Low Energies

Although high sensitivity measurements in the energy region above 130GeV will be accessible with a smaller array of 10m telescopes (e.g., 3 or 4 telescopes), the energy range from 60GeV to 130GeV is only accessible with the full array of seven telescopes. Some cosmic sources (e.g., gamma-ray pulsars or possibly gamma-ray bursts) may be detectable only in the lowest energy range which is accessible with the full seven-telescope array. However, energy measurements on all sources will benefit from having meaningful spectral measurements over a wide range of energies. The differential flux sensitivity is defined as the flux level over an energy interval of 1/4 of a decade over which an energy measurement can be made to the 20% level in 50 hours of observation. This is the meaningful level for astrophysical measurements. The flux sensitivity for the full array is shown in Figure 2 where it is contrasted with the flux sensitivity for a three telescope array (Sub-array).

  

Figure 2: The flux sensitivity, expressed as E*dF/dE, for the full VERITAS array (seven telescopes) and the sub-arrays (three telescopes) for a five-hour exposure. For reference, the spectrum of Markarian421 as seen on May 7, 1996 is shown, with an extrapolation of the spectrum below 400GeV. We also show the flux which would be seen if the source were at different redshifts, where the flux is reduced by distance and attenuated by extragalactic absorption.

The energy band from 60GeV to 130GeV is a particularly interesting one for many astrophysical measurements:

• It is the critical region for the study of AGN with z from 0.3 to 1.0. As seen in Figure 3, sensitivity in this region allows the study of objects like Mrk421 beyond redshifts of 0.3. Also, many EGRET-detected AGN are expected to have intrinsic spectral cut-offs in this energy region.

• This region is most important for measurements of the cutoff in gamma-rays from extragalactic sources due to pair production on the EBL.

• It is the region with the greatest possibility for the detection of pulsars as illustrated in Figure 4 where the extrapolated spectrum of PSR1951+32 is shown for cutoffs of 50GeV and 75GeV.

• Sensitivity at these low energies provides the highest probability of gamma-ray burst detection due to their large redshift. Information on the high energy spectra of GRBs is scarce but in Figure 5 we show the extrapolated spectra from EGRET measurements using an extragalactic infrared density model of Primack and assuming a distance to the burst source of z=0.94. If the fraction of high mass stars created during the epoch of star formation is low, the attenuation will be considerably less. A one minute duration is assumed for the very high energy emission and the sensitivity is a 5$\sigma$ detection per 1/8 decade of energy. In Figure 6 the simulated relative response of a hard-spectrum GRB is shown for the full Array and the Sub-array.

• This band covers a large portion of the allowed phase space for neutralino line emission.

• Broad-band spectral energy measurements of supernova remnants are essential to resolve the emission mechanisms at work in these objects. Effective combination of VERITAS with GLAST will provide energy spectra over 6 magnitudes in gamma-ray energy. GLAST will be able to detect gamma-rays out to 300GeV, but its primary sensitivity range will be below about 100GeV due its small effective area. With energy coverage significantly below 100GeV, VERITAS will permit accurate cross-calibration with GLAST, maximizing the scientific return of combined measurements with these instruments.

  

Figure 3: The flux sensitivity, expressed in units of E²*dF/dE, for the full VERITAS array (seven telescopes) and a sub-array (three telescopes) for a 50 hour exposure. The measured spectrum of Mrk501 from 1997 is shown, with an extrapolation below 250GeV, as well as model spectra for some other similar type of AGN at different redshifts.

  

Figure 4: The flux sensitivity, expressed in units of E²*dF/dE, for the full VERITAS array (seven telescopes) and a sub-array (three telescopes) for a 50 hour exposure. The spectrum of PSRB1951+32 is shown extrapolated from EGRET energies with exponential cut-offs at 50GeV and 75GeV.

  

Figure 5: The flux sensitivity, expressed in units of E*dF/dE, for the full VERITAS array (seven telescopes) and a sub-array (three telescopes) for a 1 minute exposure. An extrapolation of the spectra measured by EGRET for several GRBs is shown after attenuating the gamma-ray emission from interaction with the extragalactic background light.

  

Figure 6: Simulated lightcurves (without background subtraction) measured by the full VERITAS array between 55 and 180GeV (top) and a sub-array of three telescopes between 100 and 240GeV (bottom) for GRB 910503 if the spectrum follows that indicated in Fig. 5. The background dominates the sub-array measurements because the data must be accumulated much closer to the lower end of its sensitive energy range.