Cover Page for Application to DOE FOA #DE-FOA-0001358

Cover Page for Application to DOE FOA #DE-FOA-0001358

Project Title: Probing Dark Matter and Fundamental Physics with VERITAS

Applicant / Institution: Smithsonian Astrophysical Observatory

Street Address:

60 Garden St

Cambridge, MA 02138

USA

Postal Address:

60 Garden St, MS-23

Cambridge, MA 02138

USA

Lead PI:

Dr. Wystan Benbow

617-496-7597

Administrative Point of Contact:

Chris McNeil

617-496-7923

Funding Opportunity FOA Number: DE-FOA-0001358

DOE / Office of Science Program Office: High Energy Physics

DOE / Office of Science Program Office Technical Contact:

Kathy Turner

301-903-1759

PAMS Letter of Intent Number: N/A

Research Area: Experimental Research at the Cosmic Frontier in High Energy Physics

Probing Dark Matterand Fundamental Physics with VERITAS

1. General Background

This proposal requests support for the ongoing research efforts of the 50-year-old very high energy (VHE; E>100 GeV) γ-ray astrophysics group at the Smithsonian Astrophysical Observatory (SAO). The current leader of this group, and PI of this proposal, is Dr. Wystan Benbow. The PI is a federal employee at SAO and is the Project Scientistof the Very Energetic Radiation Imaging Telescope Array System (VERITAS). SAO is responsible for the day-to-day operations of VERITAS, and it hosts both the project and the VERITAS Project Office (VPO) at the F. L. Whipple Observatory (FLWO). The PI is also the director of the VPO, which consists of 8 employees, including 2 scientists. Support for the VPO and VERITAS site-operations comes from SAO and separate grants to the PI from the National Science Foundation (NSF) and the Department of Energy (DOE) Office of Science.

Since 1982, the DOE has been the primary source of external support for the SAO group’s base research. This request seeks support for the SAO group’s research efforts during a 3-year period from May 1, 2016 until April 30, 2019, during which the final legacy results of VERITAS will be generated. Dr. Benbow is the PI for one current DOE grant, the aforementioned VERITAS site-operations grant (“The Application of Two Dimensional Imaging to Very-High-Energy Gamma-Ray Astronomy”, #DE-FG02-91ER40635) which expires on April 30, 2016. The PI was previously the PI of another DOE research grant:“Expanding the Discovery Potential of VERITAS via Moonlight Observations” (#DE-FG02-09ER41619) which ended in 2014. Similar to the situation for many university groups, sources of internal (SAO) support are extremely limited and no funding for the group’s base research efforts, aside from the PI’s entire salary is provided by SAO. As a federal employee, the PI is not eligible for NSF research funding, thus the fundamental VERITAS research group (i.e. that of the Project Scientist), is dependent on continued DOE support to produce legacy results for VERITAS (from data funded by DOE).

Since Dr. Benbow became the group leader in 2008, the SAO VHE γ-ray astrophysics group’s research efforts were supported by the DOE “Moonlight” grant ($285,000 total; 2009-14), several smaller NASA grants (Fermi and Swift Guest Investigator), and the prior group leader’s DOE base grant (2007-10; ~$270k / yr, Dr. Benbow was PI after Dr. Weekes (deceased) retired in 2008). Overall, these funds supported 6 post-doctoral researchers and 2 PhD students full-time for their VERITAS research, and several 3-month stipends enabling visits to VERITAS by students working on research programs led by the PI. The two fully-supported PhD students completed their degrees and all post-docs will have departed the group by Sept. 30, 2015. The scientists currently in the SAO group are the PI, Dr. Pascal Fortin (Observatory Manager) and Dr. Veronique Pelassa (Deputy Observatory Manager). All these scientists actively participate in VERITAS research, but the latter two focus on operations and are 100% supported by the site-operations grants. This proposal requests a new 3-year base grant for the PI ($307k) to support the group’s VERITAS research efforts. The funding will pay for the stipend / health-dental insurance for one new post-doc and their travel expenses, attendance at one annual conference for the PI,and purchase materials required for VERITAS research.

2. VERITAS Overview

VERITAS [75]is an array of four 12-meter atmospheric-Cherenkov Telescopes (ACTs). It is the most-sensitive VHE γ-ray observatory in the World, and is one of two US facilities devoted to cosmic VHE γ-ray studies (HAWC [76] became fully operational in 2015). VERITASwas a high-priority experiment in DOE sponsored reports and began full-scale operations in September 2007. The DOE contributed to the development of VERITAS and funded ~42% of its ~$17.6 million construction cost. VERITAS is operated by a collaboration of ~100 scientists from 20 institutions in 4 countries, and its site operations are fully funded through 2016. VPO budget projections show site operations are possible though 2017, assuming no major failures and no-cost extensions to the existing grants from DOE and NSF. The collaboration hopes to operate VERITAS until July 2019 and will submit proposals for the necessary support in Fall 2015. VERITAS is used to study VHE γ-rays from cosmic sources. These VHE γ-rays are observed via the Cherenkov light produced as they interact in the Earth’s atmosphere. The data are taken with high-speed systems similar to those used in other particle physics experiments, and reduced using custom software employing CERN packages. VERITAS is operating well androutinely observes for ~1100 good-weather hours each year. All promised performance metrics of the experiment are achieved or exceeded [77]. These include an energy threshold of ~60 GeV, an energy resolution of ~15%, an angular resolution of ~0.1º, and a sensitivity of <1% of the Crab Nebula flux in 25 hours. In Summer 2012, a major upgrade of VERITAS was completed, improving the its sensitivity and decreasing its energy threshold. The experiment now detects sources ~2.5 times faster than in 2007 [78]. Due to its unprecedented sensitivity, VERITAS will continue to make major contributions to the field of astrophysics. While a next-generation VHE observatory is in the R&D phase, VERITAS should remain the premier VHE facility inthe Northern Hemisphere through the length of this proposal. New analysis techniques will also result in a factor of ~2 sensitivity increase for data already taken [79].

Figure 1: The VERITAS array of imaging atmospheric-Chereknov telescopes at FLWO in AZ.

3. Moonlight Observation Capabilities: SAO’s previously funded base research program

Data taking during periods of moonlight is not universal to all VHE observatories. However, several methods that enable VERITAS to operate during all levels of moonlight were developed. These methods include running with higher pixel-level triggers, running with reduced pixel gains, and/or running with custom filters that preferentially pass the UV/blue Cherenkov light. SAO led the deployment, testing, performance evaluation, and systematic-error studies of these methods, and the development of their user protocol. This development required special simulations, calibrations, etc. These new methods enable VERITAS to run every night of the year, albeit with reduced, low-energy sensitivity. VERITAS can now respond to transient phenomena accessible to no other ACTs (e.g., during the full moon). In addition, the observation yield of VERITAS is increased by 60% from ~850 hours per year to ~1350 hours. Moonlight data-taking is now routine and these data are both scientifically useful and regularly published [69]. Clearly the project’s capabilities are increased (e.g. the time available for indirect dark matter searches has ~doubled), as promised in our previous DOE research grant “Expanding the Discovery Potential of VERITAS via Moonlight Observations.”

4. VERITAS Science Introduction

VERITAS seeks to both identify new sources of VHE γ-rays, and to perform in-depth studies of the known sources to better understand their underlying fundamental processes. Since VHE γ-ray sources emit radiation over ~20 orders of magnitude in energy, these studies often involve collaboration with experiments at lower energies (e.g., radio, optical, X-ray, and MeV-GeV γ-ray). Currently >160 VHE γ-ray sources are known [80]. The primary targets of VERITAS Galactic observations are supernova remnants, pulsars, binary systems, and known VHE sources whose (likely Galactic) classification is unknown. The extragalactic targets observed by VERITAS include active galactic nuclei, radio galaxies, starburst galaxies, galaxy clusters and gamma-ray bursts. Local Group galaxies, the Galactic Center and dwarf galaxies are targets of VERITAS observations focused on the indirect detection of dark matter. The results of these studies often have broad implications beyond the physics of the objects. Other topics addressed include fundamental physics (e.g., the energy dependence of the speed of light), the origin of cosmic rays, cosmology and the nature of dark matter. The VHE observation programs also have multi-messenger implications; e.g. for wide-acceptance particle telescopes such as IceCube (neutrinos, [81]) and the Pierre Auger Observatory (UHE cosmic rays, [82]). The emission from objects detected by the Fermi γ-ray Space Telescope [83] extends from ~300 MeV to ~300 GeV, overlapping the >60 GeV regime covered by VERITAS. Results from both experiments are highly complementary, and both benefit from the simultaneous coverage of the entire γ-ray spectra of non-thermal emitters. In particular, the 105 times larger effective area of VERITAS enables time-resolved measurements of flux and spectral variability for transient sources not possible with Fermi alone, and continuation of the γ-ray spectra to TeV energies is critical for measuring the extragalactic background light and intergalactic magnetic field. Such a unique MeV-TeV viewing window is unprecedented, and it may be decades before another such opportunity due to the limited lifetime of Fermi. In addition new facilities in the X-ray (NuStar [84], Astro-H [85]) and VHE bands (HESS-II [86], HAWC [76]) have, or will soon, come online providing new opportunities. VERITAS is already following-up on new targets identified by HAWC, which are communicated under a formal MoU. The LHC is providing new constraints that guide the dark-matter search strategy. Indeed the growing number of upper limits on physics beyond the standard mode (up to TeV energies) may already suggest the scale for new physics is beyond the reach of terrestrial experiments, requiring TeV γ-ray observations.

5. VERITAS Science Plans for 2016-19

In 2014, the VERITAS Sceince Board (VSB) organized the development a long-term observation program assuming operation through at least 2019. The plan includes a de-scope option of operation through 2017, in case future funding cannot be secured. This long-term plan (LTP) was developed by members of VERITAS, with input from the scientific community, and was written by the PI. It was strongly endorsed by the VERITAS External Science Advisory Committee (ESAC), and delivered to the three agencies that fund VERITAS site operations.

The VERITAS science program will continue to follow the guidelines outlined in this long-term plan, which is based on the projected “ultimate” accomplishments of VERITAS. In other words, this is the legacy program for VERITAS. During the planning, strong consideration was given to creating a balanced program in fundamental physics and astrophysics. The plan is based on 4 major scientific themes: Particle Physics and Fundamental Laws, Cosmology, Black Holes, and Galactic Tevatrons / Pevatrons; excerpts from the three themes most relevant to SAO pursuits are described below. Under the plan ~70% of VERITAS observations are now pre-allocated to legacy programs, including ~20% of the entire budget for indirect Dark Matter detection. For the remaining time, proposals for VERITAS observations are organized annually by Science Working Groups (SWGs), and are evaluated in an open competition. SAO scientists have historically led two of the four major non-technical SWGs: the Blazar SWG and the Dark Matter, Astroparticle, Extragalactic Non-blazar (DM-AsPEN) SWG.

5.1 Particle Physics and Fundamental Laws: The origin of dark matter (DM) is one of the most compelling mysteries facing 21st century physics and astronomy. In the standard scenario, where DM is comprised of weakly interacting massive particles (WIMPs), DM annihilation in astrophysical regions with a large concentration of DM would produce a clearly recognizable signal of VHE γ-rays. This indirect detection technique provides an important complement to direct-detection experiments underground and to searches for new particle physics at the LHC. Even if the LHC detects evidence for super-symmetry, only astrophysical VHE γ-ray measurements can reveal the distribution of dark matter in haloes. In addition, the characteristic shape of the VHE spectrum provides crucial information on the particle mass and branching ratios beyond the capabilities of direct-detection experiments and the LHC [87]. As one of the world’s premier VHE detectors, VERITAS will make the most-sensitive searches for dark matter in the mass region above 200 GeV. Since the predicted signal for DM sources outside the Galactic Center are predicted to be weak, long, dedicated exposures on the most promising dark matter targets are required. To ensure a lasting legacy, a comprehensive survey of all nearby Northern dwarf spheroidal galaxies (the most promising targets) will be performed.

In addition to dark matter searches, VERITAS has significant sensitivity to other aspects of particle physics, including searches for axion-like particles and primordial black holes that could be created in the early universe. The detection of fast VHE γ-ray flares from a GRB or a blazar will allow VERITAS to make the most sensitive tests to date of Lorentz-invariance violation, especially for quadratic (~E2) and higher-order terms in the electromagnetic dispersion relation.

5.2 Cosmology: VHE observations of blazars, and possibly GRBs, are useful for cosmological measurements. Their γ-ray spectra are modified by interactions with intergalactic radiation fields through pair-production (γγ to e+e-) and subsequent cascade processes, and thus contain imprints of the extragalactic background light (EBL) and the intergalactic magnetic field (IGMF).

The EBL is the combined flux of all extragalactic sources integrated over the entire history of the universe, and its calorimetric information carries unique information regarding the epochs of galaxy formation and the history of galaxy evolution. Detailed measurements by VERITAS of a number of blazars at different redshifts enable the first reliable determination of the density and spectrum of the EBL in the optical-IR band, which is also sensitive to the dark matter content of the universe and its evolution. Since VERITAS has a catalog of 32 blazars extending to redshifts of ~1, a multi-year observing program makes an EBL-measurement possible [88].

Currently there are only weak constraints on the IGMF and no direct measurements. The γ-ray beams from AGN / GRBs provide a measurement of the IGMF strength not accessible to other techniques. A measurement of the IGMF would have profound cosmological implications because it implies a primordial field produced in the early universe. Even a good constraint on the IGMF has high value, and recent work [89,90] combining Fermi-LAT and VHE blazar measurements already provide the first reliable lower bounds on the IGMF. Using multi-year blazar studies, VERITAS will use three different observables to constrain or determine the IGMF.

5.3 Black Holes: Active galactic nuclei (AGN) are believed to be powered by the accretion of matter onto a super-massive (106-9 solar mass) black hole (SMBH). About 10% of all AGN have collimated, relativistic outflows of particles (jets), and blazars (the most numerous, identified source of VHE γ-rays) are a class of AGN where one jet is pointed directly towards Earth. The VHE γ-rays are believed to be created by these jets in a compact region near the SMBH event horizon. Thus VERITAS studies of AGN probe the innermost regions of these powerful particle accelerators, where the bulk of their luminosity is emitted, and are critical to understanding the process of astrophysical jet formation and evolution, the effect of the jet on the surrounding environment, as well as the process of matter accretion and magneto-hydrodynamics in the strong-gravity region near the central SMBH. Blazars are among the most variable objects in the universe, and the brightest and fastest variability is observed in the VHE band. During flaring episodes, it is possible for VERITAS to rapidly generate unprecedented statistics, particularly at >TeV energies, which when combined with multi-wavelength data (radio, optical, X-ray and Fermi) will conclusively address issues in source modeling, the EBL, the IGMF, and the origin of ultra-high-energy cosmic rays. Observing VHE flares is key for the program, but our goals can be accomplished without them, using deep exposures. Large flares are rare, so multiple years are needed to catch future events, and to acquire these deep data sets. In addition to detailed studies of a few sources for targeted scientific programs, we are amassing enough AGN detections that, for the first time, we will also be able perform population studies. By systematically monitoring all known Northern VHE AGN for the next 4 years we will ensure the best-possible exposure, within reason, exists on all targets for these population studies, and will maximize the likelihood of catching the key flares. These flares will be intensely observed.