Fermi Swift - benbow/PAEC_2018/  · tial arrays may begin as early as 2022. A novel -ray

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Transcript of Fermi Swift - benbow/PAEC_2018/  · tial arrays may begin as early as 2022. A novel -ray

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 -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 -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 arrays 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 taking1200 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 -ray instruments, from approximately 30 GeV to 200 TeV [5].

The future of -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) -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 -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 -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 -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-laborations 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 -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, -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 -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, -raybursts, Galactic novae and has responded to MM alerts [8]. VERITAS has also presented the most detailed-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 -rays from dark matter. Here is a brief summary of the highlights.

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Upper limitsdirect observations

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 -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.