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Particle Accelerators in Space: Recent News from VERITAS
Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 306–308 (2019) 20–27 www.elsevier.com/locate/nppp
Particle Accelerators in Space: Recent News from VERITAS T. B. Humensky, for the VERITAS Collaboration Physics Department, Columbia University, New York, NY 10027, USA
Abstract Our universe carries a small but important population of highly energetic denizens: supernova remnants with fast shocks, pulsars with powerful winds, intensely-interacting binary systems built from a compact object and a massive star, relativistic jets launched by supermassive black holes. All of these environments conspire to generate populations of nonthermal particles, and observations of the very high energy (VHE; E > 100 GeV) gamma rays produced by these particles are gradually revealing the methods by which Nature accelerates cosmic rays, as well as the ways in which those cosmic rays escape and diffuse into the interstellar medium. These observations include studies of cosmicray acceleration in the supernova remnants Cassiopeia A and IC 443, follow-up of unidentified HAWC sources, and the remarkable Fall 2017 periastron passage of VER J2032+4127, the 50-year-period binary system containing PSR J2032+4127 and a Be star. Fast TeV gamma-ray flares coincidental with the emergence of superluminal radio knots from the blazar BL Lac can be interpreted in terms of a coherent scenario of jet particle flow and radiation. The recent TeV gamma-ray discovery of the radio galaxy 3C 264 adds a new member to the small population of off-axis jets available for study. Meanwhile, the direct detection for the first time of gravitational wave (GW) transients by Advanced LIGO has motivated searches for their electromagnetic counterparts at all wavelengths. Neutrino astronomy is an emerging area of study in high-energy astrophysics, and astrophysical neutrinos are natural cousins of VHE gamma rays. The VERITAS gamma-ray observatory has an active program of follow-up observations in the directions of potentially astrophysical high-energy neutrinos detected by IceCube, as well as in the direction of GW transients. In this talk, we discuss recent results from the VERITAS galactic, extragalactic, and multi-messenger Follow-up programs.
1. Introduction
2. Galactic Science 2.1. Supernova Remnants
VERITAS is an array of four imaging air Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in southern Arizona, USA [1, 2]. VERITAS has been operating with four telescopes since 2007, with significant upgrades to the array in 2009 and 2012 [2]. VERITAS has an energy threshold for γ-rays at small zenith angles of 80 GeV, an angular resolution below 0.1° for γ-rays above 1 TeV, and the ability to detect a source with 1% of the flux of the Crab Nebula in less than 25 hours [3]. In the following pages, we cover selected recent highlights of VERITAS results in the areas of galactic, extragalactic, and multi-messenger science. https://doi.org/10.1016/j.nuclphysbps.2019.07.003 2405-6014/© 2019 Elsevier B.V. All rights reserved.
One of the historic motivators of γ-ray astronomy has been the search for the sources of galactic cosmic rays (CRs), at least up to the knee of the cosmicray spectrum around 1015 eV (so-called “PeVatrons”). Diffusive shock acceleration provides a natural theoretical mechanism for cosmic-ray acceleration [4, 5]. The forward shocks of supernova remnants (SNRs) are considered likely candidates, but direct evidence of hadronic cosmic-ray acceleration to such energies is difficult to acquire [6]. The young SNR Cassiopeia A, at an age of ∼ 340 years, shows strong non-thermal X-
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ray emission indicating the presence of multi-TeV electrons [7]. VERITAS and Fermi-LAT observations of Cas A, shown in Figure 1, strongly favor a cut-off in the γ-ray emission in the TeV range [8], a result also found recently by MAGIC [9]. Recent modeling of Cas A’s γ-ray emission tends to favor scenarios dominated by a hadronic cosmic-ray component (eg, [10]), though mixed models can work as well (eg, [11]). A forthcoming VERITAS paper will address these issues (Abeysekara et al., in prep).
(a)
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(b)
Figure 2: Gamma-ray maps of IC 443 as measured by Fermi-
LAT (left, E > 5 GeV) and VERITAS (right, E > 200 GeV). Contours tracing molecular gas are shown in yellow (12 CO) and red (HCO+).
Figure 1: Energy spectrum for Cas A as measured by Fermi-
LAT (open circles) and VERITAS (filled circles).
The slow shocks associated with older SNRs may interact with molecular clouds (MCs) in their vicinity; these clouds may even be related to the material from which the progenitor massive star was born. IC 443, the Jellyfish Nebula, is a classic example of a middle-aged (∼ 10 kyr) SNR exhibiting a shock-cloud interaction, with 12 CO maser emission observed at several points and providing a direct tracer of interaction between the SNR shock and the surrounding MCs [12, 13]. Figure 2 shows the γ-ray emission from IC 443, with 5−300 GeV emission as measured by Fermi-LAT shown on the left, and > 200 GeV emission as measured by VERITAS shown on the right. The remarkably similar variation in γ-ray intensity between the two bands argues that the energy spectra are similar all around the SNR, despite strongly varying environmental conditions. This may result from similar cosmic-ray acceleration parameters around the SNR when it was younger, or isotropization of previously accelerated and confined CRs due to streaming instabilities [14, 15, 16], or to the current γ-ray signal being dominated not by CRs created by IC 443, but rather Galactic CRs that have become trapped and reaccelerated at the radiative shocks in IC 443 [17, 18]. Taken together, these and other VERITAS SNR studies strongly support the notion that hadronic cosmic rays
are accelerated in SNRs, but do not demonstrate that SNRs are capable of accelerating CRs to the knee. It may be that the SNRs currently accessible to instruments like VERITAS are at the wrong stage of their evolution to act as PeVatrons, or it may be that the nonlinear diffusive shock acceleration responsible for particle acceleration in SNRs is incapable of reaching the knee in practice. Future observations, including followup by VERITAS of hard-spectrum sources detected by air-shower arrays such as HAWC [19] and a survey of the Galactic Plane by the forthcoming Cherenkov Telescope Array [20], will shed more light on these questions; in the meantime, we have also a candidate PeVatron associated with the diffuse emission in the inner Galaxy to consider [21, 22]. 2.2. HAWC Follow-up HAWC, the High Altitude Water Cherenkov detector, provides a wide-field, high-duty-cycle view of the multi-TeV sky. The 2HWC catalog included 39 sources, 16 of which were more than 1° from any known TeV γ-ray source [23]. Using a mixture of archival and new data totaling 187 hours, VERITAS searched for sources coincident with 13 of these 16 unassociated sources [24]. Upper limits were derived for 12 sources, and one new detection was made: 2HWC J1953+294 = VER J1952+294, which is centered on the pulsar wind nebula (PWN) DA 495, and matches its radio extent, providing a firm association. A Pass-8 analysis of 8.5 years of Fermi-LAT observations set upper limits on all 13 sources observed by VERITAS, but provided a new detection in the GeV band of the known TeV source that is the PWN associated with PSR J1930+1852 (SNR G54.1+0.3). The combined γ-ray SED of SNR G54.1+0.3 is shown in Figure 3; while the HAWC data alone cannot make a strong statement on the multi-TeV spectral shape, the combined SED favors
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Figure 3: Spectral energy distribution for SNR G54.1+0.3,
showing measurements by Fermi-LAT (green), VERITAS (red), and HAWC (blue).
a hypothesis of a power law with an exponential cut-off. 2.3. PSR J2032+4127 Periastron Passage TeV J2032+4130, discovered by HEGRA in 2004 [25], was the first extended TeV γ-ray source, and also the first TeV source for which there was no immediately obvious counterpart at longer wavelengths: the first ”dark accelerator.” It is now thought to most likely be the PWN associated with the Fermi-LAT-discovered pulsar PSR J2032+4127 [26]. This pulsar has become very interesting in its own right, as observations in radio established that it is part of a binary system with the Be star MT91 213 [27]. The binary has a highly elliptical orbit with an eccentricity of 0.95, and a 45-50 year period [28]. The system went through periastron on November 13, 2017 (MJD 58070). VERITAS and MAGIC collaborated together to monitor the system through 2016-2017 and study its variability during this once-in-an-observatory’s lifetime opportunity. Figures 4 and 5 show the light curves for PSR J2032+4127 as measured by VERITAS and MAGIC since spring 2016, along with the X-ray flux as measured by Swift [29]. The rise in flux beginning in September 2017 (approximately MJD 57900) is clear. An interesting dip approaching periastron followed by a second rise after periastron are also seen in γ-rays, and the correlation between X-rays and γ-rays is quite poor compared to model predictions (overlaid) [30], suggesting that the system geometry or timing may not yet be completely understood. The system may also be affected by interactions between the pulsar wind and the Be star’s disk, or it is possible that γ − γ absorption hasn’t been properly accounted for, again because of uncertainties in the system geometry.
Figure 4: Upper panel (left axes) shows the 0.310.0 keV SwiftXRT energy-flux light curve of PSR J2032+4127/MT91 213 for the full data set. Lower panel shows the >200 GeV photonflux light curves from VERITAS (green) and MAGIC (blue). The average flux prior to 2017 is indicated by horizontal solid lines. The solid gray lines (right axes) are the energy-flux light curve predictions from [31] for X-rays and updated predictions from [30] using the parameters from [31] (J. Takata 2018, priv. comm.) for VHE γ-rays, for an inclination angle of 60°. The vertical gray dashed line indicates periastron.
3. Extragalactic Science The TeV active galactic nucleus (AGN) population now includes roughly 48 high-frequency-peaked BL Lacs (HBLs), 10 intermediate or low-frequencypeaked BL Lacs (IBLs/LBLs), seven flat-spectrum radio quasars (FSRQs), and four radio galaxies (AGNs whose jets do not lie along the line of sight) [32]. Theoretical efforts to understand the particle acceleration and emission mechanisms in these objects, and to relate different classes of objects to each other based on geometrical differences, have recently focused on correlations in MWL emission between radio, IR/optical, X-ray, and TeV emission. One promising avenue is the correlation of TeV flares with changes in radio “knots” (distinct emission structure) observed in the jets [33]. Some jets show apparently stationary knots that are believed to be produced by recollimation shocks, some jets show moving knots that are believed to be clumps of ejecta launched from near the central super-massive black hole (SMBH), and some jets show both. Which kinds of knots are present may be related to the magnetic structure of the jets and to the class of AGN. TeV flares may be associated with either the launching of new knots from near the SMBH, or with the passage of moving knots through stationary knots.
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Figure 5: Same as Figure 4, zooming in to the months around periastron.
3.1. BL Lac: Fast Flares BL Lacertae, the AGN eponymous with the BL Lac class, is an IBL at a redshift of 0.07 from which several rapid TeV flares have been observed (eg, [34]). Most recently, VERITAS detected a powerful, fast TeV flare on October 5, 2016 [35], with a rise time of ∼140 minutes and a decay time of ∼36 minutes, as shown in Figure 6 (top), suggesting a characteristic length scale of ∼12 Schwarzschild radii. Interestingly, the observed time structure in this flare is the opposite of the typically expected fast rise/slow decay. The flare may coincide with the ejection of a superluminal radio knot, as indicated in the radio time series shown in Figure 6 (bottom). 3.2. 3C 264: New TeV Radio Galaxy With only four detections to date, radio galaxies are the rarest class of TeV AGN because they do not benefit from beaming effects since their jets are observed from significantly off axis. Ten hours of VERITAS observations of 3C 264 in 2017 led to an interesting excess and follow-up in 2018 that produced a detection [36]. 3C 264 is an FR-I radio galaxy at a redshift of 0.0216 [37], analogous in many ways to M 87 but at six times the distance. It shows a rapidly evolving knot structure and is a hard-spectrum source in the MeV-GeV range, with a spectral index in the 3FHL catalog of 1.65 [38]. With a total of 44 hours observation time, VERITAS detected 3C 264 at the eight-sigma level [36]. The flux is ∼0.5% that of the Crab Nebula and shows some indication of month-scale variability. A major MWL effort, including radio (VLBA), optical (HST, ground-based), X-ray (Chandra, Swift), and Fermi-LAT, found no major activity in the knot sub-structure.
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Figure 6: (top) Light curve of the BL Lac observations. (bottom) VLBA imaging of the BL Lac jet showing the emission of a new knot from the core at a time consistent with the observed VHE flare.
4. Multi-Messenger Science 4.1. Neutrinos Beginning in 2013, IceCube has detected O(10) neutrinos per year that are likely of astrophysical origin [39, 40], covering an energy range from ∼20 TeV to a few PeV [41, 42]. IceCube searches for point sources of astrophysical neutrinos have set limits at the level of 110% of the all-sky astrophysical neutrino flux, suggesting that the number of neutrino sources contributing to the all-sky flux may be quite large (N > 10 − 100) [41]. In collaboration with IceCube and AMON, VERITAS has developed a number of neutrino follow-up programs, covering time scales and latencies ranging from seconds/minutes to days/weeks to months/years [43]. On the longest time scales, these include searches for TeV emission at the positions of archival muon neutrinos that have a high probability (> 50%) of being astrophysical in origin (those with neutrino energies 100 TeV) and good localizations (∼1° or better). Over 40 hours of VERITAS exposure has been dedicated to this program so far; while no VHE γ-ray excesses have been found, limits are being set that are constraining on the distance or spectrum of the source population. Figure 7 shows the combined limits as a function of energy achieved so far by VERITAS, MAGIC, and HESS. On moderate time scales, observations in the direction of neutrino multiplets are made. The first such study was motivated by a neutrino triplet but yielded no significant γ-ray detection [44].
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Figure 7: Range of γ-ray flux upper limits as a function of en-
ergy for a number of IceCube astrophysical neutrino sky locations. The purple dashed line indicates 0.1% of the all-sky astrophysical neutrino flux reported in the 3-year HESE analysis [40]. The gray line indicates an extrapolation of this flux level down to the range of the γ-ray limits for comparison, and the shaded regions indicate the impact on such an extrapolated spectrum if absorption by the EBL were taken into account for sources at redshifts of 0.1 and 1.0. The solid green line and arrows indicate the median flux upper limits for neutrino locations in each energy bin, while the shaded regions indicate the full range of limits (light green) and standard deviation (medium green) for different neutrino locations. Limits vary depending on depth of observation and other factors.
Finally, on the shortest time scales, VERITAS responds to observations of prompt, online alerts from specific IceCube analysis chains that yield candidates of high astrophysical probability and good angular resolution. These include alerts for the extremely high energy (EHE) and the high-energy starting events (HESE) [45]. VERITAS observations of these alerts are given high priority, trumped only by GRB and GW alerts. IceCube began broadcasting alerts in April 2016, and the first follow-up by VERITAS came on April 27, 2016, with a latency of only 112 seconds. 4.2. IceCube-170922A and TXS 0506+056 On September 22, 2017, IceCube sent an alert regarding a neutrino, IC170922, with an energy of ∼ 290 TeV and a probability of astrophysical origin of ∼50% [46]. VERITAS observations began 12.2 hours after the event. No detection was made in 5.5 hours of data collected in the first two weeks after the neutrino, yielding an upper limit at a level of ∼4% of the Crab Nebula flux. With MAGIC reporting a detection during this time frame at a flux lower than the VERITAS limits, and Fermi-LAT reporting an enhanced flux from
Figure 8: Figures reproduced from [47]. (top) Gamma-ray SED
of TXS 0506+056 from Fermi-LAT and VERITAS data for the period MJD 58019-58155. The spectrum as measured by MAGIC within two weeks of the detection of IC 170922A is also shown [46]. The Fermi-LAT 3FGL (purple) and 3FHL (orange) catalog fluxes of the source are also shown, as well as 95% CL upper limits from VERITAS archival observations (black) [47]. Power-law fits, including color bands indicating 68% statistical uncertainties, are shown. (bottom) Power-law fit with an exponential cutoff to the de-absorbed γ-ray SED of TXS 0506+056 using Fermi-LAT data from the period MJD 58019-58155 (solid black line) and MJD 58046-60/5810158114 (dashed black line). De-absorbed flux points are in color, and observed fluxes in gray. The teal dashed line is an extrapolation of the Fermi-LAT fit to VHE that accounts for EBL absorption [49]. The pink band is the VERITAS energyscale systematic uncertainty.
the GeV blazar TXS 0506+056 over a time span including the neutrino’s arrival, VERITAS observations were continued through the fall and winter, eventually yielding a 5.8 σ detection at a location consistent with TXS 0506+056 in 35 hours of good-quality data, and a flux of 1.6% of the Crab Nebula above 110 GeV [47], ∼60% of that reported by MAGIC during its initial detection [48]. The spectrum, fit with a power law over the range 110300 GeV, is very soft with an index of 4.8 ± 1.3stat , but consistent with that reported by MAGIC (3.9 ± 0.4stat ), as shown in Figure 8 (top). To understand what we can learn from these data, Figure 8 (bottom) shows the VERITAS and Fermi-LAT spectra corrected for absorption [49] of γ-rays by the
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extragalactic background light (EBL) during their transit from the redshift of TXS 0506+056, z = 0.34 [50]. The sharp cut-off from below to above 100 GeV cannot be explained by EBL absorption alone. Even after deabsorbing the data, a cut-off remains, and the deabsorbed VHE spectrum is well fit by a power law with exponential cut-off energy of Ecut = 63.5 ± 7.4 GeV. The intrinsically soft spectrum could be explained by either a break in the parent population or, perhaps more naturally, by strong absorption at the source. Such strong γ − γ absorption at the source is likely consistent with the flux of low-energy photons required at the source to produce the observed astrophysical neutrino [48]. One caveat to the above analysis is that the VERITAS and Fermi-LAT spectra are not strictly simultaneous. LAT flux variability has an influence on the sharpness of the cut-off, but selecting LAT data from the periods with the most VERITAS signal (orange points in bottom figure) does not change the result. 4.3. Gamma-Ray Bursts Follow-up of gamma-ray bursts (GRBs) is given top priority by VERITAS. GRB alerts received via GCN are processed by the telescope tracking control software in realtime, and the telescopes repoint once observers acknowledge the alert. GRBs from Swift or Fermi-LAT are observed for 3 hours while they are above 20° elevation; Fermi-GBM bursts are observed for 1 hour due to their poorer localization. The fastest response to date has been 75 seconds from GRB trigger to VERITAS on target. The most recent impactful GRB observation by VERITAS was of GRB 150323A, a Swift-BAT burst at a redshift of 0.6. This burst had an X-ray precursor 140 s before the main peak; the precursor provided the alert. VERITAS reached target 280 s after the precursor, and thus 140 s after the BAT emission peak. No VHE emission was detected; the fluence upper limit set was < 1% of the prompt fluence. This is low enough to constrain emission models even without a detection: the lack of > 100 GeV emission favors an explosion into the stellar wind of a massive progenitor such as a Wolf-Rayet star, or into a low-density interstellar medium for which radiative cooling would be inefficient. VERITAS will continue to follow up GRB alerts at high priority, and with the association of short GRBs with binary neutron star (BNS) mergers, the excellent localization provided by Fermi-LAT or Swift argues for prioritizing them over GW alerts. 4.4. Follow-up of Gravitational Wave Alerts Over the last three years, LIGO (along with Virgo most recently) has detected 10 binary black hole (BBH)
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mergers at high significance [51], along with one BNS merger, GW170817 [52] (see also [53] and other presentations in these proceedings). BNS mergers within the LIGO/Virgo horizon (∼100 Mpc) may be detectable by TeV instruments [54, 55, 56]. The ∼10 deg2 field of view (FoV) provided by VERITAS allows for rapid, sensitive scanning of the O(100) deg2 GW localization region. Upon receipt of a GW alert, the VERITAS followup strategy is to define a list of pointings to cover the provided error regions out to some predefined containment level. The pointing centers are spaced on a 1.83° HEALPIX grid. An ordering is then defined, currently prioritized by observing westward pointings first, but other orderings (eg, based on galaxy counts) are being explored. Each pointing is observed for 5 minutes, providing a sensitivity of ∼50% Crab Nebula flux, sufficient to detect a bright transient while rapidly covering a large region. GW170104 provided the first opportunity to exercise the above scheme, and was the first systematic followup of a GW alert by an IACT array [57]. GW170104 was a 50-Msun BBH merger at a redshift of 0.2 detected by LIGO. The alert was received 6.5 hours after the merger. VERITAS covered 27% of the containment probability with 39 pointings; no signal was detected, but the data were affected by poor weather and the evaluation of limits is in progress. VERITAS was unable to follow up GW170817, the first detection of GWs from a binary neutron star merger, because it occurred during the annual monsoon shutdown. However, VERITAS is prepared to follow up GW alerts from the forthcoming LIGO/Virgo Observing Run 3, beginning in early 2019. 5. Summary and Conclusions TeV observations are revealing the processes of cosmic-ray acceleration and transport on both Galactic and extra-galactic scales. Multi-messenger programs are opening new windows into extreme environments and hold great promise for future discoveries unraveling the nature of the sources producing astrophysical neutrinos, and for understanding the connections between gravitational wave sources and high-energy astrophysics. VERITAS has established a very successful synergy with HAWC, Fermi-LAT at other γ-ray wavelengths, as well as with MAGIC and HESS and with IceCube, AMON, and the LIGO-Virgo Scientific Collaboration amongst other messengers. A VERITAS and FermiLAT follow-up study of 13 out of 16 HAWC unasso-
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ciated sources resulted in new detections of the PWNe SNR G54.1+0.3 and DA 495. For many of the other HAWC sources, the non-detections provide constraints on the source extension and/or spectral shape. The binary system containing PSR J2032+4127 is in an extreme elliptical orbit. VERITAS and MAGIC observed enhanced TeV flux during its periastron approach, taking advantage of a once-in-an-observatoryslifetime opportunity to probe the dynamics of pulsar wind / stellar wind interactions during this event. VERITAS detected a fast TeV γ-ray flare from BL Lac coincident with emergence of a radio knot from the vicinity of the central SMBH, showing again the utility of multiwavelength observations in pinpointing the location of TeV emission in jets. VERITAS also detected the radio galaxy 3C 264 in the TeV band for the first time; no strong activity from the radio knot or core were seen during an intensive MWL campaign. VERITAS welcomes proposals from external collaborators for observing time, with proposals typically due in early September for an observing season that runs mid-September through June (followed by a monsoon shutdown July through early September); contact Science Working Group Coordinators to get involved (see https://veritas.sao.arizona.edu/ for contact information). VERITAS science covers a broad range of topics, including many areas and results - on the Galactic Center, dark matter, and more - not covered here. Acknowledgements This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, and by NSERC in Canada. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. TBH acknowledges the generous support of the National Science Foundation under cooperative agreement PHY-1352567. References [1] J. Holder, et al., The first VERITAS telescope, Astroparticle Physics 25 (2006) 391–401. arXiv:arXiv:astro-ph/0604119, doi:10.1016/j.astropartphys.2006.04.002.
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Nuclear and Particle Physics Proceedings 306–308 (2019) 20–27 www.elsevier.com/locate/nppp
Particle Accelerators in Space: Recent News from VERITAS T. B. Humensky, for the VERITAS Collaboration Physics Department, Columbia University, New York, NY 10027, USA
Abstract Our universe carries a small but important population of highly energetic denizens: supernova remnants with fast shocks, pulsars with powerful winds, intensely-interacting binary systems built from a compact object and a massive star, relativistic jets launched by supermassive black holes. All of these environments conspire to generate populations of nonthermal particles, and observations of the very high energy (VHE; E > 100 GeV) gamma rays produced by these particles are gradually revealing the methods by which Nature accelerates cosmic rays, as well as the ways in which those cosmic rays escape and diffuse into the interstellar medium. These observations include studies of cosmicray acceleration in the supernova remnants Cassiopeia A and IC 443, follow-up of unidentified HAWC sources, and the remarkable Fall 2017 periastron passage of VER J2032+4127, the 50-year-period binary system containing PSR J2032+4127 and a Be star. Fast TeV gamma-ray flares coincidental with the emergence of superluminal radio knots from the blazar BL Lac can be interpreted in terms of a coherent scenario of jet particle flow and radiation. The recent TeV gamma-ray discovery of the radio galaxy 3C 264 adds a new member to the small population of off-axis jets available for study. Meanwhile, the direct detection for the first time of gravitational wave (GW) transients by Advanced LIGO has motivated searches for their electromagnetic counterparts at all wavelengths. Neutrino astronomy is an emerging area of study in high-energy astrophysics, and astrophysical neutrinos are natural cousins of VHE gamma rays. The VERITAS gamma-ray observatory has an active program of follow-up observations in the directions of potentially astrophysical high-energy neutrinos detected by IceCube, as well as in the direction of GW transients. In this talk, we discuss recent results from the VERITAS galactic, extragalactic, and multi-messenger Follow-up programs.
1. Introduction
2. Galactic Science 2.1. Supernova Remnants
VERITAS is an array of four imaging air Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in southern Arizona, USA [1, 2]. VERITAS has been operating with four telescopes since 2007, with significant upgrades to the array in 2009 and 2012 [2]. VERITAS has an energy threshold for γ-rays at small zenith angles of 80 GeV, an angular resolution below 0.1° for γ-rays above 1 TeV, and the ability to detect a source with 1% of the flux of the Crab Nebula in less than 25 hours [3]. In the following pages, we cover selected recent highlights of VERITAS results in the areas of galactic, extragalactic, and multi-messenger science. https://doi.org/10.1016/j.nuclphysbps.2019.07.003 2405-6014/© 2019 Elsevier B.V. All rights reserved.
One of the historic motivators of γ-ray astronomy has been the search for the sources of galactic cosmic rays (CRs), at least up to the knee of the cosmicray spectrum around 1015 eV (so-called “PeVatrons”). Diffusive shock acceleration provides a natural theoretical mechanism for cosmic-ray acceleration [4, 5]. The forward shocks of supernova remnants (SNRs) are considered likely candidates, but direct evidence of hadronic cosmic-ray acceleration to such energies is difficult to acquire [6]. The young SNR Cassiopeia A, at an age of ∼ 340 years, shows strong non-thermal X-
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ray emission indicating the presence of multi-TeV electrons [7]. VERITAS and Fermi-LAT observations of Cas A, shown in Figure 1, strongly favor a cut-off in the γ-ray emission in the TeV range [8], a result also found recently by MAGIC [9]. Recent modeling of Cas A’s γ-ray emission tends to favor scenarios dominated by a hadronic cosmic-ray component (eg, [10]), though mixed models can work as well (eg, [11]). A forthcoming VERITAS paper will address these issues (Abeysekara et al., in prep).
(a)
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(b)
Figure 2: Gamma-ray maps of IC 443 as measured by Fermi-
LAT (left, E > 5 GeV) and VERITAS (right, E > 200 GeV). Contours tracing molecular gas are shown in yellow (12 CO) and red (HCO+).
Figure 1: Energy spectrum for Cas A as measured by Fermi-
LAT (open circles) and VERITAS (filled circles).
The slow shocks associated with older SNRs may interact with molecular clouds (MCs) in their vicinity; these clouds may even be related to the material from which the progenitor massive star was born. IC 443, the Jellyfish Nebula, is a classic example of a middle-aged (∼ 10 kyr) SNR exhibiting a shock-cloud interaction, with 12 CO maser emission observed at several points and providing a direct tracer of interaction between the SNR shock and the surrounding MCs [12, 13]. Figure 2 shows the γ-ray emission from IC 443, with 5−300 GeV emission as measured by Fermi-LAT shown on the left, and > 200 GeV emission as measured by VERITAS shown on the right. The remarkably similar variation in γ-ray intensity between the two bands argues that the energy spectra are similar all around the SNR, despite strongly varying environmental conditions. This may result from similar cosmic-ray acceleration parameters around the SNR when it was younger, or isotropization of previously accelerated and confined CRs due to streaming instabilities [14, 15, 16], or to the current γ-ray signal being dominated not by CRs created by IC 443, but rather Galactic CRs that have become trapped and reaccelerated at the radiative shocks in IC 443 [17, 18]. Taken together, these and other VERITAS SNR studies strongly support the notion that hadronic cosmic rays
are accelerated in SNRs, but do not demonstrate that SNRs are capable of accelerating CRs to the knee. It may be that the SNRs currently accessible to instruments like VERITAS are at the wrong stage of their evolution to act as PeVatrons, or it may be that the nonlinear diffusive shock acceleration responsible for particle acceleration in SNRs is incapable of reaching the knee in practice. Future observations, including followup by VERITAS of hard-spectrum sources detected by air-shower arrays such as HAWC [19] and a survey of the Galactic Plane by the forthcoming Cherenkov Telescope Array [20], will shed more light on these questions; in the meantime, we have also a candidate PeVatron associated with the diffuse emission in the inner Galaxy to consider [21, 22]. 2.2. HAWC Follow-up HAWC, the High Altitude Water Cherenkov detector, provides a wide-field, high-duty-cycle view of the multi-TeV sky. The 2HWC catalog included 39 sources, 16 of which were more than 1° from any known TeV γ-ray source [23]. Using a mixture of archival and new data totaling 187 hours, VERITAS searched for sources coincident with 13 of these 16 unassociated sources [24]. Upper limits were derived for 12 sources, and one new detection was made: 2HWC J1953+294 = VER J1952+294, which is centered on the pulsar wind nebula (PWN) DA 495, and matches its radio extent, providing a firm association. A Pass-8 analysis of 8.5 years of Fermi-LAT observations set upper limits on all 13 sources observed by VERITAS, but provided a new detection in the GeV band of the known TeV source that is the PWN associated with PSR J1930+1852 (SNR G54.1+0.3). The combined γ-ray SED of SNR G54.1+0.3 is shown in Figure 3; while the HAWC data alone cannot make a strong statement on the multi-TeV spectral shape, the combined SED favors
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Figure 3: Spectral energy distribution for SNR G54.1+0.3,
showing measurements by Fermi-LAT (green), VERITAS (red), and HAWC (blue).
a hypothesis of a power law with an exponential cut-off. 2.3. PSR J2032+4127 Periastron Passage TeV J2032+4130, discovered by HEGRA in 2004 [25], was the first extended TeV γ-ray source, and also the first TeV source for which there was no immediately obvious counterpart at longer wavelengths: the first ”dark accelerator.” It is now thought to most likely be the PWN associated with the Fermi-LAT-discovered pulsar PSR J2032+4127 [26]. This pulsar has become very interesting in its own right, as observations in radio established that it is part of a binary system with the Be star MT91 213 [27]. The binary has a highly elliptical orbit with an eccentricity of 0.95, and a 45-50 year period [28]. The system went through periastron on November 13, 2017 (MJD 58070). VERITAS and MAGIC collaborated together to monitor the system through 2016-2017 and study its variability during this once-in-an-observatory’s lifetime opportunity. Figures 4 and 5 show the light curves for PSR J2032+4127 as measured by VERITAS and MAGIC since spring 2016, along with the X-ray flux as measured by Swift [29]. The rise in flux beginning in September 2017 (approximately MJD 57900) is clear. An interesting dip approaching periastron followed by a second rise after periastron are also seen in γ-rays, and the correlation between X-rays and γ-rays is quite poor compared to model predictions (overlaid) [30], suggesting that the system geometry or timing may not yet be completely understood. The system may also be affected by interactions between the pulsar wind and the Be star’s disk, or it is possible that γ − γ absorption hasn’t been properly accounted for, again because of uncertainties in the system geometry.
Figure 4: Upper panel (left axes) shows the 0.310.0 keV SwiftXRT energy-flux light curve of PSR J2032+4127/MT91 213 for the full data set. Lower panel shows the >200 GeV photonflux light curves from VERITAS (green) and MAGIC (blue). The average flux prior to 2017 is indicated by horizontal solid lines. The solid gray lines (right axes) are the energy-flux light curve predictions from [31] for X-rays and updated predictions from [30] using the parameters from [31] (J. Takata 2018, priv. comm.) for VHE γ-rays, for an inclination angle of 60°. The vertical gray dashed line indicates periastron.
3. Extragalactic Science The TeV active galactic nucleus (AGN) population now includes roughly 48 high-frequency-peaked BL Lacs (HBLs), 10 intermediate or low-frequencypeaked BL Lacs (IBLs/LBLs), seven flat-spectrum radio quasars (FSRQs), and four radio galaxies (AGNs whose jets do not lie along the line of sight) [32]. Theoretical efforts to understand the particle acceleration and emission mechanisms in these objects, and to relate different classes of objects to each other based on geometrical differences, have recently focused on correlations in MWL emission between radio, IR/optical, X-ray, and TeV emission. One promising avenue is the correlation of TeV flares with changes in radio “knots” (distinct emission structure) observed in the jets [33]. Some jets show apparently stationary knots that are believed to be produced by recollimation shocks, some jets show moving knots that are believed to be clumps of ejecta launched from near the central super-massive black hole (SMBH), and some jets show both. Which kinds of knots are present may be related to the magnetic structure of the jets and to the class of AGN. TeV flares may be associated with either the launching of new knots from near the SMBH, or with the passage of moving knots through stationary knots.
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Figure 5: Same as Figure 4, zooming in to the months around periastron.
3.1. BL Lac: Fast Flares BL Lacertae, the AGN eponymous with the BL Lac class, is an IBL at a redshift of 0.07 from which several rapid TeV flares have been observed (eg, [34]). Most recently, VERITAS detected a powerful, fast TeV flare on October 5, 2016 [35], with a rise time of ∼140 minutes and a decay time of ∼36 minutes, as shown in Figure 6 (top), suggesting a characteristic length scale of ∼12 Schwarzschild radii. Interestingly, the observed time structure in this flare is the opposite of the typically expected fast rise/slow decay. The flare may coincide with the ejection of a superluminal radio knot, as indicated in the radio time series shown in Figure 6 (bottom). 3.2. 3C 264: New TeV Radio Galaxy With only four detections to date, radio galaxies are the rarest class of TeV AGN because they do not benefit from beaming effects since their jets are observed from significantly off axis. Ten hours of VERITAS observations of 3C 264 in 2017 led to an interesting excess and follow-up in 2018 that produced a detection [36]. 3C 264 is an FR-I radio galaxy at a redshift of 0.0216 [37], analogous in many ways to M 87 but at six times the distance. It shows a rapidly evolving knot structure and is a hard-spectrum source in the MeV-GeV range, with a spectral index in the 3FHL catalog of 1.65 [38]. With a total of 44 hours observation time, VERITAS detected 3C 264 at the eight-sigma level [36]. The flux is ∼0.5% that of the Crab Nebula and shows some indication of month-scale variability. A major MWL effort, including radio (VLBA), optical (HST, ground-based), X-ray (Chandra, Swift), and Fermi-LAT, found no major activity in the knot sub-structure.
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Figure 6: (top) Light curve of the BL Lac observations. (bottom) VLBA imaging of the BL Lac jet showing the emission of a new knot from the core at a time consistent with the observed VHE flare.
4. Multi-Messenger Science 4.1. Neutrinos Beginning in 2013, IceCube has detected O(10) neutrinos per year that are likely of astrophysical origin [39, 40], covering an energy range from ∼20 TeV to a few PeV [41, 42]. IceCube searches for point sources of astrophysical neutrinos have set limits at the level of 110% of the all-sky astrophysical neutrino flux, suggesting that the number of neutrino sources contributing to the all-sky flux may be quite large (N > 10 − 100) [41]. In collaboration with IceCube and AMON, VERITAS has developed a number of neutrino follow-up programs, covering time scales and latencies ranging from seconds/minutes to days/weeks to months/years [43]. On the longest time scales, these include searches for TeV emission at the positions of archival muon neutrinos that have a high probability (> 50%) of being astrophysical in origin (those with neutrino energies 100 TeV) and good localizations (∼1° or better). Over 40 hours of VERITAS exposure has been dedicated to this program so far; while no VHE γ-ray excesses have been found, limits are being set that are constraining on the distance or spectrum of the source population. Figure 7 shows the combined limits as a function of energy achieved so far by VERITAS, MAGIC, and HESS. On moderate time scales, observations in the direction of neutrino multiplets are made. The first such study was motivated by a neutrino triplet but yielded no significant γ-ray detection [44].
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Figure 7: Range of γ-ray flux upper limits as a function of en-
ergy for a number of IceCube astrophysical neutrino sky locations. The purple dashed line indicates 0.1% of the all-sky astrophysical neutrino flux reported in the 3-year HESE analysis [40]. The gray line indicates an extrapolation of this flux level down to the range of the γ-ray limits for comparison, and the shaded regions indicate the impact on such an extrapolated spectrum if absorption by the EBL were taken into account for sources at redshifts of 0.1 and 1.0. The solid green line and arrows indicate the median flux upper limits for neutrino locations in each energy bin, while the shaded regions indicate the full range of limits (light green) and standard deviation (medium green) for different neutrino locations. Limits vary depending on depth of observation and other factors.
Finally, on the shortest time scales, VERITAS responds to observations of prompt, online alerts from specific IceCube analysis chains that yield candidates of high astrophysical probability and good angular resolution. These include alerts for the extremely high energy (EHE) and the high-energy starting events (HESE) [45]. VERITAS observations of these alerts are given high priority, trumped only by GRB and GW alerts. IceCube began broadcasting alerts in April 2016, and the first follow-up by VERITAS came on April 27, 2016, with a latency of only 112 seconds. 4.2. IceCube-170922A and TXS 0506+056 On September 22, 2017, IceCube sent an alert regarding a neutrino, IC170922, with an energy of ∼ 290 TeV and a probability of astrophysical origin of ∼50% [46]. VERITAS observations began 12.2 hours after the event. No detection was made in 5.5 hours of data collected in the first two weeks after the neutrino, yielding an upper limit at a level of ∼4% of the Crab Nebula flux. With MAGIC reporting a detection during this time frame at a flux lower than the VERITAS limits, and Fermi-LAT reporting an enhanced flux from
Figure 8: Figures reproduced from [47]. (top) Gamma-ray SED
of TXS 0506+056 from Fermi-LAT and VERITAS data for the period MJD 58019-58155. The spectrum as measured by MAGIC within two weeks of the detection of IC 170922A is also shown [46]. The Fermi-LAT 3FGL (purple) and 3FHL (orange) catalog fluxes of the source are also shown, as well as 95% CL upper limits from VERITAS archival observations (black) [47]. Power-law fits, including color bands indicating 68% statistical uncertainties, are shown. (bottom) Power-law fit with an exponential cutoff to the de-absorbed γ-ray SED of TXS 0506+056 using Fermi-LAT data from the period MJD 58019-58155 (solid black line) and MJD 58046-60/5810158114 (dashed black line). De-absorbed flux points are in color, and observed fluxes in gray. The teal dashed line is an extrapolation of the Fermi-LAT fit to VHE that accounts for EBL absorption [49]. The pink band is the VERITAS energyscale systematic uncertainty.
the GeV blazar TXS 0506+056 over a time span including the neutrino’s arrival, VERITAS observations were continued through the fall and winter, eventually yielding a 5.8 σ detection at a location consistent with TXS 0506+056 in 35 hours of good-quality data, and a flux of 1.6% of the Crab Nebula above 110 GeV [47], ∼60% of that reported by MAGIC during its initial detection [48]. The spectrum, fit with a power law over the range 110300 GeV, is very soft with an index of 4.8 ± 1.3stat , but consistent with that reported by MAGIC (3.9 ± 0.4stat ), as shown in Figure 8 (top). To understand what we can learn from these data, Figure 8 (bottom) shows the VERITAS and Fermi-LAT spectra corrected for absorption [49] of γ-rays by the
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extragalactic background light (EBL) during their transit from the redshift of TXS 0506+056, z = 0.34 [50]. The sharp cut-off from below to above 100 GeV cannot be explained by EBL absorption alone. Even after deabsorbing the data, a cut-off remains, and the deabsorbed VHE spectrum is well fit by a power law with exponential cut-off energy of Ecut = 63.5 ± 7.4 GeV. The intrinsically soft spectrum could be explained by either a break in the parent population or, perhaps more naturally, by strong absorption at the source. Such strong γ − γ absorption at the source is likely consistent with the flux of low-energy photons required at the source to produce the observed astrophysical neutrino [48]. One caveat to the above analysis is that the VERITAS and Fermi-LAT spectra are not strictly simultaneous. LAT flux variability has an influence on the sharpness of the cut-off, but selecting LAT data from the periods with the most VERITAS signal (orange points in bottom figure) does not change the result. 4.3. Gamma-Ray Bursts Follow-up of gamma-ray bursts (GRBs) is given top priority by VERITAS. GRB alerts received via GCN are processed by the telescope tracking control software in realtime, and the telescopes repoint once observers acknowledge the alert. GRBs from Swift or Fermi-LAT are observed for 3 hours while they are above 20° elevation; Fermi-GBM bursts are observed for 1 hour due to their poorer localization. The fastest response to date has been 75 seconds from GRB trigger to VERITAS on target. The most recent impactful GRB observation by VERITAS was of GRB 150323A, a Swift-BAT burst at a redshift of 0.6. This burst had an X-ray precursor 140 s before the main peak; the precursor provided the alert. VERITAS reached target 280 s after the precursor, and thus 140 s after the BAT emission peak. No VHE emission was detected; the fluence upper limit set was < 1% of the prompt fluence. This is low enough to constrain emission models even without a detection: the lack of > 100 GeV emission favors an explosion into the stellar wind of a massive progenitor such as a Wolf-Rayet star, or into a low-density interstellar medium for which radiative cooling would be inefficient. VERITAS will continue to follow up GRB alerts at high priority, and with the association of short GRBs with binary neutron star (BNS) mergers, the excellent localization provided by Fermi-LAT or Swift argues for prioritizing them over GW alerts. 4.4. Follow-up of Gravitational Wave Alerts Over the last three years, LIGO (along with Virgo most recently) has detected 10 binary black hole (BBH)
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mergers at high significance [51], along with one BNS merger, GW170817 [52] (see also [53] and other presentations in these proceedings). BNS mergers within the LIGO/Virgo horizon (∼100 Mpc) may be detectable by TeV instruments [54, 55, 56]. The ∼10 deg2 field of view (FoV) provided by VERITAS allows for rapid, sensitive scanning of the O(100) deg2 GW localization region. Upon receipt of a GW alert, the VERITAS followup strategy is to define a list of pointings to cover the provided error regions out to some predefined containment level. The pointing centers are spaced on a 1.83° HEALPIX grid. An ordering is then defined, currently prioritized by observing westward pointings first, but other orderings (eg, based on galaxy counts) are being explored. Each pointing is observed for 5 minutes, providing a sensitivity of ∼50% Crab Nebula flux, sufficient to detect a bright transient while rapidly covering a large region. GW170104 provided the first opportunity to exercise the above scheme, and was the first systematic followup of a GW alert by an IACT array [57]. GW170104 was a 50-Msun BBH merger at a redshift of 0.2 detected by LIGO. The alert was received 6.5 hours after the merger. VERITAS covered 27% of the containment probability with 39 pointings; no signal was detected, but the data were affected by poor weather and the evaluation of limits is in progress. VERITAS was unable to follow up GW170817, the first detection of GWs from a binary neutron star merger, because it occurred during the annual monsoon shutdown. However, VERITAS is prepared to follow up GW alerts from the forthcoming LIGO/Virgo Observing Run 3, beginning in early 2019. 5. Summary and Conclusions TeV observations are revealing the processes of cosmic-ray acceleration and transport on both Galactic and extra-galactic scales. Multi-messenger programs are opening new windows into extreme environments and hold great promise for future discoveries unraveling the nature of the sources producing astrophysical neutrinos, and for understanding the connections between gravitational wave sources and high-energy astrophysics. VERITAS has established a very successful synergy with HAWC, Fermi-LAT at other γ-ray wavelengths, as well as with MAGIC and HESS and with IceCube, AMON, and the LIGO-Virgo Scientific Collaboration amongst other messengers. A VERITAS and FermiLAT follow-up study of 13 out of 16 HAWC unasso-
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ciated sources resulted in new detections of the PWNe SNR G54.1+0.3 and DA 495. For many of the other HAWC sources, the non-detections provide constraints on the source extension and/or spectral shape. The binary system containing PSR J2032+4127 is in an extreme elliptical orbit. VERITAS and MAGIC observed enhanced TeV flux during its periastron approach, taking advantage of a once-in-an-observatoryslifetime opportunity to probe the dynamics of pulsar wind / stellar wind interactions during this event. VERITAS detected a fast TeV γ-ray flare from BL Lac coincident with emergence of a radio knot from the vicinity of the central SMBH, showing again the utility of multiwavelength observations in pinpointing the location of TeV emission in jets. VERITAS also detected the radio galaxy 3C 264 in the TeV band for the first time; no strong activity from the radio knot or core were seen during an intensive MWL campaign. VERITAS welcomes proposals from external collaborators for observing time, with proposals typically due in early September for an observing season that runs mid-September through June (followed by a monsoon shutdown July through early September); contact Science Working Group Coordinators to get involved (see https://veritas.sao.arizona.edu/ for contact information). VERITAS science covers a broad range of topics, including many areas and results - on the Galactic Center, dark matter, and more - not covered here. Acknowledgements This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, and by NSERC in Canada. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. TBH acknowledges the generous support of the National Science Foundation under cooperative agreement PHY-1352567. References [1] J. Holder, et al., The first VERITAS telescope, Astroparticle Physics 25 (2006) 391–401. arXiv:arXiv:astro-ph/0604119, doi:10.1016/j.astropartphys.2006.04.002.
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