Some more science considerations/thoughts …. P. Coppi, Yale ? ? E>5 GeV? E>30 GeV? vs.

Don’t forget absorption by infrared/optical background! Real population statistics and fully observed SED peaks would be very useful …

Numerical simulations for 3C 279. Spada et al. 2001

In case you still thought things were simple… Mkn X-ray/TeV campaign (Dieter Horns, preliminary) X-ray TeV X-ray hardness ratio (spectrum) Counts

If t_var = 6 hours (one night) - one telescope won’t do it!! Lesson from ASCA/X-ray monitoring days…. Need complete time sampling!

Typical  HESS/Veritas observation?

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 – with no coverage gaps …  …. Ideally, while GLAST is up! (HAWC won’t do this. Would be nice to have similar threshold to GLAST so see same sources. )

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.

M87 – FRI (weak jet) X-RAY Mostly synchrotron emission? Hey, there are some interesting nearby objects – jet emission (synch X-ray? => TeV e-/e+)! Resolved X-ray emission -> in situ acceleration!?

D. Harris,2003 M87 jet is not wimpy!!! X-ray variability seen in HST-1 knot too!!

An accurate measurement (upper limits) on the GeV-TeV extragalactic diffuse background. Why so interesting? GeV-TeV+ gamma-rays only produced in extreme environments or by “exotic” processes: e.g., black hole jets, supernova blast waves, cosmic strings, relict particle decays, or matter-antimatter annihilation. Background is sum of all nearby GeV-TeV activity in the Universe + all > GeV activity at z > 1. [ Gamma-ray pair production and cascading on intergalactic photon fields GLAST = calorimeter for VHE-EHE Universe! (best limits on BAU/matter-antimatter domains from gamma-rays) ]

Blazar Background Models, a la Stecker & Salamon 1996 Including IR/O absorption Don’t forget cascades! Coppi & Aharonian 1997

[~MeV] Klein-Nishina effects important? Be careful in interpreting origin of spectral features such as “bumps” and break energies! Can get spectral index harder than 0.5! ERC, blackbody targets ERC, power-law photon targets Moderski et al EGRET blazars? Some TeV blazars? [N.B.: Getting strong TeV emission not so easy!]

Fun stuff: clusters ….

Expected flux levels extremely uncertain!

Most sources can think of, even decaying/annihilating CDM particles, trace large scale structure/shocks… look for clustering signal! Bromm et al. 2003

Low threshold science objectives: GLAST AGN follow-up UV/optical EBL Diffuse gamma-ray background (extragalactic and galactic) GLAST “hotspot” follow-up GRB, high energy components Microquasars (NIR jet emission detected) SNR/Cosmic Ray accelerators Pulsed emission from plerions (pulsars ) Galaxy clusters UHECR sources/”Haloes” Star formation-related cosmic ray emission from other galaxies ??? Serendipity: Exciting particle physics? What if your “low energy” threshold is 30 GeV? Don’t go halfway or risk losing GLAST-related science! And do a bad of “TeV” science…

Aside: really pounding away at >1 TeV relatively easy and interesting too… (cosmic ray, SNR, probe EBL in micron region – most poorly constrained by direct counts & impacts star formation history

Theorist’s Wish List Rule of thumb: give a theorist a spectrum consistent with a power law (e.g., due to insufficient statistics) and he can fit any model/EBL you like. Need to detect curvature! Ideally measure both sides of low and high energy peaks, simultaneously w/good (< hour-month) time-sampling: UV-MeV, 100 MeV-TeV coverage. [Also very good to get below IR/O absorption threshold.] There will always be some special objects, e.g., Mkn 501, not accessible from a given ground-based site... Want good population statistics …. One “super” telescope not enough – want tightly coordinated space and ground-based telescopes.

As gamma-rays enter realm of mainstream astronomy, similar considerations for future progress apply as for other sub-fields of astronomy: a)Large area survey capability b)Improved Sensitivity c)Angular resolution!!! (big problem at GeV?) d)All-sky monitoring for variable sources (what will replace GLAST? Most blazars seem to be dead most of the time…) e)No gaps in time coverage/high duty cycle… f)As broadband/multiwavelength observations as possible! (Think about connections to other instruments/missions, e.g., hard X-ray telescopes like EXIST.) Given current technology, no single instrument configuration or one Instrument can do everything….