Research
Image credit: Shutterstock
Atmospheric Response for MeV Gamma Rays Observed with Balloon-Borne Detectors
The atmospheric response for MeV gamma rays (~ 0.1 − 10 MeV) can be characterized in terms of two observed components. The first component is due to photons that reach the detector without scattering. The second component is due to photons that reach the detector after scattering one or more times. While the former can be determined in a straightforward manner, the latter is much more complex to quantify, as it requires tracking the transport of all source photons that are incident on Earth's atmosphere. The scattered component can cause a significant energy-dependent distortion in the measured spectrum, which is important to account for when making balloon-borne observations. In this work we determined the full response for gamma-ray transport in the atmosphere via Monte Carlo simulations, which includes both the transmitted and scattered components. We found that the scattered component becomes increasingly more significant towards lower energies, and at 0.1 MeV it may increase the measured flux by as much as a factor of ~2−4, depending on the photon index and off-axis angle of the source. This is particularly important for diffuse sources, whereas the effect from scattering can be significantly reduced for point sources observed with an imaging telescope.
As part of this work, we publicly released our simulation and analysis pipeline, cosi-atmosphere, which is a general package for dealing with atmospheric effects in gamma-ray astronomy, and is readily applicable to other instruments and observations. Documentation for the library can be found here.
The study is published in The Astrophysical Journal (link).
Image credit: GALPROP skymap
Probing the Galactic Diffuse Continuum Emission with COSI
The Galactic diffuse continuum emission in the MeV energy band is dominated by inverse Compton radiation, generated by high-energy cosmic ray (CR) electrons up-scattering low-energy photons of the interstellar radiation field. The upcoming COSI mission will observe this emission, and thus it will probe the nature of CR electrons in the Galaxy — something that has remained a longstanding mystery in astrophysics. A big part of this mystery pertains to the observed intensity of the diffuse continuum emission towards the inner Galaxy, which is higher than what our standard models predict. This may be due to contributions from unresolved point sources, and possibly emission associated with electron-positron annihilation in the Galaxy or new physics such as dark matter.
As part of its development, in 2016 COSI had a successful 46 day flight on board NASA's Super Pressure Balloon platform. Using data from the flight, we were able to measure the Galactic diffuse continuum emission towards the inner Galaxy. These observations are the first diffuse continuum results from COSI, and they serve as a proof of principle, showing lots of promise for the upcoming satellite mission. By measuring the truly diffuse emission with unprecedented sensitivity in the MeV band, COSI will likely lead to new insights regarding the sources of CR electrons in the Galaxy. Additionally, the measurements will also play a major role in unveiling the different Galactic sources of diffuse continuum emission in the MeV band.
The study is published in The Astrophysical Journal (link).
Image credit: COSI Collaboration
The Compton Spectrometer and Imager (COSI)
The energy range between 0.1 - 100 MeV (i.e. the so-called “MeV gap”) is one of the least explored regions in the electromagnetic spectrum. However, thanks to more than 20 years of research and development, the MeV gap will soon be probed with improved sensitivity between 0.2 - 5 MeV by the Compton Spectrometer and Imager (COSI), a Small Explorer satellite mission selected by NASA and scheduled to launch in 2027. The COSI instrument utilizes Compton imaging of gamma-ray photons. Although not one of its primary science goals, COSI will be capable of measuring the Galactic diffuse continuum emission (GDCE) and extragalactic gamma-ray background (EGB) with unprecedented sensitivity. I am currently working on developing the data analysis pipelines for the COSI mission, and my science focus is measuring the GDCE and EGB.
Read the most recent mission paper here.
Image credit: Mount Lemmon SkyCenter
Gamma Rays from Fast Black-Hole Winds
Massive black holes at the centers of galaxies can launch powerful wide-angle winds that, if sustained over time, can unbind the gas from the stellar bulges of galaxies. These winds may be responsible for the observed scaling relation between the masses of the central black holes and the velocity dispersion of stars in galactic bulges. Propagating through the galaxy, the wind should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate charged particles to high energies.
Using data collected by the Fermi Large Area Telescope, we made the first detection of the average gamma-ray emission from a sample of active galactic nuclei (AGN) exhibiting fast black-hole winds (also referred to as ultra-fast outflows). Our analysis shows that the gamma-ray luminosity scales with the AGN bolometric luminosity and that these outflows transfer roughly 0.04% of their mechanical power to gamma rays. Interpreting the observed gamma-ray emission as produced by cosmic rays (CRs) accelerated at the shock front, we find that the gamma-ray emission may attest to the onset of the wind-host interaction and that these outflows can energize charged particles up to the transition region between galactic and extragalactic CRs.
The study is published in The Astrophysical Journal (link), and it can be read for free on the arXiv (link).
A press release of the study was issued by Clemson University (link), and the work was also highlighted in AAS Nova (link).
Image credit: Miguel Claro (link).
The Outer Halo of M31
The Andromeda galaxy, also known as M31, is very similar to the Milky Way. It has a spiral structure and is comprised of multiple components including a central supermassive black hole, bulge, galactic disk (the disk of stars, gas, and dust), stellar halo, and circumgalactic medium, all of which have been studied extensively. Additionally, the Andromeda galaxy, like all galaxies, is thought to reside within a massive dark matter (DM) halo. The DM halo of M31 is predicted to extend to roughly 300 kpc from its center and account for roughly 90% of the galaxy’s total mass.
Using data collected by the Fermi Large Area Telescope we made the first detailed study of the gamma-ray emission observed towards the outer halo of M31. In this study we find evidence for an excess signal that appears to be distinct from the conventional Milky Way foreground, having a total radial extension upwards of roughly 120-200 kpc from the center of M31. Although other explanations are plausible, the excess signal is found to be roughly consistent with arising from DM annihilation.
The full observational paper is published in The Astrophysical Journal (link), and it can be read for free on the arXiv (link).
A follow-up study where we give a detailed DM interpretation of the signal is published in Physical Review D (link), and it can be read for free on the arXiv (link).
Image credit: ESO/S. Brunier (link).
The Galactic Center Excess
An excess gamma-ray signal coming from the center of the Milky Way has been detected, referred to as the Galactic center (GC) excess. The signal was first identified by Lisa Goodenough and Dan Hooper in 2009, and it has since been the subject of numerous studies. Currently, the three main interpretations of the signal are that it is due to either mis-modeling of the diffuse Galactic emission, a population of unresolved millisecond pulsars, or the annihilation of cold dark matter (DM). The signal can also be due to some combination of all three. However, the true nature of the GC excess is still being actively debated. Part of what makes this debate so interesting is that the GC excess may have significant implications for the nature of DM.
The first Fermi-LAT Collaboration study of the GC excess was led by Simona Murgia (my Ph.D. advisor) and Troy Porter. This was the first study I was involved in during my Ph.D. career. The study is published in The Astrophysical Journal (link), and it can be read for free on the arXiv (link).
Using the data and models from the first Fermi-LAT Collaboration study, we characterized the excess emission as annihilating DM in the framework of an effective field theory. The full paper is published in Physical Review D (link), and it can be read for free on the arXiv (link).
Image credit: Garrison-Kimmel et al. 2018 (link)
Dark Matter
Evidence for dark matter (DM) is seen at all cosmological scales—from the local Universe, all the way back to the early Universe, as indicated by the cosmic microwave background (CMB). Collectively, this evidence indicates that DM makes up ~26% of the total matter-energy density of the Universe, whereas the baryons (i.e. the elements found in the periodic table) only amount to ~4%, and the remaining ~70% is accounted for by the so-called dark energy. Part of what makes DM so important is that it is thought to serve as the gravitational seed in the very early Universe from which all galaxies grow. The actual nature of DM remains unknown, although it is widely thought to be a fundamental particle of nature. Some of the basic DM properties we are trying to determine include its mass, its annihilation cross-section, and how it interacts with the other known particles.
I am specifically involved in DM indirect detection, using observations from Fermi-LAT. My primary interest has been the DM in the Local Group, with a focus on M31 and the Galactic center.
I had the opportunity to be a guest on The Economist Podcast for an episode about how physicists are looking for dark matter. The episode is available here.
Image credit: NASA Fermi Gamma-ray Space Telescope (link).
Fermi-LAT
The Fermi Gamma-ray Space Telescope was launched on June 11, 2008. The main instrument on board is the Fermi Large Area Telescope (Fermi-LAT), which is sensitive to photons in the energy range from roughly 20 MeV — 800 GeV. The telescope consists of an array of 16 tracker modules, 16 calorimeters modules, and a segmented anti-coincidence detector. An incident photon is converted into an electron-positron pair in the tracker modules, and the corresponding energy is deposited into the calorimeters. From these measurements one can reconstruct the direction in the sky from which the photon originated, as well as its incident energy.
Image credit: SARA Observatory
The SARA Observatory
The Southeaster Association for Research in Astronomy (SARA) is a consortium of optical telescopes. There are three telescopes: Kitt Peak, Arizona (SARA-KP, 0.9 m); Cerro Tololo, Chile (SARA-CT, 0.6 m); and Roque de los Muchahos, Spain (SARA-RM, 1m). From September 2019 - December 2020 I was involved in an observing campaign to obtain photometric redshifts for Fermi-LAT BL Lacs, as well as optical follow-up observations of gamma-ray bursts. During this time I observed on average 2 full nights per month.