Session T10: MeV Gamma-ray Science Prospects and Extragalactic Diffuse Emission

Live

Gamma-ray observations have played a critical role in every multimessenger source identified to date – including gamma-ray lines seen from SN1987A, a nearby neutrino source; a gamma- ray burst from the neutron star merger event GW170817A; and a gamma-ray flare from the active galaxy TXS 0506+056, the first identified counterpart to a high-energy neutrino source. In each of these cases, the gamma-ray observations were critical to understanding the underlying physical phenomena driving these extremely energetic sources. The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission that will provide ground-breaking new capabilities for multimessenger astrophysics - identifying and studying the astrophysical objects that produce gravitational waves and neutrinos. In this talk, I will describe how AMEGO's performance more than an order of magnitude better than previous MeV gamma-ray missions, will lead to a revolution in multimessenger astrophysics.   

Live

The possible association of the blazar TXS 0506+056 with a high energy neutrino detected by IceCube holds the tantalizing potential to answer three astrophysical questions: 1. Where do high energy neutrinos originate? 2. Where are cosmic rays produced and accelerated? 3. What radiation mechanisms produce the high-energy gamma-rays in blazars? The MeV gamma-ray band holds the key to these questions, because it is an excellent proxy for photo-hadronic processes in blazar jets, which also produce neutrino counterparts. Variability in MeV gamma-rays shed light on the physical conditions and mechanisms that take place in the particle acceleration sites in blazar jets. In addition, hadronic blazar models also predict a high level of polarization fraction in the MeV band, which can unambiguously distinguish the radiation mechanism. Future MeV missions with large field of view, high sensitivity, and polarization capability will guarantee a central role in multi-messenger astronomy, since pointed, high-resolution telescopes will follow neutrino alerts only when triggered by an all-sky instrument.   

Live

Gamma-ray bursts (GRBs) are the brightest electromagnetic transients in the universe. They are attributed to core-collapses of massive stars or mergers of compact objects such as binary neutron star mergers. Even though the origin and the physical process of GRBs have been vigorously studied, many open questions are remained such as the origin of the prompt emission. More sample of bright events, deeper studies in the MeV energy band, polarization of the prompt emission are required in order to address such questions. A sensitive MeV mission, All-sky Medium Energy Gamma-ray Observatory (AMEGO), will potentially satisfy such demands. Simulation studies suggest that AMGEO will be the most sensitive GRB detector, providing a large sample of GRBs: \textasciitilde 400/years long-duration GRBs and \textasciitilde 120/years short-duration GRBs. Also, AMEGO will be able to pinpoint the ambiguity among spectral models with sensitive MeV observations, resulting in unveiling the prompt emission mechanism. The understanding of the afterglow and polarization of bright GRBs will be greatly improved with AMEGO observations.   

Live

Magnetars are young neutron stars with high surface magnetic fields, exceeding $10^9$ Tesla, and probe the high magnetic field domain of QED. Pulsed non-thermal quiescent X-ray emission extending between 10 keV to $>150$ keV has been observed in about 10 magnetars. For inner magnetospheric models of such hard X-ray signals, resonant Compton upscattering of soft thermal photons from the neutron star surface is the most efficient process for generating the continuum radiation in high magnetic fields. Such upscattering emission is anticipated to exhibit strong polarization above around 30 keV that is pulse phase dependent. These signatures define science agendas for future hard X-ray polarimeters and Compton telescopes. We present detailed model predictions of emission spectra and polarization signals, addressing prospects for measuring the spectral cutoffs with a future Compton telescope such as AMEGO. Polarization measurements can probe fundamental strong-field QED processes operating in the magnetar magnetospheres, potentially distinguishing between spectral cutoffs due to magnetic pair production or photon splitting. Thus, magnetars can provide insights into Nature that are currently beyond the reach of current terrestrial experiments.   

Live

Gamma-rays from blazars interact with extragalactic background light (EBL) photons, and are absorbed. This allows one to use gamma-ray absorption to constrain the EBL, which depends strongly on the cosmic star formation rate. We combine results of gamma-ray absorption measurements from the Fermi Telescope with luminosity density measurements from galaxy surveys to provide a very tight constraint on the cosmic star formation rate and other cosmologically interesting parameters.   

Live

The Cosmic Infrared Background (CIB) is a prominent component of the Extra-galactic Background Light (EBL) that includes wavelengths from 5 mum to 1 mm. Most of the light is produced by stellar emission that is absorbed and reemitted by dust. As such the CIB traces stellar and galactic evolution from the time of reionization until the present. Attenuation of TeV gamma-ray spectra from extra-galactic blazars offers an alternative method of constraining the CIB density that is complementary to direct measurements and to estimation through galaxy counts. Using Fermi-LAT observations at MeV-GeV energies as a starting point, we investigate the impact of EBL attenuation on the TeV spectra of Markarian 421 and 501, using three years of TeV observations from the High Altitude Water Cherenkov Observatory (HAWC). We explore the effects of the attenuation using the EBL models of Franceschini and Gilmore, modified by a constant multiplier. A best fit value of attenuation based upon this constant is then used to constrain the EBL density. This talk will compare the HAWC CIB energy density limits with recent estimates by other TeV observatories.   

The Most Powerful Blazars with AMEGO

MeV blazars, the most luminous, persistent, sources in the Universe, are best observed in the MeV band where they release most of their radiative output. These blazars, which are typically found at high redshift, tend to host black holes with a mass in excess of 1 billion solar masses and as such are powerful probes of the formation mechanisms of massive black holes in the early Universe. The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is posed to be the best instrument to detect and study MeV blazars. Here, we discuss the prospects for a survey of powerful MeV blazars with AMEGO.   

Limits on the Diffuse Gamma-Ray Background with HAWC

The high-energy Diffuse Gamma-Ray Background (DGB) is expected to be produced by unresolved extragalactic objects such as active galactic nuclei, isotropic Galactic gamma-rays, and possibly emission from dark matter annihilations or decays in the Galactic dark matter halo. The DGB has only been observed below 1 TeV, and above this energy upper-limits have been reported. Observations or stringent limits on the DGB above this energy could have strong multimessenger consequences, such as constraining the origin of TeV-PeV astrophysical neutrinos detected by IceCube. The High Altitude Water Cherenkov (HAWC) Observatory, located in central Mexico at 4100 m above sea level, is sensitive to gamma rays from 300 GeV to above 100 TeV and continuously observes a wide field-of-view (~2 sr). With its high energy reach and large area coverage, HAWC is well-suited to significantly improve searches for the DGB at TeV energies. In this work, strict cuts have been applied to the HAWC dataset to better isolate gamma-ray air showers from background hadronic showers. The sensitivity to the DGB was then verified using Crab data and Monte Carlo simulations, thus leading to a new limit on the DGB with HAWC as well as its implications for multimessenger and dark matter studies.   

Not Participating

VERITAS is an atmospheric Cherenkov telescope in southern Arizona that is used to study astrophysical gamma rays above 100 GeV.   Since the start of the instrument's operation in 2007, VERITAS has dedicated a significant amount of time each year to the observation of active galactic nuclei (AGN).  These observations have resulted in the detection of gamma-ray emission from 39 active galaxies.  A number of these galaxies have been detected in both quiescent and flaring states, with the flaring behavior providing insight on the highly energetic particle population(s) responsible for the gamma-ray emission.  VERITAS results from a few notable flaring BL Lac type AGN, as well as detections of the radio galaxy 3C 264, will be presented.  Additionally, the long-term observations of distant BL Lac type objects will be presented and given cosmological context with the recent indirect measurement of the extragalactic background light intensity provided by VERITAS.