2020 VISION: UNIQUE OPPORTUNITIES FOR VERITAS IN THE COMING DECADE

1. Introduction to VERITAS

VERITAS (the Very Energetic Radiation Imaging Telescope Array System) is a γ-ray observatory compris-ing four imaging atmospheric-Cherenkov telescopes (IACTs) at the Fred Lawrence Whipple Observatory(FLWO) in southern Arizona (see Fig. 1) [1]. It is the most sensitive γ-ray astronomy facility for studyingastrophysical sources in the energy range from 85 GeV to 30 TeV and provides leading-edge capabilities toU.S. investigators. VERITAS was identified as a high-priority experiment in the 2000 Astronomy & Astro-physics Decadal Survey and in the DOE/NSF SAGENAP reports. VERITAS cost $17.6 million to constructand began full-scale operations in 2007. The array’s performance was significantly enhanced by improve-ments between 2009 and 2012 (costing $2.2 million), including an upgrade of the cameras. The array nowdetects a source with 1% of the Crab Nebula flux (the standard reference in TeV astrophysics) in ∼25 hours.VERITAS has a peak effective area of 100,000 m2, providing excellent instantaneous sensitivity for rapidtransient events, multi-messenger alerts and time-domain astrophysics.

VERITAS is now in its twelfth year of operations and continues to operate efficiently, typically taking∼1200 h of good-weather data per year with no significant loss of observing time to technical issues. Thescientific accomplishments of VERITAS have demonstrated that TeV astrophysics has come of age andcan address a rich variety of science including fundamental physics and cosmology, as well as Galacticand extragalactic astrophysics. Fig. 2a shows the current sky map of VERITAS source detections. Inconcert with its discovery program, VERITAS has carried out detailed spectral and morphological studiesof sources, correlated multi-wavelength (MWL) studies, especially with Fermi-LAT, Swift, NuSTAR andHAWC, correlated multi-messenger (MM) follow-ups of neutrino and gravitational wave (GW) events, andstudies of the extragalactic background light (EBL), the intergalactic magnetic field (IGMF), dark matter,the cosmic ray iron spectrum and the cosmic ray electron spectrum.

The detection of astrophysical neutrinos by IceCube [2] and gravitational waves by Advanced LIGO [3],including the electromagnetic counterpart to GW170817 [4], has opened the era of multi-messenger astro-physics (MMA). VERITAS plays a key role in MMA, and it is a top priority for the collaboration. Withits superior angular resolution, VERITAS will follow up on any observable new discoveries announced byMM partners. It complements instruments such as IceCube and LIGO, and partners effectively with HAWCand Fermi-LAT. Fig. 2b shows the differential sensitivity for non-transient sources of VERITAS comparedto current and future γ-ray instruments, from approximately 30 GeV to 200 TeV [5].

The future of γ-ray astrophysics worldwide will be the Cherenkov Telescope Array (CTA), comprisingtwo large telescope arrays, one each in the northern and southern hemispheres. It will have an order of mag-nitude better sensitivity than current generation instruments, such as VERITAS, and cover a wider energyrange (see Fig. 2b), representing a substantial leap forward for very high-energy (VHE) γ-ray astrophysics.Construction will begin soon and operation of the complete CTA will begin in 2025; observations with par-tial arrays may begin as early as 2022. A novel γ-ray Schwarzschild-Couder telescope (SCT), which uses

Figure 1: The VERITAS telescope array, located at the base camp of the Whipple Observatory in southern Arizona. Thededicated VERITAS building, located at the center of the photograph, houses the control room. The FLWO Visitor Center islocated in the green-roofed building and offers curated exhibits and guided tours of the project.

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Figure 2: (a, left) Skymap of VERITAS detections (October 2018). The shaded area shows the northern region best visible toVERITAS. The different source types detected are represented with different colored circles. (b, right) The sensitivity of VERITASin comparison to current and future γ-ray instruments.

a two-mirror design [6] that promises substantial gains in angular resolution and field of view over exist-ing instruments, is being developed for CTA. The design provides a dramatic improvement in Cherenkovtelescope optical performance, which will allow better γ-ray reconstruction and more effective rejection ofbackground protons. The NSF MRI program has supported construction of a prototype (pSCT) adjacent toVERITAS, by an “SCT Consortium” that shares a substantial fraction of its members with the VERITASCollaboration. The pSCT can be operated both as an addition to the VERITAS array, and as a stand-aloneinstrument to monitor bright, variable sources.

The VERITAS External Science Advisory Committee (ESAC) meets every ∼2 years to review the col-laboration’s science accomplishments, science plan, telescope operations and connection to the broaderastrophysics community. Its current members are Matthew Baring (Rice, Chair), Laura Cadonati (GeorgiaTech), Tom Gaisser (Delaware/Bartol), Chryssa Kouveliotou (GWU), Roger Romani (Stanford), Gus Sinnis(LANL) and Tim Tait (UC Irvine). Their report from the April 2018 meeting to review VERITAS scienceplanning for operations beyond 2019 states: “Overall the ESAC was very impressed with the state of VERI-TAS in terms of science accomplishments, science plan, telescope operations and connection to the broaderastronomy community. VERITAS is clearly an invaluable scientific facility for VHE γ-ray astronomy, pro-viding excellent return on the cumulative investment to date from the various sources.”

The case for continuing to operate VERITAS until at least 2022, three years beyond the end ofthe currently committed operations funding, is compelling. VERITAS sensitivity will continue to bestate of the art for the next several years, until CTA begins early operations in 2022 (or later). During thistime, it is expected that key MWL partners Fermi and Swift will continue to operate and VERITAS willcontinue to play an essential role in the MWL, MMA landscape, addressing a number of key scientificquestions. VERITAS will be the only observatory near its longitude capable of detecting transient VHEemission from higher-redshift extragalactic sources associated with IceCube neutrino events, γ-ray bursts,and gravitational wave signals. VERITAS also has a vital role in the education and training of the nextgeneration of astrophysicists, preparing them to exploit the future capabilities of CTA and other instruments.

2. VERITAS Accomplishments

VERITAS observations seek both to identify new VHE γ-ray sources and to study known sources in depthto understand better their underlying physics. VERITAS has detected emission from 63 sources (Fig. 2a)belonging to 8 different source classes: Galactic objects (supernova remnants, pulsars, binary systems, theGalactic Center, and unidentified) and extragalactic ones (active galactic nuclei, radio galaxies, and starburstgalaxies) [7]. In addition, VERITAS has carried out observations of galaxy clusters, globular clusters, γ-raybursts, Galactic novae and has responded to MM alerts [8]. VERITAS has also presented the most detailedγ-ray view of the Cygnus region to date, using 300 hours of data taken over 7 years [9], measured the e± andFe cosmic ray spectra, and searched for γ-rays from dark matter. Here is a brief summary of the highlights.

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Figure 3: (a, left) The VERITAS TeV light curves of BL Lac above 200 GeV in 2016 Oct. The light-blue filled circles and thedark-blue squares show the light curve in 4-min and 30-min bins, respectively (see [12]). (b, right) Near-ultraviolet to near-infraredspectrum of the EBL showing the upper limits derived from the VERITAS observations of the distant quasar PKS 1441+25 [11].

2.1 Extragalactic physics, supermassive black holes and cosmology: VERITAS has detected 36extragalactic objects with 20 being VHE discoveries. VERITAS observations have expanded the catalog ofblazars detected at VHE γ-ray energies to include intermediate- and low-frequency-peaked BL Lac objects(IBLs and LBLs) and flat-spectrum radio quasars (FSRQs), allowing tests of blazar sequence theories [10].VERITAS has collected data on most northern VHE-detected galaxies every year, providing a vast dataseton sources in both low and high emission states. Through MWL campaigns and modeling of the blazarspectral energy distribution (SED), VERITAS data have constrained the leptonic emission processes in therelativistic jets of blazars and have found evidence for the increasing role of external radiation fields inFSRQs and LBLs [11]. Target of opportunity (ToO) observations of flaring sources have led to blazardiscoveries such as the recent detection of the candidate binary black-hole blazar OJ 287, which can helpdetermine whether the emission is consistent with that predicted from a precessing jet (ATel #10051).

The excellent sensitivity of VERITAS enables probing of variability timescales down to minutes forbright blazar flares. The timescales of TeV flux changes place constraints on the sizes of emission regionsin blazar jets comparable to the Schwarzschild radius of the central black hole. Combined with VLBI,observations of TeV γ-ray flares probe the structure and emission mechanism of blazar jets. Fig. 3a shows afast flare detected from BL Lac in 2016, coincident with the emission of a radio knot measured with VLBI.A knot with superluminal apparent velocity was identified in observations at 43 GHz, passing the radio corearound the time of the γ-ray flare. Through collaborative radio observations of BL Lac, VERITAS has foundevidence that γ-ray flares are associated with the ejection or interactions of knots in blazar jets [12].

VERITAS observations of blazars provide an indirect measurement of the EBL via the absorption ofTeV photons traveling cosmological distances. The EBL is a calorimetric measure of the entire radiantenergy released by structure formation in the Universe and is difficult to measure directly. The VERITASdetection of the quasar PKS 1441+25, at a redshift of 0.939 (the second most distant VHE source), has seta stringent limit on the near-ultraviolet to near-infrared EBL intensity (Fig. 4b) [11]. The VERITAS resultssuggest that galaxy surveys have resolved most, if not all, of the sources of the EBL at these wavelengths.

In addition to blazar studies, VERITAS has detected >700-GeV γ-rays from the starburst galaxy M82and measured a cosmic ray density in the starburst core which is about 500 times the average Galacticdensity, linking cosmic ray acceleration to star-formation activity [13]. In the case of radio galaxies, thereare unique opportunities to study VHE emission in close proximity to supermassive black holes since theirinner jets can be resolved and correlated with MWL observations. VERITAS discovered an outburst ofγ-rays from the giant radio galaxy M87, simultaneous with a strong rise of the radio flux from the directvicinity of its black hole [14]. This year, VERITAS discovered VHE emission from the radio galaxy 3C 264(ATel #11436), a source with a jet <1 kpc and multiple superluminal knots, motivating an extensive MWLcampaign including Chandra, Swift, HST, VLBA, VLA, and many others.

2.2 Cosmic ray acceleration and Galactic source physics: Supernova-remnant (SNR) spectra at en-

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Figure 4: (a, left) VERITAS γ-ray counts map of IC 443 [18] (b, right) VERITAS γ-ray counts map of the pulsar wind nebulaDA 495 region. White lines are HAWC’s significance contours of 5σ. The yellow circle is the centroid of the VERITAS detection.The extension of radio emission from SNR G 65.1+0.6 is marked with a dashed black line (details in [21]).

ergies near the pion mass definitively show that SNRs accelerate hadronic cosmic rays [15]. VHE γ-raysprovide unique insight into how VHE cosmic rays are accelerated and the maximum energies achieved.VERITAS spectra from deep exposures of the young SNRs Tycho and Cas A show unexpectedly soft spec-tra above 400 GeV [16, 17], which suggest lower maximum cosmic ray energies than previously anticipatedand imply that SNRs may not power the whole Galactic cosmic ray population to the “knee” at 3× 1015 eV.Only one northern SNR is clearly resolved in VHE γ rays: the middle-aged SNR, IC 443 (see Fig. 4a).VERITAS measurements show a morphology coinciding with the shocked gas distribution and a spatiallyindependent VHE spectrum, indicating uniform cosmic ray acceleration throughout the shell [18].

VERITAS has made impactful studies of several γ-ray emitting binaries; the most recent is a once-in-a-lifetime observation during periastron passage of the 50-year-period binary PSR J2032+4127 in November2017 (ATel # 10810). It is one of only two VHE binaries with an identified compact object (a pulsar) andis also the only one with an associated VHE nebula [19]. The VERITAS light curve strongly differs frommodel predictions, suggesting that a revision is needed in our understanding of these complex systems.

A highlight result from VERITAS is the first detection of pulsed emission above 100 GeV from theCrab pulsar. The observed pulsed emission was difficult to explain with existing pulsar models [20], and theemission is unlikely to be primarily due to curvature radiation. These results imply that the γ-rays originateclose to, or outside of, the light cylinder—well beyond 10 stellar radii from the neutron star.

VERITAS has recently followed up on several sources discovered with the HAWC Observatory [21].For example, high-resolution VERITAS observations of 2HWC J1953+294 confirmed γ-ray emission andclearly showed that the object is associated with a pulsar wind nebula (DA 495) previously seen in the radioand X-ray (Fig. 4b). Although 2HWC J1953+294 is a point-like source for HAWC, VERITAS measured theextension of the TeV emission. VERITAS observed about seven times lower flux than reported by HAWC,clearly separating the true flux of the PWN from the influence of neighboring sources.

2.3 Fundamental physics: VERITAS has presented competitive limits on dark matter through a jointstatistical analysis of four dwarf galaxies using ∼230 hours of data to calculate constraints on the anni-hilation cross section of WIMP dark matter [22]. These results were derived using an optimized photonweighting statistical technique that improves on standard IACT analyses by utilizing the spectral and spatialproperties of individual photon events. VERITAS also measured the cosmic ray electron energy spectrum inthe energy range from 300 GeV to about 5 TeV [23], beyond the energy range explored by Fermi-LAT andAMS-02. At TeV energies, cosmic ray electrons provide a direct measurement of local cosmic ray accelera-tion and diffusion in our Galactic neighborhood. The recent VERITAS measurement of the energy spectrumof cosmic ray iron nuclei up to 500 TeV uses a template-based analysis method for the first time. The eventselection makes use of the “direct” Cherenkov light emitted by iron nuclei before their first interaction [24].

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2.4 Multi-messenger science: The sources of astrophysical ν detected by IceCube may emit observablefluxes of VHE γ-rays. VERITAS is a key component in identifying the sources of astrophysical ν, and hencethe sites of hadronic cosmic ray acceleration at the highest energies. VERITAS collaborates with IceCubeto observe archival νµ positions and to carry out prompt follow-up observations of ν-event alerts [25, 26,27]. Recently these observations were particularly successful, with the detection of a high-energy ν duringflares of the VHE-γ-ray-emitting blazar TXS 0506+056. A γ-ray flare observed by Fermi-LAT led to VHEdetections of the source by MAGIC [28] and soon afterwards by VERITAS [29], and the general result washeralded as the birth of high-energy ν astronomy at a press conference at NSF headquarters in July 2018.

2.5 Education and Outreach: VERITAS research has provided opportunities for the professionaldevelopment of young scientists at all stages of their academic careers. From 2007 to 2018, 60 Ph.D. theseswere written. In the same period, 104 peer-reviewed publications were published, of which ∼34% and∼64% were led (or co-led) by student authors and post-doctoral researchers, respectively. The VERITASCollaboration constitutes a diverse set of institutions, four of which are primarily undergraduate, includingone women’s college. Undergraduate students have participated in VERITAS data analysis, observing andhardware development, and made presentations at APS and AAS meetings. Recently, VERITAS developeda citizen-science project, “Muon Hunter,” hosted on the Zooniverse platform, for image classification in γ-ray astrophysics [30]. VERITAS data were classified multiple times by more than 6000 volunteers in orderto select and parameterize muon events. Muon Hunter had over a million classifications in the first week;only five out of the >120 Zooniverse projects have done so.

3. VERITAS Goals and PlansThe VERITAS Collaboration has developed a science plan beyond 2019, in the pre-CTA epoch, whichfocuses on a few key science drivers with the potential to be transformational. The collaboration has re-cently begun publishing results employing a variety of advanced analysis techniques, including maximum-likelihood techniques to match recorded shower images to templates generated from Monte Carlo simula-tions and multivariate classifiers such as “boosted decision trees.” These techniques have already producedimprovements of up to 40% in angular resolution and sensitivity, and studies indicate that combining themwill lead to at least the same improvement in performance again. Based upon previous results and new sci-ence discovery opportunities, we have revised and refocused our research goals in extragalactic astrophysicsand Galactic science, while expanding our efforts in multi-messenger and time-domain astrophysics. Jointresearch with VERITAS and the adjoining pSCT is also added to the future observing plan to further the de-velopment of the CTA project for the long-term future of VHE γ-ray astrophysics. We discuss each in turn.While minor evolution of the observing plan is expected, nearly 70% of the science program beyond 2019is already planned down to the target (i.e. specific annual allocations) or concept (e.g. neutrino follow-upallocations) levels. Many of these science programs (but not all) are discussed in this document. Remainingobservation time will be allocated annually through an internal VERITAS competition and the Fermi GuestInvestigator program.

3.1 Extragalactic Astrophysics: VERITAS has played a leading role in extragalactic γ-ray astrophysicssince its inception. The VERITAS extragalactic science program will focus on MWL and MM transientevents, including monitoring to self-trigger and initiate MWL campaigns, as well as deeper exposure onselect objects. Hence, there are strong connections to the MMA and time-domain astrophysics programs.Contemporaneous MWL and MM observations of the evolution of transient events are essential to constrain-ing the sites of emission, particle species and radiation processes in AGN jets, and VHE observations, whichconstrain the highest energies and shortest timescales, are critical. We will prioritize ToO observations ofblazars, both self-triggering and responding to flare alerts from other VHE instruments, Fermi-LAT, X-raysatellites and MM alerts. We have a unique window over the coming years to advance the field due to thebreadth of MWL and MM instruments in operation. VERITAS will also conduct a dedicated joint programwith the VLBA observatory to make unprecedented studies of the relationship between radio-knot ejectionsin jets and γ-ray flares through VERITAS-initiated coordinated observations. VERITAS will conduct deepexposures on a sample of nearby-to-moderate redshift (z < 0.5) blazars which exhibit particularly hard

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Figure 5: (a, left) VERITAS image of the Galactic Center [32]. (b, right) VERITAS sky map of the region around the blazarTXS 0506+056. The IceCube contours are shown in green, and the much smaller VERITAS PSF is displayed [29].

spectra that extend to multi-TeV energies. These extreme blazars are uniquely accessible to VERITAS, dueto its unparalleled effective area and sensitivity at TeV energies, and will be used to test models of particleacceleration deep in the Klein-Nishina regime, to constrain the EBL, and to search for evidence of Lorentzinvariance and axion-like particles. EBL constraints benefit from using multiple sources at a variety ofredshifts to study the redshift evolution and to overcome uncertainties in the intrinsic blazar source spectra.The science program will allocate 240 h/yr to exposures on select VHE blazars including deep exposuresand monitoring, with depth and cadence based on scientific priority. A further ∼70 h/yr are reserved forresponding to MWL flare alerts. Most new source discoveries are now made in response to these alerts, andthe number of high-value responses grows with the project lifetime.

3.2 Galactic Science: VERITAS Galactic observations beyond 2019 aim to address some importantquestions: What are the main cosmic ray accelerators? Where are the PeVatrons? How are particles accel-erated in binaries containing compact objects? What are the sources of the local electrons and positrons?VERITAS has discovered VHE sources useful for addressing these questions, but deep, targeted observa-tions of a few, carefully selected sources (some given below) are now required to provide precise answers.— Galactic PeVatrons: As noted above, VERITAS observations of the young SNRs expected to acceleratecosmic rays to PeV energies at the knee show softer spectra than anticipated, motivating a search for GalacticPeVatrons (sources of>PeV particles). We have identified MGRO J1908+06 as one of the two most promis-ing PeVatrons for study with VERITAS (the other is the Galactic Center, discussed below) [31]. HAWCshows no evidence for a cutoff in the MGRO J1908+06 spectrum, with photons detected at >100 TeV. Withadditional observations (∼40 h/yr), the excellent angular resolution of VERITAS will enable us to map thesource morphology and identify the source driving this powerful astroparticle accelerator.— The Galactic Center (GC): Fig. 5a shows the VERITAS map of the GC region at energies >2 TeV. Westrongly detect the diffuse emission with a hard power-law spectrum and no evidence of a cutoff. A high-statistics study (∼35 h/yr) of the GC region >2 TeV will test cosmic ray diffusion theories and address thequestion, “Where are the PeVatrons?” The same data will be used to search for signatures of dark matter.— γ-ray Galactic Binaries: VERITAS has a successful γ-ray binary program, including the recent deepobservations of PSR J2032+4127. We plan MWL campaigns including X-ray, radio, optical photometry,and optical spectroscopy of the Hα on both of our workhorse binaries. A campaign on LS I +61 303 willprobe modulation due to the 1667-day super-orbital period and is centered on October 2020. A campaignon HESS J0632+057 will occur during the 2020/2021 season to cover orbital phases 0.6–0.9 that showa secondary and less-studied peak in the VHE emission. Supplemental monitoring (∼15 h/yr each) willextend unprecedented light curves. These observations will address the questions of what causes the VHEvariability, the role of winds/mass-loss from the companion star, and the state of the circumstellar disk.— Geminga and local positrons: The cosmic ray positron excess discovered by PAMELA and confirmedby Fermi-LAT and AMS-02 is a major MMA result. The leading explanations are annihilation/decay ofdark matter or pair creation from particle acceleration by pulsars, with the most likely pulsar source being

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Figure 6: (a, left) Multi-messenger landscape until 2025. (b, right) The pSCT at FLWO (http://cta-psct.physics.ucla.edu).

Geminga. However, recent HAWC results suggest that lepton diffusion is too slow for the positrons to haveoriginated from nearby pulsars [33]. We have begun a new program of observations (∼30 h/yr) covering thefull nebula. A VERITAS detection of Geminga would extend the energy range probed down to ∼200 GeV.— Pulsars: The detection of pulsed emission from the Crab pulsar up to TeV energies has renewed theinterest of VHE observations of pulsars and VERITAS is pursuing new pulsar detections (∼30 h/yr). VHEobservations provide a unique view inside the magnetospheric accelerator, constraining its geometry andenergetics. Because only two pulsars have been detected in the VHE band so far (Crab and Vela), each newdetection has the potential to transform our understanding of pulsars.

3.3 Multi-Messenger Astrophysics: Multi-messenger astrophysics is a theme unifying many com-ponents of the VERITAS science program, as noted elsewhere in this section. Securely identifying andcharacterizing neutrino source counterparts in the VHE γ-ray band would be transformational. While theν events detected so far by IceCube [34] show no indication of clustering, it is still possible to exploit thesensitivity of VERITAS to search for their γ-ray counterparts (∼40 h/yr). Theoretical models [35] suggestthat the ν flux could correlate with the γ-ray flux in the case of flares from AGN, the most numerous typeof VHE γ-ray source. The detection of the flaring AGN TXS 0506+056 (∼30 h/yr) in the error box of Ice-Cube 170922 is likely to be the first example of this correlation. Fig. 5b shows the VERITAS sky map ofTXS 0506+056 overlaid with the IceCube contours [29]. Interestingly, MGRO J1908+06 (∼40 h/yr) is alsoa potential ν source: IceCube measures a spatially coincident neutrino excess that has a p-value 0.025 [36].VERITAS provides northern coverage with unmatched sensitivity until the start of CTA operations, filling ahole in VHE and MM coverage in the period before CTA comes online. Fig. 6a shows the MM landscapeuntil 2025. The ESAC noted in 2018: “Until the CTA Southern Observatory is operational, VERITAS willbe the only observatory near its longitude capable of detecting transient VHE emission from higher redshiftextragalactic sources associated with IceCube neutrino events, γ-ray bursts, and gravitational wave signals.”

3.4 Time-Domain Astrophysics: Several “long-shot” programs, with potentially transformative sci-ence, will be part of the portfolio. Rapid response with the VERITAS telescopes, implemented since thebeginning for GRBs (∼25 h/yr) and demonstrated for ν and GW alerts, will be key to these studies andallow the search for new source classes. Gravitational wave events: The goal is to search for electromag-netic counterparts to GW events (∼25 h/yr) and thus probe extremely energetic astrophysical processes. ANS-NS merger (possibly associated with a short GRB) within the LIGO horizon (100 Mpc) may be detectedby VERITAS [37]. The follow-up strategy for VERITAS is to tile the GW uncertainty region. The VER-ITAS FoV can cover ∼10 deg2 in a single pointing and was already used to cover an entire large northernGW uncertainty region in tens of pointings [38]. Optical transients: Fast optical transient phenomena havebeen observed in the cosmos. The requirement for coincident signals between multiple independent tele-scopes, combined with the ability to form a crude image of the light flash, allows IACTs to perform suchsearches parasitically during regular observations, with effectively no background. VERITAS may alsobe used as a unique, >10-m class, high-time-resolution optical telescope for non-γ-ray astrophysics (∼15

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h/yr). Successful path-finding studies were recently completed for eclipsing binaries, transiting exoplanets,and asteroid occultations. Fast Radio Bursts (FRBs): FRBs are mysterious transient events (at least someof which repeat) that were recently discovered by radio telescopes. The CHIME instrument [39] will detectmany FRBs and is at a similar longitude to VERITAS, enabling simultaneous observations in the 2019–2022period (∼10 h/yr). VERITAS will be able to set limits on optical counterparts to FRBs with instrumentationcapable of taking millisecond observations.

3.5 The Cherenkov Telescope Array: CTA has been in development for over a decade, although con-struction is yet to begin. U.S. participation in CTA construction, which can still be timely, was recommendedby the astronomy and astrophysics Decadal Survey and Mid-Decade Review as well as the P5 review in par-ticle physics. Most of the VERITAS members are participating in the development of CTA, contributing to(and in many cases leading) science and performance studies, instrumentation development, and organiza-tion of the project. A major component of this effort is the development of the revolutionary SCT designand implementation of the pSCT adjacent to the VERITAS telescopes (Fig. 6b), in a location where bothsystems can view the same events, allowing detailed cross-calibration. The NSF recently funded an upgradeof the current pSCT camera, covering a 2.5◦ (diameter) field of view (FoV), to the full 8◦ FoV for which theoptics are designed [40]. The upgraded camera, including improved photosensors and electronics, should becompleted in two years. This larger camera will allow rapid observations of the sky regions around transientand multi-messenger alerts, and fully exploit the exceptional optical performance of the design [6].

Simulation studies show that over much of the 100 GeV–10 TeV energy range the SCT outperforms a“conventional” single-mirror, Davies-Cotton telescope in CTA. Operating the pSCT in conjunction withVERITAS will provide data upon which to hone the analysis methods and verify the performance, whichwill be invaluable for optimizing the final implementation and performance of CTA. The long-term goal isto lead the implementation of an array of at least ten SCTs in one of the two CTA arrays, with U.S. fundingat the mid-scale level shared by NSF Physics and NSF Astronomy, closely following the recommendationsof both the Decadal Survey and P5. U.S. support of the construction and operation of CTA will unlock CTAaccess to the entire community of U.S. scientists, not only those participating directly in the construction.

3.6 Education & Outreach: VERITAS research will continue to have broader impacts in the hands-ontraining of students, the development of new capabilities for astrophysics research, and the engagement ofthe public with science and research. Student mentorship: VERITAS research provides an excellent trainingground for students at the graduate, undergraduate, and even high school level. Experiences at VERITASare transformational for undergraduate students who are still developing their career goals. Undergraduatestudents will continue to have opportunities to carry out senior thesis research on VERITAS and participatein publications. For example, in the last two years there have been 12 presentations by undergraduates atconferences. Public engagement: Individual VERITAS institutions will continue their activities at the K-12school and general public level that will involve public lectures and engaging with teachers and students.The FLWO maintains a visitor center at the VERITAS site. Activities include guided tours of the VERITAStelescopes, curated displays, time-lapse movies of VERITAS, and movies of atmospheric particle showers.VERITAS scientists are developing a successor to the highly-successful “Muon Hunter” citizen-scienceproject [30] hosted on the Zooniverse platform. VERITAS will participate with IceCube in the NSF-fundedproject, “Multi-messenger@INCLUDES: leading a community of change in multi-messenger astronomy.”This program aims to build on existing efforts of IceCube and its partners in multi-messenger astronomy toincrease diversity and inclusion in science and further develop talent for STEM research. Summer schools:We plan to collaborate with pSCT scientists to organize astroparticle schools as a way of bringing the U.S.graduate student and postdoctoral community up to speed on VHE γ-ray software, analysis methods andscience. The school will start in 2020 and be modeled after the successful Fermi School [41] organized bythe Delaware VERITAS group and NASA Goddard. New capabilities: The success of VERITAS rests onthe long history of work in VHE γ-ray astrophysics in the U.S. and at FLWO in particular. In the same way,activities honing the performance of VERITAS and the pSCT now pave the way for future discoveries withCTA and its successors.

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4. The Effectiveness and Efficiency of VERITAS OperationsVERITAS is run by an international collaboration of ∼80 scientists in 20 institutions from four coun-

tries. In addition, approximately 20 associate members participate actively in scientific studies with VER-ITAS through MWL observations or theoretical work. A signed agreement of cooperation appoints theSmithsonian Astrophysical Observatory (SAO) as the VERITAS managing organization and the host of theVERITAS Project Office (VPO). The VPO is responsible for the maintenance, day-to-day operations, andmost repairs of the experiment, so that the telescopes are ready to take data. SAO requests the fundsnecessary for VERITAS site operations.

The VERITAS Collaboration, including SAO, performs the many tasks required to obtain scientificresults: data taking itself, technical support of high-level systems, calibration, daily data quality monitor-ing, computer simulations of detector performance, analysis of the data, and writing publications. It is amoderate-sized collaboration relying on the high level of participation by all groups. These scientists re-quire base grant support, including travel to FLWO. Maintaining, or increasing, the support for thesegroups in FY20–22 is critical for both VERITAS operations and its science output to remain viable.

VERITAS recently began its twelfth year of operations. It is running well and is well-understood: arrayobservations are routine, all maintenance/repair is handled quickly by the dedicated and experienced VPOstaff, and any loss of observation time is almost exclusively due to causes outside the control of the project(e.g. poor weather). The project has always met or surpassed every long-term performance metric requiredof it, such as the total annual data yield, percentage of these data with all telescopes participating, and thetime lost to technical issues. VERITAS has always operated within its site-operations (VPO) budget, andat the same time has yielded much more data of higher quality (e.g. sensitivity) than initially envisioned.The 2014 ESAC report noted the “impressive levels of operational efficiency and expanded uptime of thetelescopes” and “the project is extremely well managed.” The report from the 2014 DOE Cosmic FrontierReview of Experimental Operations (a panel of 15 reviewers) noted VERITAS “is a mature experiment witha well-developed maintenance and operation plan that has constantly delivered a stream of quality sciencedata with high efficiency.” Delivery of the science program discussed in the previous section requiresVERITAS to sustain this high level of efficiency and performance. We propose to continue VERITASoperations following the most recent successful, streamlined support model.

VERITAS operates September to July each annual season, and is used to observe for ∼950 good-weatherhours during periods of astronomical darkness (∼83%) and partial moonlight (∼17%). In 2012, VERITASdeveloped the capability to observe during periods of bright moonlight [42], increasing the yield of theexperiment by another ∼25%. Since 2007, VERITAS has collected ∼13,600 h of data (87% are good-weather), ∼1330 h/yr since the development of bright-moon observing. It is notable that 93% of the acquiredVERITAS data have all four telescopes operational, comparable to or better than H.E.S.S. and MAGIC, withan all-time best 97% in the most recent season. Typically <2% of available time is lost to technical issues.The losses have shrunk in each of the past 4 seasons, averaging ∼25 h, and were an all-time low of 16 h in2017–18. No hardware malfunction has ever rendered a significant quantity of data unusable.

Operating from October 2019 to September 2022 (FY20–22) will yield ∼4000 h (∼30%) more VERI-TAS data. The site-operations budget projected for FY20–22 is ∼$1.3M/yr, ∼6% of the upgraded detectorcost and in line with other major facilities. It is ∼9% less than the FY14–16 budget, reflecting heavy cost cut-ting in that cycle, but ∼6% higher than the FY17–19 budget, primarily due to anticipated salary increases at∼2% inflation. Fig. 7 breaks down the operations budget, with and without ∼$300k/yr in “in-kind” supportprovided by SAO. All expense categories are well-defined after 12 years of experience. SAO has condition-ally committed to operating VERITAS at similar levels of support to FY17–19 (i.e. the in-kind support and 2technician salaries), implying the need for ∼$800k/yr in future external support. The NSF provided $525kin FY19, and an average of $475k/yr during FY17–19 (PHY-1607128). The DOE began phasing out itsdecades-long involvement in VERITAS in 2017. With only direct support from NSF and SAO, VERITASoperations were possible during FY17–19 using a cash reserve built through FY14–16 cost cutting.

The $1.0M/yr VPO cash budget primarily funds the salaries and benefits for 6 VPO employees. It

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Figure 7: Categorical breakdown of the VERITAS site-operations budget including (left) and excluding (right) in-kind supportfrom SAO consisting of all utilities, one FTE of facility-focused labor from the FLWO support group, two observer vehicles, anda variety of facility- and safety-related services. For visibility, Stipends (∼1% of the budget) are combined with Services, andShipping (0.3% of the budget) is not shown. The Project Scientist’s salary is not supported by VERITAS operations.

also purchases materials, supplies and services required for the continued operation and maintenance ofthe VERITAS array and pays for the shipping expenses of off-site repairs. It maintains a supply of themost critical spare components. Some high-level maintenance services are contracted via sub-awards toVERITAS groups. Travel funding supports the VPO staff to meet with funding agencies and collaborationmembers for project review purposes, for the Project Scientist to visit the VPO regularly, and for the threeVPO scientists to attend conferences. Stipend funding supports three-month visits by graduate students toobserve and perform technical projects under the supervision of the three VPO scientists.

The Project Scientist, Dr. Wystan Benbow, is the director of the VPO and responsible for the successfulon-site operation and management of VERITAS. He directs the VPO via the Observatory Manager, who su-pervises all operations staff and on-site activities, assisted by a Deputy Observatory Manager/IT Specialistwho also maintains all on-site computing and networking systems. These two managers are also active sci-entists. Two electronics technicians maintain all VERITAS electronic and mechanical systems. An opticaltechnician has primary responsibility for the telescope structures and their reflective surfaces; the VERITASmirror facets are re-coated on a ∼4-year cycle. An administrator (0.33 FTE) supports the VPO activities.

VPO personnel have provided some technical support to the pSCT construction. The pSCT is currentlybeing commissioned and operations will begin in FY19, likely based in the VERITAS control room. Whileit is logical that the VPO would support regular pSCT operations, and it is willing to do so, further resourcesand personnel would be required, at the level of an additional $170–200k/yr.

The VPO has performed a detailed assessment of every major component of VERITAS to assess itspotential risk for long-term operations. Generally speaking, everything is in excellent condition consideringthe project age, and there is no imminent threat to operations. There is some risk to operations from thefailure of obsolete/aging moderate cost (∼$10k) parts at a rate of ∼1 per year based on recent experience,and this is included in the budget. The project has its original three technicians, and their experience isinvaluable to continued operations. No funding for significant upgrades or for decommissioning is includedin the budget. As the VPO annual cash expenses were already reduced by ∼20% in the FY14–17 cycle,further reduction to the VPO cash budget (∼80% salaries) would be challenging to actualize.

5. Concluding Remarks

Eleven years of VERITAS operation have yielded many significant results, publications and theses.They have led to a thorough understanding of the detector and the resources required to operate the obser-vatory. The project operating costs are streamlined, the data yields are higher than ever envisioned, andthe data quality is the best ever. Continued operation of VERITAS has a guaranteed return from a newmulti-year science plan, and 2019–2022 operations will deliver ∼4000 h of new VERITAS data, an increaseof 30%. Continued operation of VERITAS will ensure the scientific achievements of complementary U.S.projects such as HAWC, IceCube, LIGO, Fermi, Swift and NuSTAR are maximized and that the U.S. VHEastrophysics community continues and develops as an active world leader, until CTA supersedes VERITAS.

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