arXiv subjects

Nicholas E. White

Publications and source records attributed to Nicholas E. White.

11 recordsLinked to original sources

arXiv2026

A Proliferated Space Architecture for Time-Domain Astrophysics

Time-Domain and Multi-Messenger Astrophysics (TDAMM) is entering a discovery-rich but follow-up-limited era, creating an urgent need for responsive, multiwavelength space-based capabilities. The Hydra constellation is a concept for a proliferated space architecture for time-domain astrophysics. The constellation would act as a disaggregated observatory composed of coordinated, relatively low-cost spacecraft that collectively provide capabilities traditionally concentrated within a single large mission. The architecture would combine persistent wide-field gamma-ray monitoring, wide-field and focused X-ray observations, and rapid-response ultraviolet, optical, and infrared imaging and spectroscopy. The constellation would both discover high-energy transients and respond to external alerts from gravitational-wave detectors, neutrino observatories, and ground- and space-based surveys, using low-latency communications, automated event prioritization, and community coordination frameworks to rapidly assign observing resources. A proliferated architecture would offer operational advantages over a single larger mission, including simultaneous observations of multiple targets, graceful degradation following individual spacecraft failures, recurring technology refresh, and opportunities for commercial, international, and philanthropic contributed nodes to join the network. The constellation would address fundamental questions concerning cosmic accelerators, the origin and evolution of the elements, the behavior of matter at extreme density, and the nature of dark energy through gravitational-wave standard sirens. This white paper presents the Hydra concept description that was submitted to NASA's ASTRA initiative for consideration by the Cosmic Origins Program Analysis Group (CoPAG) and Physics of the Cosmos Program Analysis Group (PhysPAG).

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arXiv2026

Optimizing Field Geometry for Modular Micropore-Optics Soft X-ray Time-Domain Surveys

Wide-field micropore (lobster eye) optics enable focusing soft X-ray instruments with fields of view of hundreds of square degrees, but square-format modules require careful geometric arrangement to support both wide-area surveys and sensitive pointed observations. We examine this design problem using the X-ray TRansient Array / Micropore Optics X-ray Imager trade study, developed for a NASA Small Explorer mission concept, as a worked example. Single-module performance scaling is combined with geometric survey simulations to compare modular field layouts, overlap regions, and repeated survey tilings. For realistic micropore-optic parameters, increasing module size primarily expands the field of view once the boresight-centered effective area saturates, making modular architecture and field geometry central design variables. A three-module tri-petal configuration, formed by differentially rolling and offsetting three 10 x 10 sq deg square fields, produces a compound field of approximately 240 sq deg with one-module discovery regions, two-module bridge regions, and a compact three-module core. When repeated in a dense hexagonal lattice, the footprint closes gaps, builds a more uniform exposure floor, and provides enhanced-depth survey regions. This geometry offers an efficient compromise among survey coverage, pointed sensitivity, system complexity, and resilience, and can be scaled to larger persistent soft-X-ray monitoring architectures.

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arXiv2026

The Geometric Crisis in Cygnus~X-3: Limitations of Wind-Fed Accretion and the Case for Roche-Lobe Overflow

Cygnus~X-3 is a Galactic X-ray binary with a 4.8-hr orbital period operating in the ultraluminous regime. Although the system is viewed at relatively low inclination ($i\approx28^\circ$), it exhibits a deep orbital modulation. Recent IXPE observations show strong linear polarization orthogonal to the radio jet, indicating that the X-ray emission is dominated by reflection from the inner walls of a supercritical outflow funnel. We propose a Hybrid Roche-lobe overflow (RLOF) scenario in which a massive Wolf-Rayet donor effectively fills its Roche lobe with a focused wind driving a super-Eddington accretion stream. Using a numerical synthesis of the folded light curve, we show that the modulation is reproduced when the central funnel is periodically occulted by a vertically extended, shock-heated Turbulent Wall formed by stream impact on the outer disk rim. This produces a phase lag ($Δϕ\approx0.11$) between X-ray minimum and binary conjunction, with extended attenuation by the WR wind defining a broader Suppression Region. This geometry explains the enhanced iron-line equivalent width during X-ray minimum via a coronagraphic effect. The large radial-velocity amplitude of FeXXVI measured by XRISM ($K_{\rm obs}\approx430$ km s$^{-1}$) and its zero-crossing at $ϕ_X=0.0$ arise naturally in the stream-impact region rather than from orbital motion of the compact object. Finally, we show that the observed secular orbital expansion ($\dot P>0$) follows directly from highly non-conservative mass transfer with inner-disk mass loss, indicating that Cygnus~X-3 is a stable, long-lived system in a supercritical accretion regime.

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arXiv2020

The Gamow Explorer: A Gamma-Ray Burst Mission to Study the High Redshift Universe

Long Gamma Ray Bursts (LGRBs) can be used to address key questions on the formation of the modern universe including: How does the star formation rate evolve at high redshift? When and how did the intergalactic medium become re-ionized? What processes governed its early chemical enrichment? A LGRB signals when a massive star collapses to form a black hole and in doing so provides an independent tracer of the star formation rate. The LGRB afterglow is a bright back-light to view the host galaxy and intergalactic medium in absorption. The Gamow Explorer will be optimized to search for high redshift LGRBs, with a z>6 detection rate at least ten times the Neil Gehrels Swift Observatory. Furthermore it will go beyond Swift by using the photo-z technique to autonomously identify >80% of z>6 redshift LGRBs to enable rapid follow up by large ground based telescopes and JWST for spectroscopy and host galaxy identification. The Gamow Explorer will be proposed to the 2021 NASA MIDEX opportunity for launch in 2028.

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arXiv2010

The International X-ray Observatory

The International X-ray Observatory (IXO) is a joint ESA-JAXA-NASA effort to address fundamental and timely questions in astrophysics: What happens close to a black hole? How did supermassive black holes grow? How does large scale structure form? What is the connection between these processes? To address these questions IXO will employ optics with 3 sq m collecting area and 5 arc sec angular resolution - 20 times more collecting area at 1 keV than any previous X-ray observatory. Focal plane instruments will deliver a 100-fold increase in effective area for high-resolution spectroscopy, deep spectral imaging over a wide field of view, unprecedented polarimetric sensitivity, microsecond spectroscopic timing, and high count rate capability. The mission is being planned for launch in 2021 to an L2 orbit, with a five-year lifetime and consumables for 10 years.

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arXiv2005

Discovery of Extremely Embedded X-ray Sources in the R Coronae Australis Star Forming Core

With the XMM-Newton and Chandra observatories, we detected two extremely embedded X-ray sources in the R Corona Australis (R CrA) star forming core, near IRS 7. These sources, designated as XB and XA, have X-ray absorption columns of ~3e23 cm-2 equivalent to AV ~180 mag. They are associated with the VLA centimeter radio sources 10E and 10W, respectively. XA is the counterpart of the near-infrared source IRS 7, whereas XB has no K-band counterpart above 19.4 mag. This indicates that XB is younger than typical Class I protostars, probably a Class 0 protostar or in an intermediate phase between Class 0 and Class I. The X-ray luminosity of XB varied between 29<log LX <31.2 ergs s-1 on timescales of 3-30 months. XB also showed a monotonic increase in X-ray brightness by a factor of two in 30 ksec during an XMM-Newton observation. The XMM-Newton spectra indicate emission from a hot plasma with kT ~3-4 keV and also show fluorescent emission from cold iron. Though the X-ray spectrum from XB is similar to flare spectra from Class I protostars in luminosity and temperature, the light curve does not resemble the lightcurves of magnetically generated X-ray flares because the variability timescale of XB is too long and because variations in X-ray count rate were not accompanied by variations in spectral hardness. The short-term variation of XB may be caused by the partial blocking of the X-ray plasma, while the month-long flux enhancement may be driven by mass accretion.

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arXiv2002

NASA's Future Missions in X-ray Astronomy

The NASA program in X-ray astronomy has two long term goals: 1) to achieve sufficient angular resolution to image the event horizon of a black hole (0.1 micro arc sec) and 2) to achieve sufficient collecting area (50-150 sq m) and angular resolution (0.1-1.0 arc sec) to observe in detail the first black holes and galaxies at high redshift. These ambitous goals can be used to map out a series of missions and a technology program. The next major mission will be Constellation-X which will be dedicated to high resolution X-ray spectroscopy for launch in ~2010. This mission is a critical step in the roadmap to achieve these goals. Following Constellation-X NASA is considering two very ambitious vision missions: MAXIM and Generation-X that will achieve the ultimate capabilities. The modest missions Astro-E2 and Swift address more focussed science goals on a rapid development cycle and provide important pathfinders to the larger missions.

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arXiv2001

The Discovery of a Second Luminous Low Mass X-ray Binary in the Globular Cluster M15

We report an observation by the Chandra X-ray Observatory of 4U2127+119, the X-ray source identified with the globular cluster M15. The Chandra observation reveals that 4U2127+119 is in fact two bright sources, separated by 2.7". One source is associated with AC211, the previously identified optical counterpart to 4U2127+119, a low mass X-ray binary (LMXB). The second source, M15-X2, is coincident with a 19th U magnitude blue star that is 3.3" from the cluster core. The Chandra count rate of M15-X2 is 2.5 times higher than that of AC211. Prior to the 0.5" imaging capability of Chandra the presence of two so closely separated bright sources would not have been resolved. The optical counterpart, X-ray luminosity and spectrum of M15-X2 are consistent with it also being an LMXB system. This is the first time that two LMXBs have been seen to be simultaneously active in a globular cluster. The discovery of a second active LMXB in M15 solves a long standing puzzle where the properties of AC211 appear consistent with it being dominated by an extended accretion disk corona, and yet 4U2127+119 also shows luminous X-ray bursts requiring that the neutron star be directly visible. The resolution of 4U2127+119 into two sources suggests that the X-ray bursts did not come from AC211, but rather from M15-X2. We discuss the implications of this discovery for understanding the origin and evolution of LMXBs in GCs as well as X-ray observations of globular clusters in nearby galaxies.

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arXiv2001

X-Ray Probes of Cosmic Star-Formation History

We discuss the imprints left by a cosmological evolution of the star formation rate (SFR) on the evolution of X-ray luminosities Lx of normal galaxies, using the scheme proposed by White and Ghosh (1998, WG98), wherein the evolution of Lx of a galaxy is driven by the evolution of its X-ray binary population. As indicated in WG98, the profile of Lx with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. We report here the first calculation of the expected evolution of X-ray luminosities of galaxies, updating the WG98 work by using a suite of more recently developed SFR profiles that span the currently plausible range. The first Chandra deep imaging results on Lx are beginning to probe the SFR profile of bright spirals: the early results are consistent with predictions based on current SFR models. Using these new SFR profiles, the resolution of the ``birthrate problem'' of low-mass X-ray binaries (LMXBs) and recycled, millisecond pulsars (WG98) in terms of an evolving global SFR is more complete. We discuss the possible impact of the variations in the SFR profile of individual galaxies and galaxy-types.

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arXiv1998

Low-Mass X-Ray Binaries, Millisecond Radio Pulsars, and the Cosmic Star Formation Rate

We report on the implications of the peak in the cosmic star-formation rate (SFR) at redshift z ~ 1.5 for the resulting population of low-mass X-ray binaries(LMXB) and for that of their descendants, the millisecond radio pulsars (MRP). Since the evolutionary timescales of LMXBs, their progenitors, and their descendants are thought be significant fractions of the time-interval between the SFR peak and the present epoch, there is a lag in the turn-on of the LMXB population, with the peak activity occurring at z ~ 0.5 - 1.0. The peak in the MRP population is delayed further, occurring at z < 0.5. We show that the discrepancy between the birthrate of LMXBs and MRPs, found under the assumption of a stead-state SFR, can be resolved for the population as a whole when the effects of a time-variable SFR are included. A discrepancy may persist for LMXBs with short orbital periods, although a detailed population synthesis will be required to confirm this. Further, since the integrated X-ray luminosity distribution of normal galaxies is dominated by X-ray binaries, it should show strong luminosity evolution with redshift. In addition to an enhancement near the peak (z ~ 1.5) of the SFR due to the prompt turn-on of the relatively short-lived massive X-ray binaries and young supernova remnants, we predict a second enhancement by a factor ~10 at a redshift between ~ 0.5 and ~ 1 due to the delayed turn-on of the LMXB population. Deep X-ray observations of galaxies out to z ~ 1 by AXAF will be able to observe this enhancement, and, by determining its shape as a function of redshift, will provide an important new method for constraining evolutionary models of X-ray binaries.

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arXiv1998

On the Amplitude of Burst Oscillations in 4U 1636-54: Evidence for Nuclear Powered Pulsars

We present a study of 581 Hz oscillations observed during a thermonuclear X-ray burst from the low mass X-ray binary (LMXB) 4U 1636-54 with the Rossi X-ray Timing Explorer (RXTE). We argue that the combination of large pulsed amplitudes near burst onset and the spectral evidence for localized emission during the rise strongly supports rotational modulation as the mechanism for the oscillations. We discuss how theoretical interpretation of spin modulation amplitudes, pulse profiles and pulse phase spectroscopy can provide constraints on the masses and radii of neutron stars. We also discuss the implication of these findings for the beat frequency models of kHz X-ray variability in LMXB.

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