The VERITAS data sets that were analyzed for this study are summarized in Table 8.3. All data were taken in wobble mode, with the tracking position ofset from the targets by 0.5◦, and after the T1 relocation. The data set on Arp 220 comprises 31 h, taken in February Ű April 2012, before the camera upgrade. The data set on IRAS 17208-0014 comprises 17 h. The majority (14 h) were taken between April 2011 and March 2012, also before the camera upgrade. An additional 3 h were taken in May 2015. The IC 342 data set comprises 3 h taken between November 2010 and January 2011. Only data taken under clear sky conditions, no or low moonlight, and with all four telescopes operating were selected.
All data were analyzed with the eventdisplay package by the author, using the Hillas-based standard analysis. As is customary in the VERITAS collaboration, independent secondary analyses were conducted with the VEGAS package, which conĄrmed the results presented here.
Eventdisplay provides instrument response functions for several sets of gamma/hadron separation cuts, optimized for diferent source strengths/spectral shapes. For this analysis, two sets of box cuts (moderate and hard) were used. Box cuts apply requirements on the mean scaled parameters (width and length) independently. The main diference between the cut sets is the cut on the size (total signal): moderate cuts require at least two images with a size of at least 400 d.c. (600 d.c.) before (after) the camera upgrade, whereas hard cuts require at least two images with a size of at least 1000 d.c. (1400 d.c.) before (after) the camera upgrade.
The size cut afects the energy threshold of the analysis as well as the signal and background rates. Hard cuts give the best background suppression, but at the cost of a low signal rate and high energy threshold. Thus, they are most appropriate for sources with hard spectra (index of −2.5 or harder) or long exposures on weak sources. Soft cuts (not used for this study) give the lowest possible energy threshold, at the cost of a large background rate. They are appropriate for very soft sources and/or short exposures on bright sources, for which
8.3 Results
Source Emin NON NOFF α Aeff· T Flux (E > Emin)
[GeV] [m2s] [m−2s−1]
Arp 220 500 45 282 0.17 8.35 · 109 ≤ 2.24 · 10−9
IRAS 17208-0014 600 16 124 0.17 4.76 · 109 ≤ 1.82 · 10−9
IC 342 500 37 174 0.18 940 · 106 ≤ 27.6 · 10−9
Table 8.4: Upper limits (99% conĄdence level) on the integral Ćux from the three star-forming galaxies studied here. A power law spectrum with an index of γ = 2.5 has been assumed for all sources.
background suppression is not critical. Moderate cuts provide a reasonable compromise between good background suppression and a low energy threshold.
For the Ćux estimates, the radius of the signal region was chosen to be 0.09◦, corresponding to the expected angular resolution. See Section 5.7 for more details about the analysis, particularly the Ćux estimation and the estimation of the expected limits.
8.3 Results
Data taken on the three star-forming galaxies were analyzed with two cut sets (moderate and hard box cuts). None of the objects showed a signiĄcant excess of gamma-ray like events above the background expectation. Skymaps showing the excess event count (diference between signal and background expectations) and the signiĄcance of these excesses in the Ąeld of view can be found in Figs. D.1, D.3 and D.5 in the appendix. All distributions are compatible with the background-only hypothesis.
As no evidence for gamma-ray emission was found, upper limits on the Ćux were derived. First, expected upper limits were calculated from the background counts for diferent energy thresholds to Ąnd the cut set and energy threshold that give the most sensitive limits com- pared to the Ćux estimates. Note that the energy threshold for the Ćux calculation is limited by the analysis energy threshold, which depends on the cut set and the zenith angle under which the observations were taken.
The expected upper limits can be found in Figs. D.2, D.4 and D.6 in the appendix. All limits were evaluated under the assumption of a power law energy spectrum with index 2.5. Hard cuts provide slightly more sensitive limits on Arp 220 and IRAS 17208-0014 compared to moderate cuts, while for IC 342 the diference is negligible due to the smaller exposure. Choosing a larger energy threshold for the Ćux estimate does not improve the sensitivity. Thus, the upper limits were Ąnally evaluated for hard cuts for Arp 220 and IRAS 17208-0014 and for moderate cuts for IC 342. The energy thresholds were 500 GeV for Arp 220 and IC 342 and 600 GeV for IRAS 17208-0014. The upper limits can be found in Table 8.4.
8 Limits on the Gamma-Ray Emission from Star-Forming Galaxies
8.3.1 Uncertainties
Two sources of uncertainty will be taken into account here: The shape of the source spectrum and the detector response (energy scale/efective collection area).
The shape of the source spectrum afects VERITASŠ efective collection area. Since no VHE gamma-ray emission was measured, the source spectrum is a priori unknown. However, it is expected that the gamma-ray emission from star forming galaxies follows the cosmic- ray spectrum (Lacki et al., 2011; Torres & Domingo-Santamaría, 2005; Lacki et al., 2010; Torres, 2004). Accordingly, the gamma-ray emission is expected to follow a power law with an index of 2 to 2.5. There can be deviations from this power-law behavior due to absorption in the host galaxy or by the extra-galactic background light (EBL), which would soften the spectra at the highest energies. Due to the relatively small distances, EBL absorption can be neglected in the VERITAS energy range (below some tens of TeV). Absorption in the host galaxy can lead to a cut-of at a few TeV.
VERITASŠs spectral weighted efective collection area increases for harder sources. Ac- cordingly, the limits presented here assume a spectral index of −2.5, at the softer end of the expectations. A harder index would imply a larger efective area, and thus more constraining limits for the same event counts.
If there were indeed a cutof, we would over-estimate the efective collection area. For example, Yoast-Hull et al., (2015) predict a cutof in the emission from Arp 220 for energies above 2 TeV to 5 TeV. For an energy threshold of 500 GeV and a spectral index of −2.5 (−2.0), only 13 % (25 %) of the gamma-ray photons would have energies above 2 TeV and could potentially be afected by absorption. The presence of such a cutof would afect the upper limit by less than 25 %.
The uncertainty on the VERITAS energy scale is about 20 %. The main contributions to this uncertainty arise from variations in the atmospheric density and aerosol proĄles, mirror degradation, and uncertainty on the PMT quantum eiciency. For a spectral index of −2.5, a 20 % uncertainty on the threshold energy corresponds to an uncertainty of 30 % on the integral Ćux.