arXiv subjects

Derek Fox

Publications and source records attributed to Derek Fox.

12 recordsLinked to original sources

arXiv2025

Observational strategies for ultrahigh-energy neutrinos: the importance of deep sensitivity for detection and astronomy

Detecting ultrahigh-energy neutrinos can take two complementary approaches with different trade-offs. 1)~Wide and shallow: aim for the largest effective volume, and to be cost-effective, go for wide field-of-view but at the cost of a shallow instantaneous sensitivity -- this is less complex conceptually, and has strong discovery potential for serendipitous events. However, it is unclear if any source can be identified, following detection. And 2)~Deep and narrow: here one uses astrophysical and multi-messenger information to target the most likely sources and populations that could emit neutrinos -- these instruments have deep instantaneous sensitivity albeit a narrow field of view. Such an astrophysically-motivated approach provides higher chances for detection of known/observed source classes, and ensures multi-messenger astronomy. However, it has less potential for serendipitous discoveries. In light of the recent progress in multi-messenger and time-domain astronomy, we assess the power of the deep and narrow instruments, and contrast the strengths and complementarities of the two detection strategies. We update the science goals and associated instrumental performances that envisioned projects can include in their design in order to optimize discovery potential.

astro-ph.IM↗

arXiv2024

Stochastic Variance-Reduced Iterative Hard Thresholding in Graph Sparsity Optimization

Stochastic optimization algorithms are widely used for large-scale data analysis due to their low per-iteration costs, but they often suffer from slow asymptotic convergence caused by inherent variance. Variance-reduced techniques have been therefore used to address this issue in structured sparse models utilizing sparsity-inducing norms or $\ell_0$-norms. However, these techniques are not directly applicable to complex (non-convex) graph sparsity models, which are essential in applications like disease outbreak monitoring and social network analysis. In this paper, we introduce two stochastic variance-reduced gradient-based methods to solve graph sparsity optimization: GraphSVRG-IHT and GraphSCSG-IHT. We provide a general framework for theoretical analysis, demonstrating that our methods enjoy a linear convergence speed. Extensive experiments validate

stat.ML↗

arXiv2022

Short GRB Host Galaxies I: Photometric and Spectroscopic Catalogs, Host Associations, and Galactocentric Offsets

We present a comprehensive optical and near-infrared census of the fields of 90 short gamma-ray bursts (GRBs) discovered in 2005-2021, constituting all short GRBs for which host galaxy associations are feasible ($\approx$ 60% of the total Swift short GRB population). We contribute 245 new multi-band imaging observations across 49 distinct GRBs and 25 spectra of their host galaxies. Supplemented by literature and archival survey data, the catalog contains 335 photometric and 40 spectroscopic data sets. The photometric catalog reaches $3\sigma$ depths of $\gtrsim 24-27$ mag and $\gtrsim 23-26$ mag for the optical and near-infrared bands, respectively. We identify host galaxies for 84 bursts, in which the most robust associations make up 54% (49/90) of events, while only a small fraction, 6.7%, have inconclusive host associations. Based on new spectroscopy, we determine 17 host spectroscopic redshifts with a range of $z\approx 0.15-1.6$ and find that $\approx$ 25-44% of Swift short GRBs originate from $z>1$. We also present the galactocentric offset catalog for 83 short GRBs. Taking into account the large range of individual measurement uncertainties, we find a median of projected offset of $\approx 7.9$ kpc, for which the bursts with the most robust associations have a smaller median of $\approx 4.9$ kpc. Our catalog captures more high-redshift and low-luminosity hosts, and more highly-offset bursts than previously found, thereby diversifying the population of known short GRB hosts and properties. In terms of locations and host luminosities, the populations of short GRBs with and without detectable extended emission are statistically indistinguishable. This suggests that they arise from the same progenitors, or from multiple progenitors which form and evolve in similar environments. All of the data products are available on the BRIGHT website.

astro-ph.GA↗

arXiv2019

Calibrating the cosmic distance ladder using gravitational-wave observations

Type Ia supernovae (SNe Ia) are among preeminent distance ladders for precision cosmology due to their intrinsic brightness, which allows them to be observable at high redshifts. Their usefulness as unbiased estimators of absolute cosmological distances however depends on accurate understanding of their intrinsic brightness, or anchoring their distance scale. This knowledge is based on calibrating their distances with Cepheids. Gravitational waves from compact binary coalescences, being standard sirens, can be used to validate distances to SNe Ia, when both occur in the same galaxy or galaxy cluster. The current measurement of distances by the advanced LIGO and Virgo detector network suffers from large statistical errors ($\sim 50\%$). However, we find that using a third generation gravitational-wave detector network, standard sirens will allow us to measure distances with an accuracy of $\sim 0.1\%$-$3\%$ for sources within $\le300$ Mpc. These are much smaller than the dominant systematic error of $\sim 5\%$ due to radial peculiar velocity of host galaxies. Therefore, gravitational-wave observations could soon add a new cosmic distance ladder for an independent calibration of distances to SNe Ia.

astro-ph.CO↗

arXiv2019

X-ray follow-up of extragalactic transients

Most violent and energetic processes in our universe, including mergers of compact objects, explosions of massive stars and extreme accretion events, produce copious amounts of X-rays. X-ray follow-up is an efficient tool for identifying transients because (1) X-rays can quickly localize transients with large error circles, and (2) X-rays reveal the nature of transients that may not have unique signatures at other wavelengths. In this white paper, we identify key science questions about several extragalactic multi-messenger and multi-wavelength transients, and demonstrate how X-ray follow-up helps answer these questions

astro-ph.HE↗

arXiv2018

Multi-Messenger Astrophysics: Harnessing the Data Revolution

The past year has witnessed discovery of the first identified counterparts to a gravitational wave transient (GW 170817A) and a very high-energy neutrino (IceCube-170922A). These source identifications, and ensuing detailed studies, have realized longstanding dreams of astronomers and physicists to routinely carry out observations of cosmic sources by other than electromagnetic means, and inaugurated the era of "multi-messenger" astronomy. While this new era promises extraordinary physical insights into the universe, it brings with it new challenges, including: highly heterogeneous, high-volume, high-velocity datasets; globe-spanning cross-disciplinary teams of researchers, regularly brought together into transient collaborations; an extraordinary breadth and depth of domain-specific knowledge and computing resources required to anticipate, model, and interpret observations; and the routine need for adaptive, distributed, rapid-response observing campaigns to fully exploit the scientific potential of each source. We argue, therefore, that the time is ripe for the community to conceive and propose an Institute for Multi-Messenger Astrophysics that would coordinate its resources in a sustained and strategic fashion to efficiently address these challenges, while simultaneously serving as a center for education and key supporting activities. In this fashion, we can prepare now to realize the bright future that we see, beyond, through these newly opened windows onto the universe.

astro-ph.IM↗

arXiv2015

The Dust Attenuation Curve versus Stellar Mass for Emission Line Galaxies at z ~ 2

We derive the mean wavelength dependence of stellar attenuation in a sample of 239 high redshift (1.90 < z < 2.35) galaxies selected via Hubble Space Telescope (HST) WFC3 IR grism observations of their rest-frame optical emission lines. Our analysis indicates that the average reddening law follows a form similar to that derived by Calzetti et al. for local starburst galaxies. However, over the mass range 7.2 < log M/Msolar < 10.2, the slope of the attenuation law in the UV is shallower than that seen locally, and the UV slope steepens as the mass increases. These trends are in qualitative agreement with Kriek & Conroy, who found that the wavelength dependence of attenuation varies with galaxy spectral type. However, we find no evidence of an extinction "bump" at 2175 A in any of the three stellar mass bins, or in the sample as a whole. We quantify the relation between the attenuation curve and stellar mass and discuss its implications.

astro-ph.GA↗

arXiv2015

Gamma Ray Bursts in the HAWC Era

Gamma-Ray Bursts are the most energetic explosions in the Universe, and are among the most promising for detecting multiple non-electromagnetic signals, including cosmic rays, high energy neutrinos and gravitational waves. The multi-GeV to TeV gamma-ray range of GRB could have significant contributions from hadronic interactions, mixed with more conventional leptonic contributions. This energy range is important for probing the source physics, including overall energetics, the shock parameters and the Lorentz factor. We discuss some of the latest observational and theoretical developments in the field.

astro-ph.HE↗

arXiv2012

The Co-evolution of Cosmic Entropy and Structures in the Universe

According to the second law of thermodynamics, the arrow of time points to an ever increasing entropy of the Universe. However, exactly how the entropy evolves with time and what drives the growth remain largely unknown. Here, for the first time, we quantify the evolving entropy of cosmic structures using a large-scale cosmological hydrodynamical simulation. Our simulation starts from initial conditions predicted by the leading LambdaCDM cosmology, self-consistently evolves the dynamics of both dark and baryonic matter, star formation, black hole growth and feedback processes, from the cosmic dawn to the present day. Tracing the entropy contributions of these distinct components in the simulation, we find a strong link between entropy growth and structure formation. The entropy is dominated by that of the black holes in all epochs, and its evolution follows the same path as that of galaxies: it increases rapidly from a low-entropy state at high redshift until z~2, then transits to a slower growth. Our results suggest that cosmic entropy may co-evolve with cosmic structure, and that its growth may be driven mainly by the formation of black holes in galaxies. We predict that the entropy will continue to increase in the near future, but likely at a constant rate.

astro-ph.CO↗

arXiv2011

Photometry of Some Recent Gamma-ray Bursts

We present the results of the optical, X-ray and gamma-ray analysis of some recent GRBs. The data were obtained by the automated P60 telescope and the Swift telescope (UVOT, XRT and BAT). We present some example fits for the lightcurves. The data reduction and the investigations were made by the Konkoly Observatory HEART group (http://www.konkoly.hu/HEART/index.html).

astro-ph.HE↗

arXiv2009

Discovery of Radio Afterglow from the Most Distant Cosmic Explosion

We report the discovery of radio afterglow emission from the gamma-ray burst GRB 090423, which exploded at a redshift of 8.3, making it the object with the highest known redshift in the Universe. By combining our radio measurements with existing X-ray and infrared observations, we estimate the kinetic energy of the afterglow, the geometry of the outflow and the density of the circumburst medium. Our best fit model is a quasi-spherical, high-energy explosion in a low, constant-density medium. \event had a similar energy release to the other well-studied high redshift GRB 050904 ($z=6.26$), but their circumburst densities differ by two orders of magnitude. We compare the properties of \event with a sample of GRBs at moderate redshifts. We find that the high energy and afterglow properties of \event are not sufficiently different from other GRBs to suggest a different kind of progenitor, such as a Population III star. However, we argue that it is not clear that the afterglow properties alone can provide convincing identification of Population III progenitors. We suggest that the millimeter and centimeter radio detections of \event at early times contained emission from a reverse shock component. This has important implications for the detection of high redshift GRBs by the next generation of radio facilities.

astro-ph.CO↗

arXiv2009

Astro2010 Decadal Survey Whitepaper: Coordinated Science in the Gravitational and Electromagnetic Skies

It is widely expected that the coming decade will witness the first direct detection of gravitational waves (GWs). The ground-based LIGO and Virgo GW observatories are being upgraded to advanced sensitivity, and are expected to observe a significant binary merger rate. The launch of The Laser Interferometer Space Antenna (LISA) would extend the GW window to low frequencies, opening new vistas on dynamical processes involving massive (M >~ 10^5 M_Sun) black holes. GW events are likely to be accompanied by electromagnetic (EM) counterparts and, since information carried electromagnetically is complementary to that carried gravitationally, a great deal can be learned about an event and its environment if it becomes possible to measure both forms of radiation in concert. Measurements of this kind will mark the dawn of trans-spectral astrophysics, bridging two distinct spectral bands of information. The aim of this whitepaper is to articulate future directions in both theory and observation that are likely to impact broad astrophysical inquiries of general interest. What will EM observations reflect on the nature and diversity of GW sources? Can GW sources be exploited as complementary probes of cosmology? What cross-facility coordination will expand the science returns of gravitational and electromagnetic observations?

astro-ph.CO↗