Astrophysics With the Cherenkov Telescope Array P. Coppi (for F. Aharonian, MPIK, Heidelberg)

Motivation Perhaps because: (i)strong GeV-TeV emission typically occurs in extreme physical environments, e.g., high density, compact regions like the vicinity of a black hole, (ii)the bulk motions in such extreme environments can be correspondingly extreme, e.g., relativistic, (iii) the radiative energy loss timescales for GeV-TeV emitting particles in such environments can be very short, the observed gamma-ray sky is extremely VARIABLE! [GRBs – seconds to days, blazars – minutes to days]

Motivation To understand the sources we see, we therefore need to time resolve their variability at all the relevant emission energies. Only then do we have the information required to constrain models. Ideally, our sensitivity at gamma-ray energies will be sufficient to match that at other energies, especially X-rays,

Motivation

Much Better!

Motivation - AGN Preceding example (hopefully) shows promise of adequately resolved time variability studies. Unfortunately, the gamma-rays were measured at TeV energies. Because of high energy cutoffs that are intrinsic or cutoffs due to intergalactic absorption most blazars in the Universe are GeV and NOT TeV emitters. Even for blazars with detectable TeV emission, models are greatly constrained by simultaneous GeV measurements (e.g., no uncertain EBL absorption at ~ GeV). Need same time resolution at GeV energies as at TeV energies!! Only current hope to do this is GLAST. Is it up to the job? …

GLAST and GRBs Long burst w/optical flash detected by ROTSE, BATSE flux > 99.6% BATSE bursts Briggs et al Energy Flux at MeV Peak Integration Time for Spectrum ~ 32 s Assume same energy flux at 1 GeV, collection area, photons Great GeV energy spectrum for this burst, and reasonable spectra for bursts ~ 50x fainter. A MAJOR improvement over EGRET! BUT … this is a time integrated spectrum… Look at what BATSE saw during those 32 sec

GLAST and GRBs Awesome statistics, even for 64 msec time bins. Allows detection of significant spectral variability on < 1 sec timescales. Just as for blazars, fitting time-integrated spectra when this sort of variability is going on is NOT a good idea. Can GLAST match this X-ray sensitivity?

GLAST and GRBs Assume constant GeV flux at peak count rate (optimistic!): N_photon in 1 1 GeV = o.k. N_photon in 64 1 GeV = not too useful Also, although GLAST has sensitivity at 10 GeV, N_photon in 1 > 10 GeV ~ not too useful  GLAST is marginal, and this is for a very bright burst! (N.B. OSSE detected 16 msec variability for this burst at ~ 1 MeV.)

GLAST and GRBs Another key component of GRB studies is the AFTERGLOW. Can GLAST study this? [Afterglow is much easier because there is no rapid time variability.] Bottom line: Unless we’re lucky with physics, GLAST will only see brightest bursts at ~ 1 GeV, and there is not much margin for error.

Summary (Motivation) VHE (GeV-TeV) gamma-ray emission is a highly time variable phenomenon.  We need a “Gamma-Ray Timing Explorer” (GTE) analog to the Rossi “X-Ray Timing Explorer” (RXTE) with the same relative sensitivity at ~ 1 GeV as RXTE at ~1 keV.

Proposal We need a GeV light bucket. Assume E -2 spectrum. => Relative number flux F_GeV/F_keV = 10 6! RXTE collection area is ~ 5x10 3 cm 2. => Our ideal light bucket would have an area of ~ 5x10 9 cm 2 ! Problem: conventional space-based detectors (launched by Shuttle/rocket like GLAST) unlikely to exceed 10 4 cm 2 …. Solution: Ground-based Telescopes! A souped-up VERITAS (~3x cm 1 TeV) at 5,000 m: (5 GeV 5 Km )

Why 5 km altitude? (i)Observed Cherenkov light from shower increases by factor 3! => a 20 m diameter mirror (large but not outrageously huge) is enough to collect ~50 photo-electrons from a shower with core < 100 m away. (HPD/APD/etc. would of course lower requirement more.) (ii)(Less important, but still significant) At GeV gamma-ray energies, the proton shower that produces as much light as the gamma-ray develops at lower altitudes … => some background suppression for free.

Performance

Why Do Shifts in the Atacama Desert? 1. Most of the required infrastructure will already be there! Big Road!

Why Do Shifts in the Atacama Desert? 2. In terms of water vapor content, it has perhaps the best atmospheric conditions in the world (better than Mauna Kea).  higher observing duty cycle (no surprise thunderstorms!) 3. Can probably piggy-back of atmospheric monitoring that will already be going on there => better absolute energy calibration. 4. ** (still in progress) Potentially huge benefit: The site is on the geomagnetic equator => rigidity cutoff of 15 GeV for electrons trying to enter the Earth’s atmosphere. [N.B. Cosmic ray electrons are the primary background!]

What Other “Tricks” Are We Relying On? 1.Array of telescopes operated in co-incident trigger, stereoscopic imaging mode. (Kills muon background and imaging enables strong suppression of cosmic ray proton background.) Optimized cuts for low-energy (GeV) regime degree camera pixels to image small GeV showers and improve night sky rejection. 3.Fast timing: >5< nsec gate. Conclusions which follow based on HEGRA/HESS Monte Carlo (Konopelko) code which reproduces well HEGRA shower parameters.

Backgrounds

Performance

Performance – Long Observations

Performance - AGNs

Performance - GRBs

Open Invitation: We need manpower, ideas, partners, and MONEY! [workshop coming this winter: contact [International!]