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A survey of Active Galaxies with the HAWC Gamma-ray Observatory

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Three years of HAWC data detect only Mrk 421 and Mrk 501 among 138 nearby AGN, and push M87 below the Fermi extrapolation.

desk verdict Useful survey result rather than a breakthrough: HAWC confirms the two known TeV blazars and sets a set of new TeV upper limits, with the M87 and IC310 limits being the most valuable, provided you accept the fixed spectral-index caveat. read the letter →

arxiv 1908.06831 v1 pith:UZKWGWDS submitted 2019-08-19 astro-ph.HE

Alberto Carramiñana , Daniel Rosa González , Sara Coutiño de León , Anna Lia Longinotti (for the HAWC Collaboration) This is my paper · ORCID
classification astro-ph.HE
keywords activegalacticnucleivery-high-energygamma-rayastronomyHAWCObservatoryBLLacobjectsM87Mrk421501extragalacticbackgroundlight
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Almost all very-high-energy observations of active galactic nuclei come from short, pointed runs with Cherenkov telescopes, which can catch flares but say little about what the sources do on average. This paper analyzes 1017 days of continuous HAWC exposure of 138 AGN selected from the 3FHL catalog with redshift z <= 0.3, and establishes the first systematic wide-field picture of their persistent TeV emission. The result: only the two nearest BL Lacs, Mrk 421 and Mrk 501, are detected at high significance, while all 136 other AGN are consistent with no steady emission, with 95% confidence upper limits reaching N(>0.5 TeV) around $10^{-12}$ $cm^{-2}$ $s^{-1}$. The most pointed claim concerns the radio galaxy M87, whose HAWC limit lies below the spectral extrapolation of the 3FHL data and below the 2005-2010 pointed-telescope measurements, implying that M87's quiet TeV baseline is fainter than flaring-state observations suggest. If this holds, it means the TeV sky's extragalactic population is dominated by a handful of nearby objects, and that the variability seen by pointed telescopes is not representative of steady output.

What carries the argument

The load-bearing instrument is HAWC's wide-field water-Cherenkov array, whose 300 tanks survey two-thirds of the sky every sidereal day with a median energy response around a few TeV; events are split into nine fhit bins (fhit being the fraction of channels recording a signal, used as an energy proxy), with gamma-hadron separation applied per bin. The analysis uses a standard maximum-likelihood fit of an intrinsic power-law spectrum, (dN/dE)_intr = phi_1 (E/1 TeV)^{-$\alpha$}, with EBL attenuation $e^{{-tau(E,z)}}$ folded in. For upper limits, $\alpha$ is fixed to 2.5 and the normalization phi_1 is fit, with 95% confidence intervals from a Feldman-Cousins prescription; the EBL attenuation is reduced to the exponential approximation N_obs(>0.5 TeV) = $e^{{-z/0.108}}$ N_intr using the EBL model adopted by the paper. This machinery converts HAWC counts into astrophysical flux limits that can be placed on the same plot as 3FHL spectra and pointed-telescope measurements.

What would settle it

Reanalyze the same 1017 days of HAWC data at the position of M87 with simulated source injections at the 3FHL-extrapolated flux: if the likelihood analysis cannot recover such a source at the claimed sensitivity, the upper limit is not a valid test of that extrapolation. Alternatively, a future wide-field TeV exposure with longer integration that measures M87's time-averaged N(>0.5 TeV) above 0.95 x $10^{-12}$ $cm^{-2}$ $s^{-1}$ would directly contradict the reported limit, assuming the source's baseline state is unchanged.

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Extended reading notes

Core claim

The authors report that after 1017 effective days, HAWC detects time-averaged TeV emission from Mrk 421 (significance +45.8) and Mrk 501 (significance +19.9), the only two AGN in the survey with $\sqrt$(TS) >= 5 in all five 3FHL energy bands. For every other source, the fit returns a 95% confidence upper limit on the observed flux above 0.5 TeV, computed for an assumed intrinsic power law dN/dE = phi_1 (E/1 TeV)^{-$\alpha$} with $\alpha$ fixed at 2.5 and with EBL attenuation approximated by N_obs = $e^{{-z/0.108}}$ N_intr; the deepest limits reach N(>0.5 TeV) ~ $10^{-12}$ $cm^{-2}$ $s^{-1}$. The paper's sharpest result concerns the radio galaxies M87 and IC 310: the HAWC limit on M87 is more than a factor of two below the 3FHL spectral extrapolation in the 0.5-2 TeV overlap and an order of magnitude below the 3FHL upper limit, and it is also below the 2005-2010 pointed-telescope measurements of M87 in high and intermediate states. For IC 310, the HAWC upper limit is about a factor of five below the hardest 3FHL point and below the reported flaring-state measurements. The conclusion drawn is that the persistent, time-averaged TeV output of these radio galaxies lies below the levels seen in activity-biased pointed observations.

Load-bearing premise

Every non-detection upper limit assumes the source shines with a simple power-law spectrum of index 2.5 and that EBL absorption follows one exponential scale zh = 0.108; if any of these sources has a curved, harder, or softer spectrum, or if the EBL model is wrong, the limits and the M87 comparison shift.

Editorial extensions

If this is right

  • If the HAWC upper limits are correct, then the vast majority of z <= 0.3 AGN have persistent N(>0.5 TeV) at or below the 10^-12 to 10^-11 cm^-2 s^-1 range, so any future TeV survey that wants to see quiescent AGN must reach below this flux.
  • For M87, the result implies its 0.5-2 TeV baseline is below the 3FHL extrapolated spectrum, meaning the catalog average is likely dominated by activity states rather than steady emission.
  • For IC 310, the hard 3FHL spectrum must soften or switch off in the TeV band during quiescence, since HAWC sees less flux than the hardest 3FHL point.
  • Continued HAWC exposure should either convert the best upper limits (M87, IC 310, 1ES 2344+514) into detections or push them deeper, distinguishing steady emitters from one-sided flaring sources.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A natural extension is to re-fit all 136 non-detections with a free spectral index and the full EBL optical depth rather than the fixed alpha = 2.5 exponential; the paper's own numbers show the redshift scale shifts from zh = 0.095 at alpha = 2.0 to zh = 0.118 at alpha = 3.0, so the tightest limits could move by tens of percent.
  • If the M87 baseline is genuinely this low, then monitoring campaigns that combine wide-field surveys with fast follow-up could measure the duty cycle of radio-galaxy TeV flares, i.e., the fraction of time these sources spend in high states, rather than just their peak outputs.
  • Stacking the non-detected AGN by redshift could turn this survey into a population-level probe of the EBL or of the average TeV luminosity function of nearby AGN, independently of any individual flaring episode.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. This paper reports a survey of active galactic nuclei conducted with the HAWC gamma-ray observatory using 1017 days of data. From a sample of 138 Fermi-LAT 3FHL AGN with redshift z≤0.3, the analysis detects the two nearest BL Lacs, Mrk 421 and Mrk 501, at high significance, and reports 95% confidence-level upper limits for the ten best non-detected candidates under the assumption of a fixed intrinsic spectral index α=2.5. The paper highlights that the HAWC limit on M87 lies below the 3FHL spectral extrapolation and below historical high/intermediate VHE states, and makes analogous statements for IC 310.

Significance. If the results are robust, the survey provides a valuable long-term, time-averaged census of TeV emission from nearby AGN, complementing pointed observations with Cherenkov telescopes. The two detections are extremely significant and consistent with known sources, and the upper limits reach integrated fluxes of order 10^-12 cm^-2 s^-1 above 0.5 TeV. The M87 and IC 310 limits are of particular interest for constraining the quiescent emission of radio galaxies. The analysis uses standard maximum-likelihood methods and the Feldman-Cousins prescription, which are appropriate for this type of search. However, the quantitative cross-comparisons between HAWC limits and Fermi/ACT spectral extrapolations are conditional on the assumed spectral shape, and the paper does not currently demonstrate that key conclusions are robust to that assumption.

major comments (2)
  1. [Section 5.1, Eq. (5.1); Table 1; Section 5.2] The upper limits are computed with the spectral index fixed to α=2.5. The paper's own fits show that this choice has a substantial effect on the integrated flux: for Mrk 501, fitting α=1.96 yields φ1=(6.8±1.1)x10^-12 TeV^-1 cm^-2 s^-1, whereas the α=2.5 fit gives φ1=12.7x10^-12 TeV^-1 cm^-2 s^-1, changing N_obs(>0.5 TeV) by roughly a factor of 1.7. For non-detected sources the true indices are unknown, and the 3FHL catalog provides per-source spectral indices that often differ from 2.5. The highlighted claim in Section 5.2 that the HAWC upper limit for M87 lies more than half below the 3FHL extrapolation is therefore not demonstrated until the M87 limit is recomputed with M87's actual 3FHL spectral index or a range of plausible indices. Please provide this recomputation, state the resulting factor relative to the 3FHL extrapolation, and assess whether the conclusion survives.
  2. [Section 5.1, Eq. (3.1); Table 1] The reported N_obs(>0.5 TeV) limits also depend on the exponential EBL approximation with z_h=0.108, which is calibrated for α=2.5 and E0=0.5 TeV. The paper notes that z_h varies from 0.095 (α=2.0) to 0.118 (α=3.0), which propagates into a small but not entirely negligible uncertainty in the quoted limits. The paper should state explicitly that all integrated-flux limits in Table 1 are defined for α=2.5 only, and ideally quote a systematic range from the α-dependence of the EBL attenuation and the integral conversion.
minor comments (5)
  1. [Section 5.2 and Table 1] Several significances differ between the text and Table 1: the text quotes s=+45.6 for Mrk 421 and s=+20.2 for Mrk 501, while Table 1 lists +45.8 and +19.9. Please make these values consistent.
  2. [Abstract and Section 5.2] The abstract states that the analysis 'sets limits for the rest of the sample', but the paper presents limits only for the ten best non-detected candidates (Table 1). Please clarify whether upper limits were computed for all 138 sources and either show a summary of those limits or revise the wording to specify that limits are reported for the selected best candidates.
  3. [Section 2] The phrase 'with a & 20kHz rate' appears to be a typesetting error; it should read 'with a ≥ 20 kHz rate'.
  4. [Section 5.2] In the M87 discussion, the phrase 'more than half below the LAT extrapolation' would be clearer as 'more than a factor of two below the 3FHL spectral extrapolation', and the paper should specify which spectral model and energy band are used for the comparison.
  5. [References] Reference [13] lists the first author as 'Abeysekara, U.' but the standard author name is 'Abeysekara, A.U.'; please correct this in the bibliography.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the survey limits rest on an external EBL model and an explicitly disclosed fixed spectral index; the two detections are validated against external sources.

full rationale

The paper's central claims are a high-significance detection of Mrk 421 and Mrk 501 and upper limits for the remaining AGN sample. The two detections are validated against external TeV spectra and against Fermi-LAT 3FHL detections, not against quantities derived from the same HAWC fit. The non-detection upper limits assume an intrinsic power-law index alpha=2.5 and use the Dominguez et al. (2011) EBL model, which is an external benchmark rather than a parameter fitted to HAWC data. The exponential attenuation approximation in Eq. (3.1) is computed from that external model, and the redshift cutoff z<=0.3 is chosen consistently with it; this is an internal consistency choice, not a circular derivation. The comparison of the M87 limit with the 3FHL extrapolation depends on the assumed spectral index, but the paper explicitly states that alpha=2.5 is fixed, so this is a disclosed assumption and a robustness caveat rather than a fitted value masquerading as a prediction. Self-citations to HAWC calibration and catalog papers provide instrument response and background characterization, which are necessary inputs but not load-bearing evidence for the astrophysical conclusion. No equation reduces to its own input, and no fitted parameter is renamed as a prediction.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The central claim borrows three external building blocks: the Dominguez EBL model, the Fermi 3FHL catalog spectral models, and the HAWC instrument calibration. None of these are derived in the paper, and the fixed spectral index and energy threshold are choices made by the authors rather than outputs of the data. No new particles, forces, or physical entities are introduced.

free parameters (4)
  • Fixed spectral index for upper limits = 2.5
    Section 5.1 fixes alpha=2.5 when computing the 95% upper limits in Table 1; a different index would change N(>0.5 TeV) and the comparison with Fermi extrapolations.
  • Energy threshold for reported fluxes = 0.5 TeV
    All limits and the EBL scale are quoted above E0=0.5 TeV; the choice reflects HAWC's low-energy response, and the EBL attenuation scale z_h depends strongly on E0.
  • Redshift attenuation scale z_h = 0.108 (E0=0.5 TeV, alpha=2.5)
    Appears in Eq. (3.1) and in the flux-limit formula in Section 5.1; it is taken from the Dominguez EBL model rather than fitted to HAWC data, so it carries the EBL model uncertainty.
  • Candidate flux threshold = 30 mCrab
    Used in Section 4 to select the best candidates from extrapolated Fermi and ACT fluxes; it is an arbitrary indicative threshold that determines which objects appear in the highlighted list.
assumptions (5)
  • domain assumption EBL attenuation follows the Dominguez et al. 2011 model, approximated by N_obs = e^(-z/z_h) N_intr.
    Used in Section 3 to justify the z<=0.3 limit and in Section 5.1 to derive upper limits; the model is external and not verified by HAWC data in this paper.
  • domain assumption Non-detected AGN emit a power-law spectrum in the HAWC band, and alpha=2.5 is representative for all of them.
    Equation (5.1) and the upper-limit run assume this; individual AGN can be softer or harder, and intrinsic cutoffs would change the inferred limits.
  • domain assumption The Fermi-LAT 3FHL catalog spectral models can be extrapolated from the LAT band up to TeV energies.
    Section 4 extrapolates 3FHL fluxes to E>0.5 TeV for candidate selection and for the M87 comparison; LAT power laws may not continue unchanged into the HAWC band.
  • standard math The Feldman-Cousins procedure gives valid 95% confidence upper limits.
    Used in Section 5.1; this is a standard statistical method applied to the HAWC likelihood.
  • domain assumption HAWC's point-spread, energy response, and background model are accurate as calibrated in the cited Crab and 2HWC papers.
    The analysis relies on HAWC internal likelihood software and standard cuts; no event data are given to check systematic biases.

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Pith. "Pith review of A survey of Active Galaxies with the HAWC Gamma-ray Observatory." pith.science (2026). https://pith.science/paper/UZKWGWDS

@misc{pith2026190806831,
  author       = {Pith},
  title        = {Pith review of: A survey of Active Galaxies with the HAWC Gamma-ray Observatory},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/UZKWGWDS}},
  note         = {Machine review of arXiv:1908.06831}
}
abstract

The High Altitude Water Cherenkov Gamma-Ray Observatory has been accumulating a progressively deeper exposure of the TeV sky since its inauguration in March 2015. Located at geographical latitude $+19^\circ$N, HAWC has been able to perform a deep and unbiased survey of two thirds of the sky. We analyzed three years of HAWC data searching for long term persistent emission from a redshift limited ($z\leq 0.3$) sample of active galactic nuclei drawn from the {\it Fermi}-LAT 3FHL catalog. The HAWC dataset confirms the high significance detection of the two nearest BL Lac objects, \mbox{Mrk 421} and \mbox{Mrk 501}, and sets limits for the rest of the sample, down to integrated photon fluxes of order $N(>0.5~{\rm TeV})\lesssim 10^{-12}\,\rm cm^{-2}s^{-1}$. We present and discuss some of the results of this survey, focusing on individual objects of particular interest.

Figures

Figures reproduced from arXiv: 1908.06831 by the authors.

Figure 1
Figure 1. The exponential dependence on redshift of Nobs/Nintr for different values of the starting energy E0 (in color) and the spectral index α, which ranges from 2.0 (lower for each E0) to 3.0 (upper for each E0). The yellow lines correspond to E0 = 0.5TeV which best describes the HAWC data. starburst galaxy (NGC 1068), 6 radiogalaxies, 117 BL Lac objects, 6 flat-spectrum-radio-quasars and 8 blazars of uncertain nature (“b… view at source ↗
Figure 2
Figure 2. High-energy and long term averaged spectra of M 87 (left) and IC 310 (right). The points and darker band represent the Fermi-LAT 3FHL data and spectral fits. The HAWC 95% CL limits are shown as a dotted line for the intrinsic spectra, with the shaded region including the EBL attenuation. 6. Conclusions and summary HAWC detects with high significance the time-averaged TeV emission of Mrk 421 and Mrk 501, the two near… view at source ↗

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