About Me

I am a professor of physics and astronomy at James Madison University in Harrisonburg, Virginia, U.S.A.. My research focuses on astroparticle and high energy astrophysics. I am interested in astrophysical constraints that can be placed on fundamental physics, particularly on quantum gravity theories.

I am originally from the Twin Cities where most of my family still resides. I received my Bachelors, Masters and Ph.D. degrees from the University of Minnesota. Following that, I had a postdoctoral fellowship at the Institute d'Astrophysique de Paris during 1997 - 1998 and an NSF Postoctoral Fellowship (NRC) at the Goddard Space Flight Center in Greenbelt, MD from 1998 - 2000. Prior to my current position, I was a visiting assistant professor at Valparaiso University from 2000 - 2004.

In my spare time you may find me swimming, lifting weights, hiking or biking the roads and trails of the Shenandoah Valley. I am a big movie buff and enjoy both modern films and the classics. I am a fan of most of the Minnesota professional sports teams but my favorite sport is association football (soccer). I especially enjoy the English Premier League and am a supporter of Arsenal FC.


Gamma-Ray Opacity of the Universe

Gamma-rays that originate from extragalactic sources will experience absorption as they propagate to Earth via photon-photon interactions with the lower energy ambient photons. I and my collaborators (Stecker, Malkan & Scully 2006 ) applied data from Spitzer and GALEX (and other astronomical data) along with some modeling to fill in the gaps to estimate the redshift dependence of low energy photon backgrounds in the universe. With the advent of many more deep galaxy surveys, we have crafted the first fully empirical extragalactic background light (EBL) determination out to redshifts of ~8 spanning the far-IR through to the Lyman limit. We first calculated this from the far UV through to the I band (Stecker, Malkan & Scully 2012) and have extended this work up to the mid IR (Scully, Stecker & Malkan 2014) and all the way through to the FIR (Stecker, Scully, & Malkan 2016). We used continuum colors between the bands to fill in the gaps where there is still a paucity of data. In the mid to far IR we directly integrated the luminosity functions available in the the literature. We determined the resulting pair-production absorption features to be expected in the spectra of gamma-ray sources at various redshifts. This is critical for interpreting the data from Fermi of distant blazars and gamma-ray bursts and the handful of distant sources observed by the ground-based H.E.S.S. and MAGIC Čerenkov atmospheric gamma-ray telescopes. Below are the downloadable major results of this work which extends through to PeV energy gamma-rays providing an energy range of interest to the upcoming Čerenkov Telescope Array.

The +/- 68% confidence band of the co-moving photon energy densities shown as a continuous function of photon energy and redshift. This is derived from our confidence bands for the rest frame luminosity densities, ${\rho}_{\nu}$: $$u_{\nu}(z)= \int_{z}^{z_{\rm max}}dz^{\prime}\,{\rho}_{\nu^{\prime}}(z^{\prime}) \frac{dt}{dz}{(z^{\prime})}$$ where $\nu^{\prime}=\nu(1+z^{\prime})/(1+z)$ and $z_{\rm max}$ is the redshift corresponding to initial galaxy formation while $$\frac{dt}{dz}{(z)} = {[H_{0}(1+z)\sqrt{\Omega_{\Lambda} + \Omega_{m}(1+z)^3}}]^{-1}$$ We smoothly interpolate between the bands from the far UV through to 8 microns. We artificially set 10 microns to be a factor of 5 (lower limit) and 3 (upper limit) lower than 8 microns to account for PAHs features. We then smoothly interpolate from 12 microns through to 90 micron bands. We create a 160 micron band from the 90 micron band by normalizing to the local measurement and similarly for 350, 550, and 850 microns from the 250 micron band.

Our empirically-based determination of the EBL together with a selection of measurements and limits. We consider data based on galaxy counts as lower limits and those data are distinguished by their color (blue). Also shown are the results of Dominguez et al. 2011 (red dashed line) and Franceschini et al. 2008 (solid black line) for comparison. There is excellent consistency except in the far IR where the models are on or just slightly above our upper limit. We attribute that to their assumption that redshift evolution follows the K-band at these wavelengths whereas we used direct measurements of the luminosity functions.

The resulting +/- 68% confidence bands based on our photon uncertainties for the gamma-ray opacities for redshifts of 0.1, 0.5, 1, 3, 5. $$\tau_\gamma=c\int_{0}^{z_{e}}dz\,\frac{dt}{dz}\int_{0}^{2} dx\,\frac{x}{2}\int_{0}^{\infty}d\nu\,(1+z)^{3}\left[\frac{u_{\nu}(z)} {h\nu}\right]\sigma_{\gamma\gamma}$$ The result includes the contribution from the CBR (which is why the opacity uncertainties narrow at higher energies since its contribution is exact): $$\tau_{CBR} = 5.00 \times 10^5 \sqrt{{1.11 PeV}\over {E_{\gamma}}} \int_0^z {dz'~(1 + z') ~{e^{-\left({1.11 PeV}\over {E_{\gamma}(1 + z')^2}\right)}} \over {\sqrt{\Omega_{\Lambda} + \Omega_{m}(1+z')^3}}}$$ The dashed lines indicate the opacities τ = 1 and τ = 3. We have calculated the opacities from z = 0 out to z = 5 spaced every 0.02 in z and 0.02 in Log gamma-ray energy (tables available below).

A τ = 1 energy-redshift plot (Fazio & Stecker 1970) showing our uncertainty band results compared with the Fermi plot of their highest energy photons from FSRQs (red), BL Lacs (black) and and GRBs (blue) vs. redshift (from Abdo et al. 2010). There is little evidence to suggest that additional modifications may be in play such as cosmic-ray interactions along the line of sight to the source (Essey & Kusenko 2014) or line-of-sight photon-axion oscillations during propagation (e.g., De Angelis et al. 2007; Mayer & Horns 2013)


File Description
comovehi.txt Co-moving Density Upper Limit     -2.84 < Log ε < 1.14 spaced 0.02     0 < z < 5 spaced 0.02
comovelo.txt Co-moving Density Lower Limit     -2.84 < Log ε < 1.14 spaced 0.02     0 < z < 5 spaced 0.02
opachi.txt Upper Opacity Limit
opaclo.txt Lower Opacity Limit
ebl.txt Integrated Extragalactic Background Light
horizon.txt Gamma-ray Horizon (τ = 1)

The following code in Mathematica will read in the co-moving densities correctly, interpolate them, and plot them:

CoMovingEnerDensHi = Import["comovehi.txt", "Table"];
CoMovingEnerDensLo = Import["comovelo.txt", "Table"];
DensityHi = 
   Log[10, CoMovingEnerDensHi], {{-2.84, 1.14}, {0, 5}}, 
   Method -> "Spline"];
DensityLo = 
   Log[10, CoMovingEnerDensLo], {{-2.84, 1.14}, {0, 5}}, 
   Method -> "Spline"];
plothi = Plot3D[
  DensityHi[logepsi, z] , {logepsi, -2.84, 1.1}, {z, 0, 5}, 
  AxesStyle -> AbsoluteThickness[1.1], ImageSize -> 500, 
  AspectRatio -> 1, 
  BaseStyle -> {FontFamily -> "Times", FontSlant -> "Plain", 
    FontSize -> 11}, 
  AxesLabel -> {"Log[\[Epsilon](eV)]", "z", 
    "Log[\!\(\*SubscriptBox[\(u\), \(\[Nu]\)]\) (erg/Hz\[CenterDot]\!\
\(\*SuperscriptBox[\(cm\), \(3\)]\))]"}, PlotPoints -> 100]
plotlo = Plot3D[
  DensityLo[logepsi, z] , {logepsi, -2.84, 1.1}, {z, 0, 5}, 
  AxesStyle -> AbsoluteThickness[1.1], ImageSize -> 500, 
  AspectRatio -> 1, 
  BaseStyle -> {FontFamily -> "Times", FontSlant -> "Plain", 
    FontSize -> 11}, 
  AxesLabel -> {"Log[\[Epsilon](eV)]", "z", 
    "Log[\!\(\*SubscriptBox[\(u\), \(\[Nu]\)]\) (erg/Hz\[CenterDot]\!\
\(\*SuperscriptBox[\(cm\), \(3\)]\))]"}, PlotPoints -> 100]

Astrophysical Constraints on Fundamental Physics

There are various proposed scenarios for new physics at the Planck scale some of which imply a possible small violation of Lorentz invariance. Examples can be found within the realm of string theory, non-commutative field theories, doubly (deformed) special relativity, stochastic space-time foam, loop quantum gravity, Hôrava-Lifshitz gravity, and brane-world models. In addition, there is an important fundamental connection between Lorentz invariance and CPT violation: a local interacting theory that violates CPT invariance will also violate Lorentz invariance. How can we test these proposals when terrestrial accelerators fall many times many orders of magnitude short of the Planck scale? Observations of high energy cosmic neutrinos, gamma-rays, and cosmic rays from nature's cosmic accelerators achieve energies and propagation lengths that may provide the possibility of detecting the traces of Planck-suppressed Lorentz invariance violation (LIV), as may arise in a quantum gravity theory. This is the focus of my work in this area. Below I describe some of my efforts.

The Pierre Auger observatory in Argentina detects cosmic rays with energies beyond EeV (1018 eV). Since such high energy particles have an estimated arrival rate of just 1 particle per square kilometer per century, the Auger Observatory has created an effective detection area of 3000 square kilometers (or about 1200 square miles). Construction began in 2000 and was officially completed in 2008.

The Fermi Gamma-ray Space Telescope is a space-based observatory that was launched in June of 2008. Its mission is to detect the highest energy photons known as gamma-rays up to a few hundred GeV in energy. Fermi data is being used to study supermassive black-hole systems, pulsars, cosmic rays, and hints of new physics.

The IceCube Neutrino Observatory is a neutrino telescope constructed at the Amundsen-Scott South Pole Station. It consists of thousands of sensors distributed over a cubic kilometer of volume under the Antarctic ice. The array was completed in December 2010. IceCube has detected 37 neutrinos in the TeV to PeV range that are likely extragalactic in origin.

Traces of LIV may be found by examining the spectra of very high-energy neutrinos with detectors such as IceCube at the South Pole. We (Stecker, Scully, Liberati, & Mattingly 2015) explored the physics and cosmology of superluminal neutrino propagation based on the dominance of mass dimension [d] = 5 or 6 operators with Planck mass suppression that may arise in an effective field theory (EFT) with CPT violation or CPT conservation. If LIV is produced by Planck scale physics, it has been suggested that in an EFT framework, LIV terms can be expressed by Planck-suppressed operators in the free particle Lagrangians. We considered a conservative generic scenario for the redshift distribution of cosmic neutrino sources and employed Monte Carlo techniques to take account of energy losses, treating potential losses by vacuum pair emission (VPE) and neutrino splitting both separately and together - reactions that are kinematically forbidden but become viable above a certain threshold if neutrinos are superluminal. We compared the spectra derived using our Monte Carlo calculations with that observed by IceCube to determine the implications of our results regarding Planck-scale physics.This was an extension of earlier work in which I was involved (Stecker & Scully 2014) that considered the VPE process alone placing the strongest constraint to date on LIV in the neutrino sector using the recent IceCube observations of cosmic neutrinos with Eν > 60 TeV and up to ~2 PeV, most of which are likely of extragalactic origin. The results suggest that the apparent cutoff in the observed spectrum above ∼2 PeV can conceivably be an effect of Lorentz invariance violation. Furthermore, the detection of a pronounced pileup feature just below the cutoff energy, primarily a result of the splitting mechanism, would be prima facie evidence of Planck scale physics. Such a determination would require the detection of many more astrophysical neutrinos above 100 TeV energy by IceCube or other neutrino experiments.

The Pierre Auger array provides the best statistics on the spectrum of ultrahigh energy cosmic rays (UHECRs) and therefore allows us the best constraints in the proton sector on LIV. A small violation of Lorentz invariance at ultrahigh energies can increase or even eliminate the threshold energy for photopion production interactions of ultrahigh energy protons with photons of the cosmic background radiation thus attenuating the reactions that lead to the expected GZK cutoff. I along with my colleague Floyd Stecker placed strong constraints on LIV from UHECRs using the spectrum from the Auger array (Stecker & Scully 2009). This paper, Searching for New Physics with Ultrahigh Energy Cosmic Rays, was selected as the IOP Best of 2009 paper in Cosmology in the New Journal of Physics. We then later extended this work (Scully & Stecker 2010) to expand the calculation to corresponding cosmogenic neutrino production. The high energy hadrons lose energy through photopion production and therefore there should be a reasonable flux of neutrinos detectable on Earth that result from the decay of these pions under the standard energy-loss picture. Under LIV, there would be a dampening of the neutrino flux from this process whose signature may be detectable by future neutrino experiments.

Astronomy 320
Fall Semester

Fall semesters I usually teach ASTR 320: Astronomical Techniques. Astronomy 320 is an overview of modern astronomical techniques with an emphasis on quantitative data collection and analysis and is intended for upper level physics majors and astronomy minors. The course covers 5 major areas: solar observing, astrophotography, temporal photometry, color photometry, and spectroscopy. I have been gradually building up the department's observing capabilities. We currently operate six 10" Meade telescopes and two 14" Celestrons. We also have two state of the art SBIG 8300 CCDs and an SBIG Spectrograph. For solar observing we are equipped with a Lunt 80 mm solar telescope and imager. This picture was taken this Fall (2015) semester using our Lunt!

Star Parties
JMU Astronomy Park

When the department moved into its new location on the East Campus in Fall semester of 2005, JMU opened the adjacent Astronomy Park. It consists of 6 permanent piers to support our Celestron and Meade telescopes. The facility sees extensive use in our astronomy courses both in general science and those in support of the minor and we also host public star parties at the Park using our 10" and 14" telescopes. These events are scheduled for the last Friday (with Saturday as a backup) of each month during the regular semester or for special astronomical events.

John C. Wells Planetarium

The John C. Wells Planetarium at James Madison University is a $2 million, hybrid facility, which is the only one of its kind in the world. It hosts both an Evans & Sutherland Digistar 5, an ultra-high resolution digital projection system, and a Goto Chronos opto-mechanical star projector that provides visitors with a superior and realistic night sky. Our digital system allows us to present unique full dome movies as well as project the multi-wavelength universe on the dome. I am working to develop several demonstrations and programs involving this system for use in my astronomy lecture and lab courses.

Eclipse 2017!

I drove down to Loudon State Park just south of Knoxville for the August 21, 2017 eclipse! I traveled along with Prof. Harold Butner (JMU) and met up with some colleagues from NASA/GSFC and NRL. To the right are a few of the images I took of the event using our Lunt Solar Telescope and Solar Imager (pictured). We had excellent weather for viewing and totality lasted almost two and a half minutes at this location. There were a couple of nice sunspot groups on the Sun visible during the partial eclipse. I was also able to capture some Baily's Beads during the time of totality (4th frame - the famous "diamond ring effect"). Traffic and parking were crazy but it was worth it in the end. Words can't describe how impressive it is to see a total solar eclipse in person - looking forward to April, 2024!

Curriculum Vitae

You'll find here an abbreviated version of my cv including education, appointments, publications, recent talks, and grant support. Papers link to their arXiv versions.

  • Education

    • Ph.D. in Astrophysics, University of Minnesota, August 1997 Advisor: Prof. Keith A. Olive

    • M.S. in Astronomy, University of Minnesota, 1993

    • B.S. in Astrophysics, University of Minnesota, 1990

    • B.S. in Physics, minor in Mathematics, University of Minnesota, 1990

  • Appointments

    • Professor of Physics & Astronomy, James Madison University 2016 - present.

    • Associate Professor of Physics & Astronomy, James Madison University 2009 - 2016.

    • Assistant Professor of Physics & Astronomy, James Madison University 2004 - 2009.

    • Visiting Assistant Professor, Valparaiso University 2000 – 2004.

    • National Research Council Research Associate, NASA/GSFC 1998 - 2000.

    • Postdoctoral Research Fellow, Institut D'Astrophysique, Paris 1997 - 98.

  • Publications

    • Stecker, F. W., Scully, S. T. , Malkan, M. 2016 An Empirical Determination of the Intergalactic Background Light from UV to FIR Wavelengths Using FIR Deep Galaxy Surveys and the Gamma-ray Opacity of the Universe, ApJ, 827,3.  

    • Stecker, F. W., Scully, S. T. , Mattingly, D., & Liberati, S. 2015, Searching for Traces of Planck-Scale Physics with High Energy Neutrinos, Phys. Rev. D 91, 045009.  

    • Stecker, F. W., Scully, Sean T. 2014 Propagation of superluminal PeV IceCube neutrinos: A high energy spectral cutoff or new constraints on Lorentz invariance violation, Phys. Rev. D 90, 043012.  

    • Scully, S.T., Malkan, M., & Stecker, F.W. 2014, An Empirical Determination of the Intergalactic Background Light Using NIR Deep Galaxy Survey Data out to 5 μm and the Gamma-ray Opacity of the Universe, ApJ, 784:138.  

    • Stecker, F.W., Malkan, M., & Scully, S.T. 2012, A Determination of the Intergalactic Redshift Dependent UV-Optical-NIR Photon Density using Deep Galaxy Survey Data and the Gamma-ray Opacity of the Universe, ApJ 761, 128.  

    • Scully, S.T., Stecker, F.W. 2011, Testing Lorentz Invariance with Neutrinos from Ultrahigh Energy Cosmic Ray Interactions, Astroparticle Physics, Volume 34, Issue 7, p. 575-580.  

    • Stecker, F.W. & Scully, S.T. 2010, Derivation of a Relation for the Steepening of Tev-Selected Blazar Gamma-ray Spectra with Energy and Redshift, Astrophysical Journal Letters 709, L124.  

    • Stecker, Floyd W.; Scully, Sean T. 2009 ,Searching for new physics with ultrahigh energy cosmic rays, New Journal of Physics, Volume 11, Issue 8, pp. 085003 (2009).  

    • Stecker, F.W. & Scully, S.T. 2009, Is the Universe More Transparent to Gamma Rays Than Previously Thought?, Astrophysical Journal Letters 691, L91.  

    • Stecker, F.W. & Scully, S.T. 2008, The Spectrum of 1ES0229 + 200 and the Cosmic Infrared Background, Astronomy and Astrophysics 478, L1.  

    • Stecker, F. & Scully, S.T., 2006. A Simple Analytic Treatment of the Intergalactic Absorption Effect in Blazar Gamma-Ray Spectra, Astrophysical Journal, 652, L9.  

    • Stecker, F. , Malkan, M., & Scully, S.T., 2006. Intergalactic Photon Spectra from the Far IR to the UV Lyman Limit for 0 < z < 6 and the Optical Depth of the Universe to High Energy Gamma-Rays, Astrophysical Journal, 648, 774.  

    • Stecker, F., & Scully, S.T., 2004. Lorentz Invariance Violation and the Spectrum and Source Power of Ultrahigh Energy Cosmic Rays, Astroparticle Physics , 23, 203.  

    • Scully, S.T., & Stecker, F., 2002. Gamma Ray Bursts as Sources of the UHECRs - Revisited, Astroparticle Physics, 16, 271.  

    • Prunet, S., Teyssier, R., Scully, S.T., Bouchet, F.R., Gispert, R., 2001. Error Estimation for the MAP Experiment, Astronomy & Astrophysics, 373, L13-L16.  

    • Ramaty, R., Scully, S.T., Lingenfelter, R., Kozlovsky, B., 2000. Light Element Evolution and Cosmic Ray Energetics, Astrophysical Journal, 534, 747.  

    • Vangioni-Flam, E., Casse, M., & Scully, S.T. 1998. Origin and Evolution of the Light Elements Li, Be, and B, in Proc. 2nd INTEGRAL Workshop, ed. C. Winkler et al., ESA SP-382.

    • Vangioni-Flam, E., Casse, M., & Scully, S.T. 1998. The Gamma-Ray Line Emission of Orion, in Proc. 2nd INTEGRAL Workshop, ed. C. Winkler et al., ESA SP-382.

    • Hogan, C.J., Olive, K.A., & Scully, S.T. 1997. A Bayesian Estimate of the Primordial Helium Abundance, Astrophysical Journal, 489, L119.  

    • Olive, K.A., Schramm, D.N., Scully, S. T., & Truran, J. 1996. Low Mass Stars and the 3He Problem, Astrophysical Journal, 479, 752.  

    • Scully, S.T., Casse, M., Olive, K.A., & Vangioni-Flam, E. 1996. The Effects of an Early Galactic Wind on the Evolution of D, 3He and Z, Astrophysical Journal, 476, 521.  

    • Scully, S.T., Casse, M., Olive, K.A., Schramm, D.N., Truran, J., & Vangioni-Flam, E. 1996. The Local Abundance of 3He: A Confrontation Between Theory and Observation, Astrophysical Journal, 462, 960.  

    • Olive, K.A., & Scully, S.T. 1996. Big Bang Nucleosynthesis: An Update, Int. Journal of Modern Physics, vol. 11, p. 409.  

    • Scully, S.T., & Olive, K.A. 1995. The Deuterium Abundance and Nucleocosmochronology, Astrophysical Journal, 446, 272.  

    • Olive, K.A., Prantzos, N., Scully, S. T., & Vangioni-Flam, E. 1994. Neutrino Process Nucleosynthesis and the 11B/10B Ratio, Astrophysical Journal, 424, 666.  

  • Recents Talks

    • Probing Fundamental Physics with Astrophysics: Lorentz Invariance and CPT Violation, From the Big Bang to Black Holes: A celebration of the distinguished careers of three Goddard scientists, NASA/GSFC Greenbelt Maryland, November 17, 2016

    • The Intergalactic mid IR - far IR Luminosity Density and the γ-ray Opacity of the Universe, Special Session, "The Cosmic History of Light: New Results and Future Outlook" at the 227th AAS Meeting, January 4-8 2016

    • Closing in on the NIR Background and gamma-ray Opacities, NIRB II Meeting, June 2015

    • An Empirical Determination of the EBL and the Gamma-ray Opacity of the Universe, 4th Fermi Symposium, October 28 2012

    • The Extragalactic Background Light and the Opacity of the Universe, Fermi Science and Proposals Workshop, December 14th, 2011

    • Probing the Extragalactic Background Light, Fermi Science and Proposals Workshop, November 16th, 2010

    • A Relation for the Steepening of TeV Selected Blazar Gamma-ray Spectra with Energy and Redshift, High Energy Astrophysics Division March Meeting, March 6th, 2010

    • Astrophysical Limits on Lorentz Invariance Violation and Quantum Gravity Models, NASA/Goddard Astrophysics Division Seminar, December 4th, 2009

  • Funding

    • NASA/Fermi Newly Discovered Ionizing Photons and the Gamma Ray Opacity of the Universe, Funded $52,000

    • NASA/ADP X-ray Absorbers as Probes of AGN Unification, Funded $274,952


    • NASA/Fermi/GI New Studies of the Extragalactic Background Light, 2012 - 2013, Funded, $80,000

    • NASA/Fermi/GI AGN Unification and their Fermi γ−Ray Spectra, 2010 - 2011, Funded, $80,000

    • NASA/GLAST-1 Guest Investigator: The Redshift Evolution of Blazars and the Extragalactic Background Light, 2008, Funded, $80,000

    • Jeffress Trust Foundation Grant: Using Neutrinos from the Propagation of Ultrahigh Energy Cosmic Rays to Identify their Sources and Potential New Physics, 2005 - 2007, Funded, $40,000

  Fall 2018 Schedule

For the Fall Semester 2017, I am teaching PHYS 240: University Physics I, which is the first semester of introductory physics with calculus covering mechanics and waves. I am also teaching a section of PHYS 240L, which is the accompanying lab course. Additionally, I am teaching ISCI 104: Exploring the Daytime & Nighttime Sky, which is a general eduation astronomy lab course. I am also teaching a section of PHYS 240L, which is the accompanying lab course.