark matter model, such a dip spectrum may help explain both increased large scale structure and earlier galaxy formation. If we consider a general inflaton potential, it is possible to invert the slow-roll equations of motion and find the potential corresponding to almost any desired fluctuation spectrum. In models with multiple inflatons such as double inflation, de Sitter fluctuations in the second inflaton generated during the inflationary period controlled by the first inflaton make it impossible to preset the value of the second inflaton, and thus to get structure in the fluctuation spectrum on a cosmologically interesting scale. Regarding non-Gaussian fluctuations (nGf), we distinguish between local and non-local nGf. With a single inflaton, the inflationary nGf are negligible if the fluctuation amplitude lies below the upper limit from nonobservation of cosmic background radiation fluctuations. Several nonstandard possibilities arise in the generation of non-local nGf such as cosmic strings and texture, with the corresponding scalar fields either free or coupled to the inflaton or to curvature. In particular, such cosmic strings typically correspond to a strongly Type I superconductor, a case that should be investigated further.

Modeling GeV Observations of Gamma-ray Bursts

Fermi has shown GRBs to be a source of >10 GeV photons. We present an estimate of the detection rate of GRBs with the future Cherenkov Telescope Array (CTA). Our predictions are based on the observed properties of GRBs detected by Fermi, combined with the spectral properties and redshift determinations for the bursts population by in struments operating at lower energies. We develop two model for high energy prompt and early afterglow emission, and show how the probability of detection is affected by instrument effecti ve area, response time, and energy threshold. While detection of VHE emission from GRBs has eluded ground-based instruments thus far, our results suggest that ground-based detection may be within reach of CTA, though detections would be infrequent even with prompt followup to all valid satellite triggers. We estimate a rate of one GRB every 2 ‐ 3 years based on the trigger rate from the Swift satellite, provided that no spectral softening or cutoff features below 100 GeV exist in a significant number of GRBs. Such a detection would help constrain the emission mechanism of gamma-ray emission from GRBs. Photons at these energies from distant GRBs are affected by the UV-optical background light, and a ground-based detection could also provide a valuable probe of the Extragalactic Background Light (EBL) in place at high redshift.

Dark Matter and Large Scale Structure III.

We discuss how measurements of the absorption of γ-rays from GeV to TeV energies via pair production on the extragalactic background light (EBL) can probe important issues in galaxy formation. We use semi- analytic models (SAMs) of galaxy formation, set within the hierarchical structure formation scenario, to obtain predictions of the EBL from 0.1 to 1000/im. SAMs incorporate simplified physical treatments of the key processes of galaxy formation — including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production — and have been shown to reproduce key observations at low and high redshift. Here we also introduce improved modelling of the spectral energy distributions in the mid-to-far-IR arising from emission by dust grains. Assuming a flat ACDM cosmology with Ωm0.3 and Hubble parameter h — 0.65, we investigate the consequences of variations in input assumptions such as the stellar initial mass function (IMF) and the efficiency of converting cold gas into stars. We conclude that observational studies of the absorption of γ-rays with energies from ∼ Gev to ∼10 TeV will help to determine the EBL, and also help to explain its origin by constraining some of the most uncertain features of galaxy formation theory, including the IMF, the history of star formation, and the reprocessing of light by dust.

Interpreting High-Redshift Galaxies in the Hubble Deep Field

Recent observations of “Lyman-break” galaxies in the Hubble Deep Field imply surprisingly large comoving number densities of bright galaxies at redshifts of z~3 — up to several times the density of L ≥ L * galaxies today (Lowenthal et al. 1997). We present the predictions of semi-analytic hierarchical galaxy formation models for the comoving number densities of bright galaxies at high redshift in a variety of CDM-based cosmologies. Our results suggest that a significant fraction of the Lyman-break galaxies may be low-mass starbursts, rather than the massive progenitors of present day bright galaxies. If this scenario is correct, we predict that the NICMOS observations of the Lyman-break galaxies will yield a broad distribution of (I — K colors.

Dark Matter, Cosmic Strings, and Large Scale Structure

Although the hypothesis of cold dark matter with a Zeldovich spectrum of primordial Gaussian fluctuations appears to give a picture of galaxy and cluster formation that is in reasonably good agreement with the available observations, there are indications that this model leads to less structure on very large scales than is observed. This paper gives a progress report on our efforts to study the formation of large scale structure in models with either (a) an additional feature in the fluctuation spectrum on large scales, such as can arise in a hybrid model with comparable amounts of baryonic and cold dark matter, or (b) non-Gaussian fluctuations, for example those that arise from cosmic strings. Although we cannot yet tell whether either approach will ultimately be sucessful, our preliminary results confirm that cosmic strings can lead to the sorts of rich cluster correlations that are observed.