Some recent highlights from VERITAS

Nuclear Instruments and Methods in Physics Research A 692 (2012) 24–28

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Some recent highlights from VERITAS K. Ragan n Department of Physics, McGill University, Montreal, QC, Canada H3A 2T8

For the VERITAS Collaboration a r t i c l e i n f o

a b s t r a c t

Available online 18 January 2012

VERITAS is an array of four 12-m imaging atmospheric Cherenkov telescopes operated by an international collaboration in Arizona for the study of very high-energy (VHE, E 4 100 GeV) gamma rays. The array has been in regular operation since 2007 and was recently reconfigured to improve the sensitivity and resolution. Here we give a brief update on the performance of the instrument, and highlight several recent results from our program of observation of galactic and extragalactic sources, as well as from our indirect searches for dark-matter annihilation. & 2011 Elsevier B.V. All rights reserved.

Keywords: VERITAS Cherenkov Observations: gamma rays Galactic Extragalactic Dark matter

1. Introduction Very-high energy gamma-ray astronomy has advanced dramatically in recent years. The field now counts more than 120 sources [1]—more than double the number of just five years ago, and a 10-fold increase since the start of the millenium. The majority of these sources fall into just a few categories: active galactic nuclei (AGN), supernovae renmants (SNR), and pulsar wind nebulae (PWN), with smaller numbers of X-ray binaries, starburst galaxies, and massive star clusters. About 25 of them are unidentified, with no unambiguous counterpart at other wavelengths. A good recent review of the field is in Ref. [2]. This rapid progress is primarily due to the advent of new multi-telescope ground-based instruments based on the proven atmospheric Cherenkov technique: HESS (High Energy Stereoscopic System) [3], MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov [Telescope]) [4], and VERITAS (Very Energetic Radiation-Imaging Telescope Array System) [5]. In view of the space limitations of these proceedings, this paper will concentrate on just a few recent results from VERITAS observations.

100 GeV implies charged particle acceleration to even higher energies still. Some of the critical questions are:

 What are the accelerators?  How do they work, and to what energies do they accelerate charged particles?

 What is being accelerated—electrons or protons? In addition to these questions (which are linked to the origin of cosmic rays), the study of VHE gamma rays provides a discovery space for new physics. In many models in which dark matter is in the form of weakly interacting massive particles (WIMPs), these particles decay or annihilate to gamma rays that may be observable in the VHE regime. Astrophysical sources of VHE gamma rays can also be used as probes of fundamental physics. One good example is the use of rapid flaring of AGNs to establish limits on the dispersive nature of the vacuum [6].

3. VERITAS 2. The scientific questions The scientific questions addressed by VHE gamma-ray astronomy pertain primarily to the fact that VHE sources are extreme accelerators. The presence of gamma rays at energies above n

Tel.: þ1 514 398 6518. E-mail address: [email protected] URL: http://veritas.sao.arizona.edu

0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.12.068

VERITAS is an array of atmospheric Cherenkov telescopes (ACT) to detect and study VHE gamma rays. Very-high energy particles striking the upper atmosphere generate showers of secondaries which emit Cherenkov radiation. The Cherenkov light propagates to the ground where it can be detected in focal-plane cameras composed of photomultipliers, effectively imaging the shower of secondaries. The effective area of the telescope is approximately equal to the size of the Cherenkov light pool on the ground—  105 m2 . Imaging the same shower from different

K. Ragan / Nuclear Instruments and Methods in Physics Research A 692 (2012) 24–28

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Fig. 1. A recent view of the VERITAS array.

Fig. 2. The VERITAS sky at the time of this meeting (from TeVCat [1]). The blue shaded region is the part of the sky observable in southern Arizona at small zenith angles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

locations with an array of telescopes provides enhanced sensitivity and resolution compared to a single telescope. VERITAS is composed of four 12-m telescopes situated at the basecamp of the SAO Fred Lawrence Whipple Observatory in Amado, Arizona (elevation  1270 m a.s.l.). The cameras are composed of 499 photomultipliers, equipped with light concentrators, providing a field of view of 3.51. The array is sensitive to gamma-rays between approximately 50 GeV and 30 TeV, with maximum sensitivity between a few hundred GeV and a few TeV. One of the telescopes was relocated in the summer of 2009 to provide a more uniform array spacing; together with a new mirror alignment system [7], the sensitivity of the array was improved by  15%. The current sensitivity of VERITAS allows a 5s detection of the Crab Nebula, the standard reference in the field of VHE gamma rays, in less than 1 min. A source with a flux of 1% of the Crab Nebula can be detected at the same significance in under 30 h of observations. The angular resolution of the array is better than 0.11, and the energy resolution is approximately 20%. Fig. 1 shows a recent view of the array (after the 2009 reconfiguration). At the time of this symposium, VERITAS had 39 firm source detections (see Fig. 2), including 22 extragalactic sources, 12 galactic sources, and five unidentified sources.

4. Recent VERITAS results 4.1. Extragalactic observations Extragalactic objects currently constitute the majority of VERITAS sources. The current 22 detections include 20 blazars and the first detection of a starburst galaxy, M82 [8]. In turn, the

majority of the blazars detected at VHE energies are high-energy peaked BL Lacs (HBLs). The scientific goal of these blazar observations is to understand jet production – and the physics of the production of VHE gamma rays – by the supermassive black holes that drive the AGN. To this end, VERITAS has a regular monitoring campaign on TeV blazars, as well as frequent multi-wavelength campaigns with other observatories across the EM spectrum. Another major goal is to measure the extragalactic infrared background light (EBL) through its effect on blazar spectra. As VHE gamma rays propagate through the EBL – the red-shifted residue of stellar radiation from the early Universe – they are attenuated through direct scattering via gTeV þ gEBL -e þ e . The effects of this attenuation are difficult to deconvolve from the astrophysics of the VHE emission by the source AGN; having a large sample of VHE-emitting AGN at a range of redshifts z will aid in statistical studies of the EBL. Of the 20 VERITAS blazar detections, nine have been VERITAS discoveries in the VHE range. Most are at flux levels of a few percent of the Crab Nebula flux, requiring tens of hours of observation time. Thus, the development of a large catalog of blazars at moderate redshifts will require years of observation by multiple observatories. That work is ongoing by VERITAS and other instruments; the current redshift range of VHE-observed blazars extends to  0:5. An unusual and noteworthy recent result was the observation by VERITAS of the first triple-AGN field. 1ES1218þ304, discovered by the MAGIC collaboration in 2006 [9], is only  1:51 from W Comae, well within our field of view of 3.51. After the VERITAS discovery of VHE emission from W Comae [10], subsequent deep observations revealed another AGN within 11 of 1ES1218þ304: 1ES1215þ303. For a field which had only a handful of confirmed

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K. Ragan / Nuclear Instruments and Methods in Physics Research A 692 (2012) 24–28

observations of the galactic centre, at large zenith angle, were shown separately at this conference and are presented in Ref. [11]. The sky survey was conducted between 2007 and 2009, and comprised an initial 112 h base survey in the region 661 ol o821 and 21o b o51 where l, b are galactic longitude and latitude. The area of deepest coverage, to a level of approximately 3% of the Crab Nebula flux (for point sources above  200 GeV), was 671 ol o821 and 11 ob o 41. Analysis of the sky survey data is ongoing, and results will appear soon.

sources a decade ago, a single observational exposure with three visible sources marks a coming-of-age! A demonstration of the sensitivity of modern ACT arrays is seen in Fig. 3, where we show the light curve for the AGN Mrk421 during late 2009 and early 2010. Regular monitoring of this – and many other – AGNs allows the detection of flaring behavior, which may then trigger deeper observations and multi-wavelength campaigns. In the case of Mrk421, flaring was observed in February 2010 when the flux increased to more than 10 times the flux of the Crab Nebula, triggering extensive deep observations. The flux was intense enough to allow high-significance detection in several tens of seconds; the figure inset shows the flux binned in 2-min time bins. This unprecedented temporal resolution will allow a detailed study of the spectral evolution of the source during the flare, which can inform AGN emission models.

4.2.1. CTA1 CTA1 is a relatively young (age  13 kyr) nearby (distance  1:4 kpc) composite SNR: a radio shell has clearly been detected, as well as X-ray emission. It was detected by EGRET, and has since been detected with a pulsed signal by both Fermi [12] and XMMNewton [13], although no radio or optical pulsed counterpart has been seen. It is a strong candidate for VHE emission, but had not been seen by ground-based gamma-ray detectors. VERITAS observed CTA1 for a total of 26.5 h in the 2010–2011 observing season. The left panel of Fig. 4 shows the resulting significance skymap, with a clear detection of extended emission at the location of CTA1. The detection is at a nominal level of 7:3s,

4.2. Galactic observations In Fig. 2, a concentration of sources can be seen along the galactic plane. VERITAS has performed extensive targeted observations on galactic sources, as well as a sky survey in the Cygnus region. To date, 13 galactic sources of several types have been detected, including SNR, PWN, binary systems, a pulsar, and the galactic centre. VERITAS

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Fig. 3. The observed flux of Mrk421 at energies above 300 GeV in late 2009 and early 2010 (MJD: Modified Julian Date). The inset shows the observations of February 2010, binned in 2-min time intervals.

Fig. 4. Left panel: the significance skymap resulting from 26.5 h of VERITAS observations of CTA1 in 2010–2011. The point spread function of the instrument is shown in the lower right-hand corner. Right panel: the VERITAS excess counts skymap (colors and green contours) with the radio contours (black) and the location of the Fermi pulsar in red. See Ref. [14] for more details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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and 6:3s post-trials. Above 1 TeV, the observed flux is approximately 4% of the Crab Nebula flux. The rightmost panel of Fig. 4 shows the VERITAS excess counts skymap (colors, with green contours) with radio intensity contours overlayed in black and the location of the Fermi pulsar shown by a red circle. There is obviously good agreement between the Fermi and VERITAS results. In addition, the ratio of gamma-ray to X-ray luminosity is approximately in the middle of the known PWN population [14]. The overall picture is consistent with that expected for a young pulsar wind nebula—a new VHE source. 4.2.2. Tycho’s SNR (G120.1þ1.4) Tycho’s supernova remnant is the result of a Type Ia supernova, observed in 1572 and historically important for providing significant impetus to revise ancient models of the heavens. It is now known to be a VHE emitter. VERITAS data shown here comes from 68 h of observations between 2008 and 2010 [15], resulting in a detection of 5:0s significance (post-trials). The flux fits a power law spectrum dN=dE  EG with G ¼ 1:95 70:5 ðstatÞ 7 0:3 (sys). Fig. 5 shows the sky map with VERITAS data (colored) overlayed with Chandra X-ray data and with 12CO emission data. The VERITAS excess has peak significance close to where a molecular cloud may be interacting with the expanding shell of the supernova. We have modelled this VHE emission using both leptonic and hadronic models; the results of these models are shown in the right-hand panel of Fig. 5. Both models describe our data well, but they differ at lower energies where further observations may be able to distinguish between them. The models require minimum magnetic fields well above the expectations from shock compression of the interstellar medium; VHE gamma-ray detection represents additional evidence for magnetic field amplification in this SNR. Enhanced magnetic fields are considered a signature of cosmic ray acceleration, and thus this detection appears to support SNRs as a source of cosmic rays. 4.3. Dark matter searches Dark matter is believed to make up about 23% of the energy density of the Universe, but we know little about it. It must be non-baryonic, cold, heavy, and gravitationally bound. A good dark matter candidate would be a WIMP (weakly interacting massive

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particle), such as a neutralino in most super-symmetric (SUSY) models. Cherenkov arrays like VERITAS are well-suited to search for signals of WIMP self-annihilation in the dark-matter halos of nearby galaxies. Good candidate galaxies would be those with high mass-to-light ratios, such as dwarf spheroidals. VERITAS has observed several dwarf spheroidals, including Ursa Minor, Draco, ¨ Willman 1, Bootes 1, and Segue 1; typical exposures for each are approximately 20 h. No detections have been made, leading to flux upper limits (at the 95% confidence level) of 1–2% of the Crab Nebula flux [16]. Turning the flux limits into a limit on dark matter requires both particle physics and astrophysics modelling. The particle physics factors contain the information about the dark matter particle, including its mass, velocity-weighted annihilation crosssection /svS, and final state branching ratios. The astrophysical factors include the dark matter density squared integrated along the line of sight, and require a modelling of the dark matter density distribution. A commonly used profile for dwarf spheroidals is that of Navarro, Frenk, and White (NFW, [17]). Fig. 6 shows the VERITAS upper limits (95% confidence level) on the velocity-weighted annihilation cross-section /svS as a function of the mass of the dark matter particle, mw , for each of the five dwarf spheroidals that we have observed. We use a composite spectrum with branching ratios to bb of 90% and to t þ t of 10%. The best limits are for Segue 1, which has a slightly deeper exposure and one of the highest astrophysical factors. The grey band in Fig. 6 is the range of values of the annihilation cross-section expected for thermally produced dark matter. In the region mw  500 GeV, Fermi has published similar limits [18].

5. The VERITAS upgrade An upgrade of the VERITAS instrument is underway in order to enhance its sensitivity and flexibility. The upgrade has two major aspects: replacement of the entire complement of photomultipliers with new tubes of higher quantum efficiency, and a replacement of the level-2 pattern trigger with a new FPGA-based device. 5.1. High quantum efficiency photomultipliers VERITAS’s original phototubes were Photonis model XP2970 tubes, with typical quantum efficiency (QE) of 15–20% at 420 nm.

Fig. 5. Left: skymap of VERITAS excess counts (colors) overlayed with Chandra X-ray data (black contours) and 12CO emission (magenta contours), for Tycho’s SNR. Right: hadronic and leptonic spectral models superimposed on the spectral energy distribution. For details of the models, see Ref. [15]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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6. Summary and conclusions This article presents a few of the many recent results from the VERITAS instrument and its diverse program of observations. VERITAS is operating exceptionally well, and an upgrade is in progress that will further improve the sensitivity of the instrument. We continue to discover new VHE sources – both galactic and extragalactic – as well as to perform follow-up observations on known sources. A key part of our program is participation in multi-wavelength studies with other instruments across the EM spectrum.

Acknowledgements

Fig. 6. VERITAS upper limits (95% confidence level) on the velocity-weighted annihilation cross-section /svS as a function of the dark matter particle mass, mw . The grey band is the range of values expected for thermally produced dark matter.

We are currently acquiring new Hamamatsu model R10560-10020 tubes, which boast a peak QE of approximately 35% between 300 nm and 400 nm. The R10560 also has a narrower pulse width than the Photonis tubes by about 30%, allowing a better discrimination between Cherenkov pulses and night-sky background. Phototube delivery and acceptance tests are ongoing, and we expect to install the new phototubes on all four cameras during the summer of 2012. The increased quantum efficiency of the upgraded cameras will be equivalent to a  30% increase in mirror area or a corresponding decrease in observation time for the same flux sensitivity. 5.2. Pattern trigger The current VERITAS level-2 trigger system is a pattern trigger that embodies a nearest-neighbor pixel (phototube) requirement, using technology developed for the Whipple 10-m telescope in the 1990s. The new trigger will use modern fast FPGAs, providing better pixel-to-pixel timing alignment, allowing a narrower time coincidence gate and thus lower backgrounds. Tests have indicated that use of the new system allows the pixel timing to be adjusted to the level of a few nanoseconds, compared to  75 ns in the current design. Ultimately this will allow the use of a coincidence gate as short as 3 ns. The hardware for the new pattern trigger is currently constructed and we have run one of the units parasitically during regular VERITAS observations. We plan to install new triggers on all four telescopes before the 2012 summer shutdown.

The VERITAS Collaboration gratefully acknowledges support from the Office of Science of the US Department of Energy, the US National Science Foundation, and the Smithsonian Institution; from the Natural Sciences and Engineering Research Council (NSERC) in Canada; from Science Foundation Ireland (SFI); and from STFC in the UK. We acknowledge the excellent work of the technical support staff at the FLWO and at the collaborating institutions in the construction and operation of the instrument. This author thanks the organizers of RICAP 2011 for an enjoyable and stimulating workshop. References [1] See, for example, tevcat.uchicago.edu. [2] J.A. Hinton, W. Hoffman, Annual Review of Astronomy and Astrophysics 47 (2009) 523. [3] J.A. Hinton, et al., the HESS Collaboration, New Astronomy Reviews 48 (2004) 331. [4] D. Ferenc, et al., the MAGIC Collaboration, Nuclear Instruments and Methods in Physics Research Section A 553 (2005) 274. [5] J. Holder, et al., the VERITAS Collaboration, Proceedings of the 4th International Symposium on High-Energy Gamma-Ray Astronomy, Heidelberg, Germany, July 2008. [6] J. Bolmont, A. Jacholkowska, Advances in Space Research 47 (2011) 380. [7] A. McCann, et al., Astroparticle Physics 32 (2010) 325. [8] V. Acciari, et al., the VERITAS Collaboration, Nature 462 (2009) 770. [9] J. Albert, et al., the MAGIC Collaboration, The Astrophysical Journal 642 (2006) L119. [10] V. Acciari, et al., the VERITAS Collaboration, The Astrophysical Journal 684 (2008) L73. [11] See presentation by M. Beilicke, these proceedings. [12] A. Abdo, et al., Science 322 (2008) 1218A. [13] P. Caraveo, et al., The Astrophysical Journal 725 (2010) L6. [14] B. McArthur, et al., the VERITAS Collaboration, Proceedings of the Fermi Symposium, Rome, Italy, May 2011. [15] V. Acciari, et al., the VERITAS Collaboration, The Astrophysical Journal 730 (2011) L20. [16] M. Vivier, et al., the VERITAS Collaboration, Proceedings of the Fermi Symposium, Rome, Italy, May 2011. [17] J. Navarro, et al., The Astrophysical Journal 490 (1997) 493. [18] A. Abdo, et al., The Astrophysical Journal 712 (2010) 147.