Science with the New Generation of High Energy Gamma-Ray Experiments

Introduction

CONTENTS

After 4 years of operation, ESA's INTEGRAL mission has produced a remarkable portfolio of results, ranging from the inventory of the high energy sources, to the discovery of dozens of variable sources to the mapping of the Al and annihilation line in the Galaxy. A brief review of the most dramatic achievements of the mission is presented.

Suzaku satellite, the Japan-US collaborative mission, was successfully launched on July 10, 2006. It is equipped with 5 soft X-ray telescopes (XRT), one microcalorimeter (XRS), 4 CCD cameras (XIS, and one hard X-ray detector (HXD). Though XRS is not operational, XIS and HXD provide us with new views of thermal and non-thermal phenomena. After the performance verification phase with 70 targets, the first term of general observer program (AO-1) has been started from April 1.

The Swift Gamma-ray Burst Explorer mission, launched on 2004 November 20, is a multiwavelength observatory for gamma-ray burst astronomy. Here we report on the status of the mission at 2 years from the launch. We use the exceptional dataset of GRB 060814 as an example to illustrate the kind of temporal and spectral analysis that Swift data allow.

The AGILE Mission will explore the gamma-ray Universe with a very innovative instrument combining for the first time a gamma-ray imager (sensitive in the range 30 MeV - 50 GeV) and a hard X-ray imager (sensitive in the range 15-45 keV). AGILE will be operational at the beginning of 2007 and it will provide crucial data for the study of Active Galactic Nuclei, Gamma-Ray Bursts, unidentified gamma-ray sources, Galactic compact objects, supernova remnants, TeV sources, and fundamental physics by microsecond timing.

The Gamma-ray Large Area Space Telescope (GLAST), scheduled for launch in late 2007, is a satellite based observatory to study the high energy gamma-ray sky. There are two instruments on GLAST: the Large Area Telescope (LAT) which provides coverage from 20 MeV to over 300 GeV, and the GLAST Burst Monitor (GBM) which provides observations of transients from 8 keV to 30 MeV. GLAST will provide capabilities well beyond those achieved by the highly successful EGRET instrument on the Compton Gamma-Ray Observatory, with dramatic improvements in sensitivity, angular resolution and energy range. The very large field of view will make it possible to observe ∼20% of the sky at any instant, and the entire sky on timescales of a few hours.

The CANGAROO-III telescope system for very-high-energy gamma-ray astrophysics consists of four 10-m atmospheric Cherenkov telescopes located near Woomera, South Australia. The construction of the fourth telescope was completed in summer 2003, and stereoscopic observations have been in progress since March 2004. Here we report on the status of the system and some recent results from CANGAROO-III observations.

The H.E.S.S. Imaging Atmospheric Cherenkov Telescope Array is currently the most sensitive instrument for Very High Energy (VHE) γ-ray observations in the energy range of about 0.1-10 TeV. While the detector is well suited for studies of extended galactic sources, an extensive program of observations of variable point-sources has also been carried out in the past three years of operation. Many galactic and extragalactic VHE γ-ray sources have been discovered, and a selection of recent H.E.S.S. results is presented in this proceeding.

The Magic Cherenkov Telescope is located at the Observatory of the Roque de los Muchachos (ORM) and since 2004 is operative, in may 2005 was started the first yearly campaign of data taking named Cycle I. During this period the telescope reported signals from galactic and extragalactic AGN sources. In this report we will describe the technique for the detection of Very High Energy (VHE) gammas by this class of experiments and the first results from Cycle I observation.

VERITAS is an atmospheric Cherenkov telescope array sited in Tucson, Arizona. The array is nearing completion and consists of four, 12 m diameter telescopes. The first telescope in the array has been operating since February 2005, while observations with the full array are expected to begin in January, 2007. We report here in some detail on the performance of the first VERITAS telescope, and briefly discuss the first stereo observations.

The characteristics of the largely-unidentified Galactic sources of gamma rays that were detected by EGRET are reviewed. Proposed source populations that may have the correct spatial, spectral, luminosity, and variability properties to be the origins of the EGRET sources are also presented. Finally, the prospects for studying Galactic gamma-ray sources with the GLAST LAT are reviewed.

H.E.S.S., a system of imaging Cherenkov telescopes, is dedicated to the observation of TeV gamma-rays. Within the first years of operation a number of objects were detected, most of these objects were previously not known to be TeV emitters. The observed TeV emission is crucial for the understanding of particle acceleration in the sources.

Here I will review the results obtained on Galactic sources expected to show variable emission. Variable emission was detected with high significance from the binary systems PSR B1259−63 / SS 2883 and LS5039. The emission of the latter object appears to be periodic in accordance with the orbit. No pulsed emission from pulsars was detected so far. For three pulsars (PSR B0531+21, PSR B0833−45, PSR B1706−44) upper limits on the TeV emission at the EGRET pulse phases were derived.

Pulsars are useful tools for probing the law of physics under extreme conditions. Since pulsars emit radiation across the whole electromagnetic spectrum they are ideal targets for multiwavelength studies. Today we know a large number of radio pulsars but unfortunately our current knowledge of gamma-ray emitting pulsars is still lacking. Presently there are only seven high-confidence detections of gamma-ray pulsars, four of them detected by the instruments of the Compton Gamma Ray Observatory. The launch of GLAST and AGILE mission will lead to a major breakthrough in our understanding of gamma-ray pulsars. The general properties of the presently known gamma-ray pulsars are reviewed and the impact of the future gamma-ray mission on pulsar science is presented.

Unidentified objects dominate current catalogues of high-energy (MeV-to-GeV) sources. Solving the mystery of such unidentified γ-ray sources is a major challenge for astronomy. Different approaches towards source identification have been tried in the past, with limited success. The wealth of gamma-ray sources expected in the near future calls for a novel approach.

We review the observational status of the Supernova/Gamma-Ray Burst connection. Present data suggest that a significant fraction of long-duration GRBs (although not all of them) are associated with bright SNe of type Ib/c. Current estimates of SN and GRB rates yield ratios GRB/SNe-Ibc in the range ~ 0.4% − 3%. In the few SN/GRB associations so far discovered the SN and GRB events appear to be simultaneous events. For the association GRB 060218/SN 2006aj, X and optical observations suggest that the SN and GRB are coeval events within ~ 0.1 days.

I review the results of the galactic observations performed during the first 15-month operation cycle of MAGIC Very High Energy (VHE) Gamma Ray telescope. Highlights of this first cycle are the study of the Crab Nebula and its pulsar, and two unidentified HESS sources (in particular the tentative identification of HESS J1834-087 with a molecular cloud), the measurement of the Galactic Center energy spectrum up to 10 TeV and the discovery of the variable gamma-ray binary LS I +61 303.

The recent detection of high energy gamma-ray emission from some galactic Supernova Remnants (SNRs) provided the first observational support to the long standing hypothesis that SNRs are the sources of galactic Cosmic Rays (CRs). The possible detection of neutrinos coming from the same sources would be valuable to confirm the hadronic origin of that radiation. Having this scenario in mind we evaluate the neutrino-photon flux ratio to be expected from pion decay in the TeV range. We applied our results to estimate the neutrino flux from the Galactic Centre region which was showed by HESS and MAGIC to be an active gamma-ray source, most likely powered by the SNR Sgr A East. According to our results that source should be detectable by a km³ Mediterranean Neutrino Telescope.

A persistent, thin (a solid angle less than 0.1μsr.), gamma jet, may be ejected from BH and Pulsars, powered by ultra-relativistic electron pairs. These γ jet while precessing and spinning are originated by Inverse Compton⁷ and-or Synchrotron Radiation at pulsars or micro-quasars sources. They are most powerful at Supernova birth blazing, once on axis, to us and flashing GRB detector. The trembling of the thin jet explains naturally the observed erratic multi-explosive structure of different GRBs. The jets are precessing (by binary companion) and decaying on time scales of a few hours but they are usually staying inside the observer cone view only a few seconds (GRB) duration times; the jets whole lifetime, while decaying in output, could survive as long as thousands of years, linking huge GRB-SN jet apparent Luminosity to more modest SGR relic Jets (at corresponding X-Ray pulsar output). Therefore long-life SGR may be repeating and if they are around our galaxy they might be observed again as the few known ones and a few rarer extragalactic XRFs. The orientation of the beam respect to the line of sight plays a key role in differentiating the wide GRB morphology. The relativistic cone is as small as the inverse of the electron progenitor Lorentz factor (γ_e ≃ 10³ – 10⁴ for I.C.⁷ and γ_e ≃ 10⁸ – 10⁹ for Synchrotron radiation). The hardest and brightest gamma spectra are hidden inside the inner gamma jet axis. To observe the inner beamed GRB events one needs the widest SN sample and largest cosmic volumes (red-shift ≥ 1). The most beamed are hardest. On the contrary the nearest ones, within tens Mpc distances, are mostly observable on cone periphery. Their consequent large impact crossing angle, leads to longest anomalous SN-GRB duration, with lowest fluency and the softest spectra, as in GRB060218 signature. A majority of GRB jet blazing much later (weeks, months after) may hide their progenitor explosive SN after-glow and therefore are so called orphan GRB. The late GRBs are called now local SGRs (or in outer extragalactic XRF) and are linked to anomalous X-ray AXPs. Conical shape of few nebulae and the precessing jet of few known micro-quasar describe in space the model 3D signature. Recent outstanding episode of X-ray precursor, ten minutes before the main GRB event, cannot be understood otherwise.

Multiwavelength (MW) observations are an essential diagnostic tool to study the physics of blazars. Blazars, the most extreme objects among the Active Galactic Nuclei (AGNs), are characterized by rapid variability at all wavelengths from radio through TeV γ-rays. Considerable progress has been made in recent years thanks to multiwavelength monitoring campaigns. In this contribution, I will give a brief overview of the MW campaigns carried out in the past by large collaboration and their impact on the knowledge of the most important source/jet parameters needed by the physical models used to fit the observed blazar spectral energy distribution (SED). Furthermore, I will discuss some of the perspectives for multiwavelength observations during the operation of the gamma-ray mission GLAST.

MAGIC is presently the imaging atmospheric Cherenkov telescope with the largest reflecting surface and the lowest energy threshold. MAGIC concluded its first year of regular observation in April 2006. During this period and the preceding commissioning phase, 25 Active Galactic Nuclei have been observed and VHE γ-ray emission has been confirmed by 4 of them. Two more AGNs have been detected as γ-ray sources with high statistical significance for the first time. We report in this paper the results obtained analyzing data of the detected sources. Temporal and spectral properties of detected signals are shown and discussed.

I present a short review of Gamma–Ray Bursts studies, including observations, standard pictures, recent discoveries and the future perspectives of this very intriguing field of modern astrophysics.

Observations are showing that X-ray flares are very common features in GRB light curves. X-ray flares occur from about 100 s up to thousands of seconds after the burst, when the prompt-to-afterglow transition is taking place. In the context of External Shock, X-ray flares are produced by thick shell fireballs emitted by a long duration central engine activity, and represent the beginning of the afterglow emission. The delayed flare photons are expected to interact with the forward shock electrons by Inverse Compton producing high energy counterparts that potentially will be detected by GLAST LAT. Such high energy components could explain the delayed GeV emission detected by EGRET in GRB 940217. Observations with GLAST LAT will give useful information to constrain the origin of X-ray flares.

Gamma-ray bursts are amazing phenomena involving relativistic jets and the formation of black holes. Our understanding of these explosions and of the physics of their extreme environments would be greatly expanded by the observation of high energy gamma-rays. However, such observations of these unpredictable, rare, and short duration events are difficult. The Milagro detector has been used to search for gamma-ray bursts, but no significant emission has been observed. The HAWC (High Altitude Water Cherenkov) observatory is a proposed next generation detector based on the technology of Milagro that will have more than 15 times the sensitivity of Milagro to search for the highest energy emission from gamma-ray bursts.

MAGIC is presently the imaging atmospheric Cherenkov telescope with the largest reflecting surface and the lowest energy threshold. MAGIC concluded its first year of regular observation in April 2006. During this period and the preceding commissioning phase, several Gamma-Ray Bursts were observed by the MAGIC telescope thanks to its very fast slewing capabilities. The observations were triggered by GCN alerts received from SWIFT, HETE2 and INTEGRAL satellites and were performed various minutes following the alert. The threshold energies depend on the zenith angle of the burst location and span between 80 GeV and 200 GeV. Using standard MAGIC analysis, no evidence of gamma signals was found.

GRB 031203 and GRB 980425 are the two outliers with respect to the E_p – E_iso correlation of long GRBs. These two bursts share several properties with GRB 060218, a nearby burst detected by Swift which instead obeys the E_p – E_iso correlation. The SED of the prompt emission of this burst presents both thermal and non–thermal components: the former can be interpreted as due to the emission from the hot cocoon surrounding the GRB jet and the latter can be explained as synchrotron emission. During its long duration the prompt spectrum evolves from hard–to–soft and the peak energy of the time integrated spectrum occurs at ∼5 keV, i.e. in the energy range of the XRT instrument. Observations at these low energies were unavailable for the two outliers, but the comparison of the available spectral informations suggests that they might be twins of GRB 060218 and, therefore, they could be only apparent outliers with respect to the E_p – E_iso correlation. This interpretation underlines the importance of studying the broad band spectra of GRBs in order to monitor their spectral evolution throughout the entire duration of the prompt phase.

The GLAST Large Area Telescope (LAT) is a high-energy gamma-ray telescope that will be flown in late 2007. The LAT tracker (TKR) consists of an array of tower modules, composed of planes of silicon-strip detectors (SSDs) interleaved with converter tungsten layers. The photon detection is based on the pair conversion process and silicon-strip detectors will reconstruct tracks of electrons and positrons.

The instrument is actually assembled and the full LAT has been tested and integrated at Stanford Linear Accelerator Center (SLAC). Experimental results from the pre-launch cosmic ray data taking will be discussed.

The Gamma-ray Large Area Space Telescope (GLAST¹) will be launched in autumn 2007 to observe the γ-ray sky from 20MeV to 300GeV. The spare flight modules, integrated in a Calibration Unit (CU), underwent a beam test campaign at CERN during summer 2006. An online monitoring software was specifically developed to check for hardware performance and beam features.

The ARGO-YBJ detector is an Extensive Air Shower (EAS) array located in Tibet (P.R. China) at 4300 m a.s.l., performing a continuos sky observation by shower reconstruction with an energy threshold of a few hundreds of GeV. To lower this energy threshold to E∼1 GeV the detector has been designed to work in scaler mode, i.e. recording the counting rate of each module of the detector at fixed time intervals. At these energies, signals due to local (e.g. solar events) and cosmological (e.g. Gamma Ray Bursts, GRBs) phenomena are expected as a significant enhancement of the counting rate over the background. Results on the search for GRBs in coincidence with satellite detections are presented.

One of the milestones in the preparation of the software of the Gamma-ray Large Area Space Telescope (GLAST) was the second Data Challenge (DC2). The results of the analysis of simulated pulsars in DC2 is presented together with the description of the automated procedure used in the analysis.

In view of the upcoming launch of new generation gamma-ray satellites, new analysis methods for pulsar detection are in development. We have formulated a new kind of period test that self contain the photon selection cut optimization for each single pulsar. Comparing these tests with the already known period tests, it results in an overall increase in power of ∼20% with respect to the Z test and ∼50% with respect to the H test.

The Gamma-ray Large Area Space Telescope (GLAST), scheduled to be launched in late of 2007, is the next generation satellite for high-energy gamma-ray astronomy. The Large Area Telescope (LAT), the main instrument of GLAST, will survey the sky in the energy range between 20 MeV to more than 300 GeV, shedding light on many issues left open by its highly successful predecessor EGRET (Energetic Gamma Ray Experiment Telescope). The Gamma-Ray Bursts (GRBs) are one of the most exiting science topic for the LAT, which will open an new window, exploring these sources in a energy range never observed before. Even if little is know at high energy, we would like to address, in this contribution, some of the features of the GRB spectrum that theories predict and that LAT can observe. Finally we would like to present some results about the study of the sensitivity of the LAT to GRBs.

We describe a general method for time resolved gamma-ray spectroscopy of astrophysical transients - the Global Fit Analysis. Instead of parameterizing individual spectra and analyzing spectral evolution of a transient in terms of numerous individual spectral fit parameters we define a spectral evolution model with a set of constant global parameters and just a few (N=1-2) time dependent variables.

In case of GRBs, Global fits explain spectral variability in terms of single variable component with much simpler properties than normally assumed for GRB emission, being are statistically as good as traditional bin-to-bin fits.