Some more science considerations/thoughts ….
P. Coppi, Yale
?
?
E>5 GeV?
E>30 GeV?
vs.
Don’t forget absorption byinfrared/opticalbackground!
Real population statistics and fully observed SED peaks would be very useful …
Numerical simulations for 3C 279. Numerical simulations for 3C 279. Spada et al. 2001Spada et al. 2001
In case you still thought things were simple…
Mkn 421 2002 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 timevariable 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 similarthreshold to GLAST so see same sources. )
GLAST and GRBsLong burst w/optical flash detected by ROTSE, BATSE flux > 99.6% BATSE bursts
Briggs et al. 1999
Energy Flux at MeV Peak6 -2 -14 10 erg cm s
Integration Time for Spectrum ~ 32 s
Assume same energy flux at 1 GeV,4 210 cm collection area,
~ 800 photons
Great GeV energy spectrum forthis burst, and reasonable spectrafor 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 GRBsAwesome statistics, even for64 msec time bins.
Allows detection of significantspectral variability on < 1 sectimescales.
Just as for blazars, fitting time-integrated spectra when thissort of variability is going on isNOT a good idea.
Can GLAST match this X-ray sensitivity?
GLAST and GRBsAssume constant GeV flux at peak count rate (optimistic!):
N_photon in 1 sec @ 1 GeV = 25 -- o.k. N_photon in 64 msec @ 1 GeV = 1.6 -- not too useful
Also, although GLAST has sensitivity at 10 GeV, N_photon in 1 sec @ > 10 GeV ~ 2.5 -- 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
1 -7 -2 -1Assume standard decay rate, and GeV flux at 100 sec of 4 10 erg cm s
(same as X-ray flux), then N_photon in 180 at 1 GeV (good), and
~18 at 10 GeV.
O.K. for spectrum up to 10 GeV. Good news
t
t t
! ... But might be very optimistic
because emission peak could move quickly through GeV observing window.
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 seebrightest 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’tforgetcascades!
Coppi & Aharonian 1997
[~MeV]
Klein-Nishinaeffects important?
Be careful in interpreting originof spectral featuressuch as “bumps” and break energies!
Can get spectral indexharder than 0.5!
ERC,blackbodytargets
ERC,power-lawphotontargets
Moderski et al. 2005
EGRETblazars?
Some TeVblazars?
[N.B.: Getting strongTeV emission not so easy!]
Fun stuff: clusters ….
Expected flux levels extremelyuncertain!
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 10-60 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 oflow 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 absorptionthreshold.]
There will always be some special objects,e.g., Mkn 501, not accessible from a givenground-based site...
Want good population statistics ….
One “super” telescope not enough – want tightly coordinatedspace and ground-based telescopes.
As gamma-rays enter realm of mainstream astronomy, similar considerationsfor 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? Mostblazars 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 oneInstrument can do everything….