About

Nathaniel Butler is an associate professor and associate director of undergraduate initiatives at the School of Earth and Space Exploration. He is an astrophysicist with a focus on the properties and evolution of the early universe. His current research involves experiments to observe and study astrophysical explosions, using these phenomena to probe the first stars and galaxies and their environments. Butler's educational background includes a Ph.D. in Physics from the Massachusetts Institute of Technology and a B.A. in Physics from Princeton University. His work contributes to understanding the early universe through observational and experimental approaches related to astrophysical transients and cosmic reionization.

Research topics

• Astronomy
• Physics
• Astrophysics
• Computer Science
• Mathematics

Selected publications

• The EXoplanet Climate Infrared TElescope (EXCITE): A balloon-borne mission to measure spectroscopic phase curves of transiting hot Jupiters

  Open MIND · 2026-02-04

  preprint

  The EXoplanet Climate Infrared TElescope (EXCITE) is a balloon-borne mission dedicated to measuring spectroscopic phase curves of hot Jupiter-type exoplanets. Phase curve measurements can be used to characterize an exoplanet's longitude-dependent atmospheric composition and energy circulation patterns. EXCITE carries a 0.5 m primary mirror and moderate resolution diffraction-limited spectrograph with spectral coverage from 0.8--3.5 um. EXCITE is designed to fly from a long-duration balloon (LDB). EXCITE will observe through the peak of a target's spectral energy distribution (SED) and through spectral signatures of hydrogen and carbon-containing molecules. In this paper, we present the science goals of EXCITE, detail the as-built instrument, and discuss its performance during a 2024 engineering flight from Fort Sumner, New Mexico.

  DOI

• The EXoplanet Climate Infrared TElescope (EXCITE): A balloon-borne mission to measure spectroscopic phase curves of transiting hot Jupiters

  ArXiv.org · 2026-02-04

  articleOpen access

  The EXoplanet Climate Infrared TElescope (EXCITE) is a balloon-borne mission dedicated to measuring spectroscopic phase curves of hot Jupiter-type exoplanets. Phase curve measurements can be used to characterize an exoplanet's longitude-dependent atmospheric composition and energy circulation patterns. EXCITE carries a 0.5 m primary mirror and moderate resolution diffraction-limited spectrograph with spectral coverage from 0.8--3.5 um. EXCITE is designed to fly from a long-duration balloon (LDB). EXCITE will observe through the peak of a target's spectral energy distribution (SED) and through spectral signatures of hydrogen and carbon-containing molecules. In this paper, we present the science goals of EXCITE, detail the as-built instrument, and discuss its performance during a 2024 engineering flight from Fort Sumner, New Mexico.

  PublisherOA PDF

• Evidence of energy injection in the short and distant GRB 250221A in a high density environment

  Monthly Notices of the Royal Astronomical Society · 2026-01-27 · 1 citations

  preprintOpen access

  ABSTRACT We present the photometric and spectroscopic analysis of the short-duration GRB 250221A ($T_{90}=1.80\pm 0.32$ s), using a data set from the optical facilities COLIBRÍ, the Harlingten 50 cm Telescope, and the Very Large Telescope. We complement these observations with data from the Neil Gehrels Swift Observatory and the Einstein Probe, as well as radio observations from the Very Large Array. GRB 250221A is among the few short GRBs with direct afterglow spectroscopy, which gives a secure redshift determination of $z=0.768$ and allows the unambiguous identification of the host as a galaxy with a star formation rate of $\sim 3\, \mathrm{M}_\odot \, {\rm yr}^{-1}$. The X-ray and optical light curves up to $T_0+3\times 10^4$ s (where $T_0$ refers to the GRB trigger time) are well described by forward-shock synchrotron emission in the slow-cooling regime within the standard fireball framework. However, at $T_0 \sim 5\times 10^4$ s, both the X-ray and optical bands exhibit an excess over the same interval, which we interpret as evidence of energy injection into a jet with a half-opening angle of $\theta _j=11.5^{\circ }$ through a refreshed shock powered by late central engine activity or a radially stratified ejecta. The burst properties (duration, spectral hardness, peak energy, and location in the Amati plane) all favour a compact binary merger origin. However, our modelling of the afterglow suggests a dense circumburst medium ($n\sim 80$ cm$^{-3}$), which is more typical of a collapsar environment.

  PublisherDOI

• Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-significance Gravitational Wave Event S250818k

  The Astrophysical Journal Letters · 2025-11-25 · 4 citations

  articleOpen accessCorresponding

  Abstract Kilonovae, the ultraviolet/optical/infrared counterparts to binary neutron star mergers, are an exceptionally rare class of transients. Optical follow-up campaigns are plagued by contaminating transients, which may mimic kilonovae but do not receive sufficient observations to measure the full photometric evolution. In this work, we present an analysis of the multiwavelength dataset of supernova (SN) 2025ulz, a proposed kilonova candidate following the low-significance detection of gravitational waves originating from the potential binary neutron star merger S250818k. Despite an early rapid decline in brightness, our multiwavelength observations of SN 2025ulz reveal that it is a type IIb SN. As part of this analysis, we demonstrate the capabilities of a novel quantitative scoring algorithm to determine the likelihood that a transient candidate is a kilonova, based primarily on its three-dimensional location and light-curve evolution. We also apply our scoring algorithm to other transient candidates in the localization volume of S250818k and find that, at all times after the discovery of SN 2025ulz, there are ≥4 candidates with a score comparable to SN 2025ulz, indicating that the kilonova search may have benefited from the additional follow-up of other candidates. During future kilonova searches, this type of scoring algorithm will be useful to rule out contaminating transients in real time, optimizing the use of valuable telescope resources.

  PublisherDOI

• Assembly, integration, and laboratory testing of the EXCITE spectrograph (Erratum)

  2024-11-08

  erratumOpen access

  Publisher's Note: This paper, originally published on 30 July 2024, was replaced with a corrected/revised version on 8 November 2024. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

  PublisherOA PDFDOI

• PTF10iya: a short-lived, luminous flare from the nuclear region of a star-forming galaxy

  Figshare · 2024-01-01 · 58 citations

  articleOpen access

  We present the discovery and characterization of PTF10iya, a short-lived (Δt≈ 10 d, with an optical decay rate of ∼0.3 mag d−1), luminous ( mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fitted by a blackbody with T≈ (1–2) × 104 K and peak bolometric luminosity LBB≈ (1–5) × 1044 erg s−1 (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z= 0.224 05 ± 0.000 06) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic ‘big blue bump’ seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early ‘super-Eddington’ phase of a solar-type star being disrupted by a ∼107 M⊙ black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence.

  PublisherDOI

• Assembly, integration, and testing of the Star-Planet Activity Research CubeSat (SPARCS)

  2024-06-17

  article

  We discuss the final assembly, integration, and testing of the Star-Planet Activity Research CubeSat. SPARCS is a 6U CubeSat mission designed to monitor the dual-channel, far-UV (153-176 nm) and near-UV (258-308 nm) photometric activity of nearby low mass stars to advance our understanding of their evolution, activity, and the habitability of surrounding exoplanets. This paper details the assembly of the SPARCS instrument and the testing process to characterize and validate the performance of the payload prior to spacecraft integration. To test SPARCS, we have established a customized CubeSat AIT laboratory and thermal vacuum chamber at ASU equipped to handle CubeSats requiring meticulous contamination control for work in the FUV. After a brief overview of these facilities and the testing plan, we will detail the methods and data used to verify the performance of SPARCS and generate calibration products to reduce raw flight data to high-quality science products. The result will be the delivery of the first highly sensitive FUV astrophysics CubeSat which will inform exoplanet environments and future observations of these systems by facilities like the Habitable Worlds Observatory.

  PublisherDOI

• The Exoplanet Climate Infrared Telescope (EXCITE): gondola pointing and stabilization qualification

  2024-06-14 · 1 citations

  article

  High precision sub-arcsecond pointing stability has become a capability widely utilized in the balloon-borne community, in particular for high resolution optical systems. However, many of these applications are also pushing the state-of-the-art with regards to detector technology, many forms of which require some level of cryogenic cooling and active dissipative cooling systems to achieve target performance specifications. Built on the success of the Super-pressure Balloon-borne Imaging Telescope (SuperBIT) experiment, we present the results of improved technologies and design methodologies applied to the EXoplanet Infrared TElescope (EXCITE), which uses active cryogenic systems to achieve detector performance while requiring pointing stability at the 100 milliarcsecond level. Results from EXCITE's recent balloon-borne campaign are presented within the context of Super-pressure Balloon (SPB) and Long Duration Balloon (LDB) applications.

  PublisherDOI

• Taxonomy of Subkilometer Near-Earth Objects from Multiwavelength Photometry with RATIR

  The Astronomical Journal · 2024-03-18 · 2 citations

  articleOpen access

  Abstract We present results from observations of 238 near-Earth objects (NEOs) obtained with the RATIR instrument on the 1.5 m robotic telescope at San Pedro Martir’s National Observatory in Mexico, in the frame of our multiobservatory, multifilter campaign. Our project is focused on rapid response photometric observations of NEOs with absolute magnitudes in the range 18.1–27.1 (diameter ≈ 600 and 10 m, respectively). Data with coverage in the near-infrared and visible range were analyzed with a nonparametric classification algorithm, while visible-only data were independently analyzed via Monte Carlo simulations and a 1-Nearest Neighbor method. The rapid response and the use of spectrophotometry allows us to obtain taxonomic classifications of subkilometer objects with small telescopes, representing a convenient characterization strategy. We present taxonomic classifications of the 87 objects observed in the visible and near-infrared. We also present the taxonomic distribution of an additional 151 objects observed in the visible. Our most accurate method suggests a nonfeatured-to-featured ratio of ≈0.75, which is consistent with the value found by the Mission Accessible Near-Earth Object Survey, which conducted a similar study using a spectral analysis. The results from the Monte Carlo method suggest a ratio of ≈0.8, although this method has some limitations. The 1-Nearest Neighbor method showed to be not suitable for NEO classifications.

  PublisherOA PDFDOI

• An optical gamma-ray burst catalogue with measured redshift – I. Data release of 535 gamma-ray bursts and colour evolution

  Monthly Notices of the Royal Astronomical Society · 2024-06-20 · 18 citations

  articleOpen access

  ABSTRACT We present the largest optical photometry compilation of gamma-ray bursts (GRBs) with redshifts (z). We include 64813 observations of 535 events (including upper limits) from 28 February 1997 to 18 August 2023. We also present a user-friendly web tool grbLC which allows users to visualize photometry, coordinates, redshift, host galaxy extinction, and spectral indices for each event in our database. Furthermore, we have added a Gamma-ray Coordinate Network (GCN) scraper that can be used to collect data by gathering magnitudes from the GCNs. The web tool also includes a package for uniformly investigating colour evolution. We compute the optical spectral indices for 138 GRBs, for which we have at least four filters at the same epoch in our sample, and craft a procedure to distinguish between GRBs with and without colour evolution. By providing a uniform format and repository for the optical catalogue, this web-based archive is the first step towards unifying several community efforts to gather the photometric information for all GRBs with known redshifts. This catalogue will enable population studies by providing light curves (LCs) with better coverage since we have gathered data from different ground-based locations. Consequently, these LCs can be used to train future LC reconstructions for an extended inference of the redshift. The data gathering also allows us to fill some of the orbital gaps from Swift in crucial points of the LCs, e.g. at the end of the plateau emission or where a jet break is identified.

  PublisherOA PDFDOI

Frequent coauthors

• J. S. Bloom

  New York University

  428 shared

• E. Troja

  University of Rome Tor Vergata

  374 shared

• A. Kutyrev

  Goddard Space Flight Center

  366 shared

• A. M. Watson

  361 shared

• J. X. Prochaska

  328 shared

• E. Ramirez‐Ruiz

  University of California, Santa Cruz

  310 shared

• C. Román-Zúñiga

  310 shared

• M. G. Richer

  307 shared

Education

• Ph.D.

  Massachusetts Institute of Technology

  2003

• B.A.

  Princeton University

  1998