VERITAS Pulsar Program Overview

• The VERITAS Pulsar Program is a coordinated observational initiative using atmospheric Cherenkov telescopes to detect very-high-energy gamma rays from pulsars and their environments.
• Instrumental upgrades such as improved trigger systems and relocated telescopes have reduced detection times and lowered energy thresholds to enhance sensitivity.
• The detection of pulsed emission from the Crab pulsar above 100 GeV challenges conventional curvature radiation models and constrains the geometry of the emission region.

Searching arXiv for VERITAS pulsar program papers to ground the article in the relevant literature. The VERITAS Pulsar Program is the set of observational and analysis efforts within the Very Energetic Radiation Imaging Telescope Array System directed toward pulsars, pulsed very-high-energy (VHE; $E>100$ GeV) gamma-ray emission, and related systems such as pulsar wind nebulae and gamma-ray binaries. Within the published record, the program is defined above all by the first definitive detection of pulsed VHE gamma-ray emission from the Crab pulsar, by systematic archival searches for comparable emission from other northern pulsars, and by instrumentation upgrades that lowered the energy threshold and improved sensitivity for faint pulsed signals [(McCann, 2011); (Archer, 2015); (Ong, 2013)]. Its scientific scope also includes constraints on magnetospheric emission models, searches for correlations with giant radio pulses, Lorentz invariance violation tests using phase alignment, and targeted observations of specific systems such as Geminga, PSR J1023+0038, CTA 1, and PSR J2032+4127 [(Zitzer, 2012); (Richards, 2015); (Park, 2012); (Richards, 2019)].

1. Program definition and instrumental setting

VERITAS is a major ground-based detector of very high energy gamma rays and cosmic rays, consisting of an array of four 12 m-diameter atmospheric Cherenkov telescopes that has been fully operational since September 2007 (Ong, 2013). In the pulsar context, the array is described as sensitive to gamma rays from $\sim 100$ GeV to 30 TeV, with later summaries quoting an energy range of $0.085$ to $>30$ TeV, angular resolution of $0.1^\circ$ at 68% containment, pointing accuracy $<50$ arcseconds, and a wide field of view of $\sim 3.5^\circ$ [(Holder et al., 2011); (Richards, 2019)].

The program operates within a broader Galactic science portfolio that includes pulsar wind nebulae, supernova remnants, and binary systems, but its distinctive objective is the search for and characterization of pulsed gamma-ray emission above 100 GeV [(Ong, 2013); (Richards, 2019)]. A recurrent organizing principle in target selection is the observability metric $\dot{E}/d^2$, where $\dot{E}$ is the spin-down luminosity and $d$ is distance; northern-hemisphere pulsars with high $\sim 100$0 were prioritized, and many were also observed incidentally in fields centered on pulsar wind nebulae or supernova remnants (Archer, 2015, Richards, 2019).

Two instrumental upgrades shaped the program’s capabilities. In 2009, relocation of one telescope improved array geometry and optical point spread function, and the needed observation time for a 1% Crab flux detection dropped from 50 to $\sim 100$1 hours (Ong, 2013). A further upgrade completed in summer 2012 introduced a new Level 2 trigger system and replaced all photomultiplier tubes with high quantum efficiency models, reducing the threshold from $\sim 100$2 GeV to $\sim 100$3 GeV, increasing the all-events trigger rate by 60%, and projecting the time to detect a 1% Crab flux source at $\sim 100$4 hours (Ong, 2013). This directly expanded the range over which pulsed signals could be sought.

2. Detection of the Crab pulsar above 100 GeV

The central result of the program is the detection of pulsed gamma-ray emission from the Crab pulsar above 100 GeV (McCann, 2011). The original VERITAS data set comprised 107 hours of observations, or 97 hours dead-time corrected, and yielded a $\sim 100$5 detection of pulsed emission above 100 GeV (McCann, 2011). Subsequent reports describe the detection as high significance, citing $\sim 100$6 in an initial analysis and $\sim 100$7 with further data, while an updated 130-hour sample is summarized as containing $\sim 100$8 pulsed excess events [(Zitzer, 2012); (Ong, 2013)]. The 2015 update expanded the exposure to 194 hours and reported a $\sim 100$9 detection significance for the combined P1 and P2 signal (Nguyen, 2015).

Phase-resolved analysis identified two sharp peaks in the phaseogram, the main pulse P1 and interpulse P2, at phases 0.0 and 0.4, matching the lower-energy pulse structure [(McCann, 2011); (Nguyen, 2015)]. In the updated analysis, the signal windows were P1: phases $0.085$0 to $0.085$1, P2: phases $0.085$2 to $0.085$3, and the background region: $0.085$4 to $0.085$5 (Nguyen, 2015). The pulse profile above 120 GeV is narrower than at 100 MeV by a factor of $0.085$6–$0.085$7, with measured full widths at half maximum $0.085$8 for P1 and $0.085$9 for P2 (McCann, 2011). The P2/P1 amplitude ratio above 120 GeV is $>30$0, so P2 becomes dominant at VHE [(McCann, 2011); (Zitzer, 2012)].

This detection is repeatedly characterized as the first unambiguous observation of pulsed gamma rays above 100 GeV from any pulsar [(McCann, 2011); (Holder et al., 2011)]. It also established the Crab pulsar as, at that stage, the only pulsar robustly detected in the VHE band by VERITAS and other Cherenkov arrays [(Holder et al., 2011); (Nguyen, 2015)]. Later programmatic papers retain the same benchmark status, noting that the Crab remains the only pulsar with detected pulsed emission above 100 GeV in the VERITAS archival analyses of that period (Nguyen, 2015, Richards, 2015, Archer, 2015).

3. Spectral characterization and implications for emission physics

The high-energy Crab spectrum is the primary empirical basis for the program’s theoretical impact. Between 100 GeV and 400 GeV, the VERITAS pulsed emission spectrum is well fitted by a simple power law of the form

$>30$1

with $>30$2 and $>30$3 in the 2011 analysis (McCann, 2011). A later formulation above 100 GeV quoted $>30$4 and $>30$5, with an analysis threshold of 120 GeV (Zitzer, 2012). In the 194-hour update, the phase-averaged differential energy spectrum was again fit by a power law,

$>30$6

with $>30$7 and $>30$8 (Nguyen, 2015).

When combined with Fermi-LAT data from 100 MeV to 400 GeV, the Crab pulsar spectrum is strongly preferred to follow a broken power law rather than a power law with an exponential cutoff [(McCann, 2011); (Zitzer, 2012)]. The broken power-law fit is

$>30$9

with $0.1^\circ$0, $0.1^\circ$1 GeV, $0.1^\circ$2, and $0.1^\circ$3 (McCann, 2011). The broken power law yields $0.1^\circ$4 for 15 degrees of freedom, whereas a power law with exponential cutoff gives $0.1^\circ$5 for 16 degrees of freedom and fit probability $0.1^\circ$6, excluding the exponential cutoff description [(McCann, 2011); (Zitzer, 2012)].

The consequence stated throughout the literature is that curvature radiation cannot be the dominant gamma-ray-producing mechanism above 100 GeV [(McCann, 2011); (Zitzer, 2012); (Nguyen, 2015)]. One formulation uses the curvature-radiation break estimate

$0.1^\circ$7

with $0.1^\circ$8 and $0.1^\circ$9 the curvature radius in units of the light-cylinder radius, to emphasize that even favorable parameters predict a sharp cutoff above a few tens of GeV (McCann, 2011). The observational absence of such a cutoff therefore requires either a different emission mechanism at all energies or a second component that becomes dominant above the break; inverse-Compton scattering, synchrotron self-Compton processes, and outer-gap or beyond-light-cylinder scenarios are explicitly discussed in the cited papers [(McCann, 2011); (Nguyen, 2015); (Richards, 2015)]. The same data are also taken to imply that the emission region must be far out in the magnetosphere, with one lower bound placing it beyond 10 stellar radii from the neutron star and later summaries locating it close to or even beyond the light cylinder [(McCann, 2011); (Nguyen, 2015)].

No significant detection above 400 GeV was obtained in the 194-hour data set: the combined P1+P2 excess above 400 GeV is only $<50$0, with no significant excess in P1 and any pulse-like excess localized to P2 (Nguyen, 2015). This non-detection did not change the conclusion that the spectrum extends to at least 400 GeV, but it limited the strength of any claim regarding emission toward 1 TeV in that data release (Nguyen, 2015).

4. Timing analysis, correlation studies, and fundamental-physics applications

The program’s timing methodology is phase resolved. Event arrival times are barycentered and folded using precise pulsar ephemerides; for different analyses, contemporaneous timing solutions, Jodrell Bank ephemerides including glitch corrections, and software such as Tempo2 and VEGAS are explicitly referenced [(Holder et al., 2011); (Richards, 2015); (Nguyen, 2015); (Wong et al., 23 Sep 2025)]. Statistical assessments of pulsed emission employ the H-Test, the Li and Ma method, and in broader survey work also $<50$1 and Helene-type upper limits (Nguyen, 2015, Richards, 2015, Archer, 2015, Richards, 2019).

For archival pulsar searches, phase gates are defined using Fermi-LAT pulse profiles. An algorithm, with visual inspection and multi-pass checks for complex pulse shapes, optimizes the placement of signal and background gates for each pulsar (Archer, 2015). Background estimation uses the reflected-region method, with

$<50$2

and significance calculated from the Li and Ma formula, Equation (17) (Archer, 2015). Event selections were optimized for sources at 1% of the Crab Nebula flux, with typical energy threshold ranging from $<50$3 GeV upwards and best sensitivity above $<50$4–200 GeV (Archer, 2015).

The Crab data were also used to search for gamma-ray enhancement correlated with giant radio pulses. In one campaign, VERITAS performed 11.6 hours of simultaneous observations with the Green Bank Telescope, recording over 15,300 overlapping giant radio pulses; no significant enhancement in the gamma-ray signal was found (Zitzer, 2012). Another summary of the same campaign states that 15,366 giant radio pulses at 8.9 GHz were recorded, each at least $<50$5 brighter than the average pulse, and that no significant gamma-ray enhancement was seen in any of 72 search windows spanning 1 to 2187 pulsar rotations (McCann, 2013). The upper limits constrain any gamma-ray enhancement associated with giant radio pulses to less than 10 times the average pulsed flux on single-pulse time scales, and for the widest windows to below $<50$6–$<50$7 times the average flux (Zitzer, 2012).

The phase alignment of the VHE pulses with lower-energy pulses also enabled Lorentz invariance violation tests. No measurable time delay was found between the VERITAS $<50$8 GeV pulse profile and the Fermi-LAT $<50$9 MeV profile, and the 95% upper limit on the time difference was $\sim 3.5^\circ$0 ms in one report and 100 $\sim 3.5^\circ$1s in a later preliminary treatment using phase uncertainty of $\sim 3.5^\circ$2 or $\sim 3.5^\circ$3 $\sim 3.5^\circ$4s as the dominant error [(Zitzer, 2012); (McCann, 2013)]. For a linear Lorentz invariance violation scenario, this gives a constraint of approximately $\sim 3.5^\circ$5 GeV on the effective quantum-gravity scale, and the later summary also quotes $\sim 3.5^\circ$6 GeV for the quadratic case [(Zitzer, 2012); (McCann, 2013)]. The papers emphasize that these limits are less stringent than those derived from gamma-ray bursts, but notable because they use periodic, persistent sources (Zitzer, 2012).

5. Archival survey and targeted non-detections beyond the Crab

After the Crab discovery, the program expanded into a systematic search for pulsed VHE emission from other pulsars. One archival survey reports that VERITAS had observed the locations of 19 known pulsars since 2007, 11 of them with more than 20 hours of exposure, accumulating more than 700 hours of high-quality four-telescope data (Archer, 2015). The sample included the top 10 northern-hemisphere pulsars ranked by $\sim 3.5^\circ$7, as well as some millisecond pulsars (Archer, 2015). The principal result was null: apart from the Crab, no evidence for pulsed VHE emission above 100 GeV was found (Archer, 2015).

A related overview of young northern pulsars summarized a sample of 13 gamma-ray pulsars observed in archival data and likewise reported no pulsed VHE detections; in many cases the upper limits reached below 1% of the Crab Nebula flux and below the level of the Crab pulsar’s VHE pulsed emission (Richards, 2019). The same papers caution against a simple extrapolation of Fermi-LAT spectral energy distributions through the GeV break into the VHE regime, since for most targets the extrapolated fluxes would not be detectable unless a new spectral component were present (Archer, 2015). This is used to motivate two competing interpretations already explicit in the data: either Crab-like VHE emission is rare, or it requires very specific source conditions (Archer, 2015, Richards, 2015).

Geminga and PSR J1023+0038 illustrate the targeted null-result component of the program. For Geminga, VERITAS obtained 71.6 hours of high-quality data, defined phase regions from the Fermi-LAT pulse peaks, and found no significant signal, with H-test $\sim 3.5^\circ$8 and $\sim 3.5^\circ$9 (Richards, 2015). The 95% confidence upper limits above 135 GeV are $\dot{E}/d^2$0 for P1 and $\dot{E}/d^2$1 for P2 (Richards, 2015). These are described as among the most stringent for any pulsar except the Crab, and as consistent either with a strong exponential cutoff at a few GeV or with any VHE component being much fainter than in the Crab (Richards, 2015).

For the transitional binary pulsar PSR J1023+0038, VERITAS observed 18.1 hours in the radio millisecond-pulsar state and 8.2 hours in the accretion/LMXB state, finding no pulsed or steady emission in either state (Richards, 2015). The H-statistics are 0.28 and 0.18, respectively; pulsed flux upper limits above 166 GeV and steady-emission upper limits above 300 GeV are reported for both source states (Richards, 2015). These constitute the first ground-based VHE constraints on that system (Richards, 2015).

Taken together, these null results are presented not as anomalies but as model constraints. They limit the general applicability of Crab-like power-law tails, inverse-Compton scenarios, or other non-curvature processes across the pulsar population, while remaining compatible with exponential-cutoff behavior in most known gamma-ray pulsars (Archer, 2015, Richards, 2015).

6. Associated systems: pulsar wind nebulae and binaries in the program

Although the program’s defining theme is pulsed emission, several papers place pulsar wind nebulae and binary systems within the same observational framework. CTA 1 is a composite supernova remnant containing the radio-quiet Fermi pulsar PSR J0007+7303, and VERITAS observations over 2010–2012 yielded about 41 hours of good-quality data and a $\dot{E}/d^2$2 detection of extended TeV emission designated VER J0006+729 (Park, 2012). The centroid is at RA $\dot{E}/d^2$3, Dec $\dot{E}/d^2$4 (J2000), about 5 arcminutes from the pulsar; the source has $\dot{E}/d^2$5 angular extension $\dot{E}/d^2$6, an integral flux above 1 TeV of approximately $\dot{E}/d^2$7, about 4% of the Crab Nebula flux above 1 TeV, and spectral index $\dot{E}/d^2$8 (Park, 2012). The morphology coincides with the non-thermal X-ray pulsar wind nebula rather than the radio shell, and the interpretation favored in the paper is inverse-Compton emission from PWN electrons rather than direct magnetospheric pulsar emission (Park, 2012).

Program summaries also identify CTA 1 and CTB 87 as examples where VERITAS discovered or characterized VHE gamma-ray emission from nebulae surrounding powerful pulsars, helping to elucidate particle acceleration and emission in these environments (Ong, 2013). This indicates that the term “pulsar program” in VERITAS usage includes both pulsed magnetospheric searches and studies of the nebular environments of energetic pulsars.

A separate branch of the program concerns pulsar binaries. PSR J2032+4127/MT91 213, a $\dot{E}/d^2$9-year-period system containing a young energetic gamma-ray pulsar and a 15 $\dot{E}$0 Be star, was monitored around periastron in late 2017 (Richards, 2019). VERITAS and MAGIC detected an increase in VHE flux beginning in September 2017, coincident with the periastron approach, and the source was identified as a new, variable, point-like gamma-ray source associated with the binary (Richards, 2019). The papers interpret this as confirmation that PSR J2032+4127/MT91 213 is a gamma-ray binary and note that the observed time evolution was poorly correlated with theoretical models, implying significant complexity in geometry and wind interaction (Richards, 2019).

7. Evolution toward long-baseline TeV searches

Later work extends the program from VHE into the explicit TeV and multi-TeV domain. A 2025 overview reports nearly two decades of VERITAS data, nearly 25,000 hours on northern-hemisphere pulsars with some double count due to overlapping fields of view, and high-energy analysis techniques optimized for emission up to 100 TeV (Wong et al., 23 Sep 2025). The Crab pulsar data set had grown to 329 hours from 2007–2025, and the analysis used phase-resolved background separation, Li and Ma significances, and relaxed image-loss cuts to recover high-energy events whose images would otherwise be truncated on small cameras (Wong et al., 23 Sep 2025).

In that study, the Crab interpulse P2 was detected at $\dot{E}$1 using moderate cuts, with spectral index $\dot{E}$2 and normalization $\dot{E}$3 at 0.15 TeV (Wong et al., 23 Sep 2025). A new spectral point at 781 GeV had significance $\dot{E}$4, and no statistically significant point could be reconstructed above 1 TeV (Wong et al., 23 Sep 2025). This suggests continuity with the earlier VHE results while showing that the significance drops rapidly above $\dot{E}$5 TeV in the VERITAS data set analyzed there.

The same paper applied dual optimization for “Crab-like” soft spectra and “Vela-like” hard spectra to PSR J2229+6114, using 242 hours of exposure (Wong et al., 23 Sep 2025). No pulsed emission was detected. The hard search excluded emission at the absolute Vela flux level at $\dot{E}$6, though not at a Vela-like spectrum scaled by the Fermi-LAT flux ratio, while the soft search excluded both emission at the Crab pulsar flux level and at the Crab-pulsar-scaled level at $\dot{E}$7 at 130 GeV (Wong et al., 23 Sep 2025). The paper explicitly frames these results as guidance for future catalog construction and for next-generation instruments such as CTA and LHAASO (Wong et al., 23 Sep 2025).

A plausible implication is that the VERITAS Pulsar Program evolved from a discovery-oriented VHE search centered on the Crab into a population-constraining enterprise in which deep exposures, dual spectral hypotheses, and null results are all treated as essential data. That interpretation is consistent with the stated goals of determining whether the Crab is unique, constraining the location and mechanism of pulsed gamma-ray production, and using the resulting limits to inform future searches for the next TeV pulsars (Archer, 2015, Richards, 2019, Wong et al., 23 Sep 2025).