arXiv subjects

Kristin K. Madsen

Publications and source records attributed to Kristin K. Madsen.

At least 19 recordsLinked to original sources

arXiv2025

Measuring the Evolution of the NuSTAR Detector Gains

The memo describes the methods used to track the long-term gain variations in the NuSTAR detectors. It builds on the analysis presented in Madsen et al. (2015) using the deployable calibration source to measure the gain drift in the NuSTAR CdZnTe detectors. This is intended to be a live document that is periodically updated as new entries are required in the NuSTAR gain CALDB files. This document covers analysis up through late-2024 and the gain v011 CALDB file released in version 20240226.

astro-ph.IM↗

arXiv2023

The High Energy X-ray Probe (HEX-P): Instrument and Mission Profile

The High Energy X-ray Probe is a proposed NASA probe-class mission that combines the power of high angular resolution with a broad X-ray bandpass to provide the necessary leap in capabilities to address the important astrophysical questions of the next decade. HEX-P achieves breakthrough performance by combining technologies developed by experienced international partners. HEX-P will be launched into L1 to enable high observing efficiency. To meet the science goals, the payload consists of a suite of co-aligned X-ray telescopes designed to cover the 0.2 - 80 keV bandpass. The High Energy Telescope (HET) has an effective bandpass of 2 - 80 keV, and the Low Energy Telescope (LET) has an effective bandpass of 0.2 - 20 keV. The combination of bandpass and high observing efficiency delivers a powerful platform for broad science to serve a wide community. The baseline mission is five years, with 30% of the observing time dedicated to the PI-led program and 70% to a General Observer (GO) program. The GO program will be executed along with the PI-led program.

astro-ph.IM↗

arXiv2022

Orbital decay in M82 X-2

M82 X-2 is the first pulsating ultraluminous X-ray source (PULX) discovered. The luminosity of these extreme pulsars, if isotropic, implies an extreme mass transfer rate. An alternative is to assume a much lower mass transfer rate, but with an apparent luminosity boosted by geometrical beaming. Only an independent measurement of the mass transfer rate can help discriminate between these two scenarios. In this Paper, we follow the orbit of the neutron star for seven years, measure the decay of the orbit ($\dot{P}_{orb}/{P}_{orb}\approx-8\cdot10^{-6}\mathrm{yr}^{-1}$), and argue that this orbital decay is driven by extreme mass transfer of more than 150 times the mass transfer limit set by the Eddington luminosity. If this is true, the mass available to the accretor is more than enough to justify its luminosity, with no need for beaming. This also strongly favors models where the accretor is a highly-magnetized neutron star.

astro-ph.HE↗

arXiv2022

Reconstruction of the NuSTAR point spread function using single-laser metrology

This paper describes a method by which the metrology system of the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray space observatory, which uses two lasers to characterize the relative motion of the optics and focal plane benches, can be approximated should one laser fail. The two benches are separated by a ten-meter-long rigid mast that undergoes small amounts of thermal flexing which need to be compensated for in order to produce a non-blurred image. We analyze the trends of mast motion by archival observation parameters in order to discover whether the mast motion in future observations can be predicted. We find that, by using the solar aspect angle (SAA), observation date, and orbital phase, we can simulate the motion of one laser by translating the track produced by the other and applying modifications to the resulting mast aspect solution, allowing the reconstruction of a minimally distorted point spread function in most cases. We will implement the generation of simulated mast files alongside the usual NuSTAR data reduction pipeline for contingency purposes. This work has implications for reducing the risk of implementing laser metrology systems on future missions that use deployable masts to achieve the long focal lengths required in high-energy astronomy by mitigating the impact of a metrology laser failure in the extended phase of a mission.

astro-ph.IM↗

arXiv2021

2021 Effective Area calibration of the Nuclear Spectroscopic Telescope ARray (NuSTAR)

We present here the updated calibration of The Nuclear Spectroscopic Telescope ARray NuSTAR, which was performed using data on the Crab accumulated over the last 9 years in orbit. The basis for this new calibration contains over 250ks of focused Crab (imaged through the optics) and over 500ks of stray-light Crab (not imaged through optics). We measured an epoch averaged Crab spectrum of the stray-light Crab data and define a canonical Crab spectrum of Gamma = 2.103 +- 0.001 and N = 9.69 +- 0.02 keV-1 cm-2 s-1 at 1 keV, which we use as our calibration standard. The new calibration, released in the CALDB update 20211020, provides significant updates to: 1) the detector absorption component, 2) the detector response function, and 3) the effective area vignetting function. The calibration improves agreement between FPMA and FPMB across detectors with a standard deviation of 1.7% for repeat observations between off-axis angles of 1-4 arcmin, and the measured flux has increased by 5-15%, with 5% below 1 arcmin off-axis angle, 10% between 1-2 arcmin, and 15 above 4arcmin.

astro-ph.IM↗

arXiv2021

Photospheric Radius Expansion and a double-peaked type-I X-ray burst from GRS 1741.9-2853

We present analysis of two type-I X-ray bursts observed by NuSTAR originating from the very faint transient neutron star low-mass X-ray binary GRS 1741.9-2853 during a period of outburst in May 2020. We show that the persistent emission can be modeled as an absorbed, Comptonized blackbody in addition to Fe K$α$ emission which can be attributed to relativistic disk reflection. We measure a persistent bolometric, unabsorbed luminosity of $L_{\mathrm{bol}}=7.03^{+0.04}_{-0.05}\times10^{36}\,\mathrm{erg\,s^{-1}}$, assuming a distance of 7 kpc, corresponding to an Eddington ratio of $4.5\%$. This persistent luminosity combined with light curve analysis leads us to infer that the bursts were the result of pure He burning rather than mixed H/He burning. Time-resolved spectroscopy reveals that the bolometric flux of the first burst exhibits a double-peaked structure, placing the source within a small population of accreting neutron stars which exhibit multiple-peaked type-I X-ray bursts. We find that the second, brighter burst shows evidence for photospheric radius expansion (PRE) and that at its peak, this PRE event had an unabsorbed bolometric flux of $F_{\mathrm{peak}}=2.94^{+0.28}_{-0.26}\times10^{-8}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. This yields a new distance estimate of $d=9.0\pm0.5$ kpc, assuming that this corresponds to the Eddington limit for pure He burning on the surface of a canonical neutron star. Additionally, we performed a detailed timing analysis which failed to find evidence for quasiperiodic oscillations or burst oscillations, and we place an upper limit of $16\%$ on the rms variability around 589 Hz, the frequency at which oscillations have previously been reported.

astro-ph.HE↗

arXiv2021

Timing Calibration of the NuSTAR X-ray Telescope

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission is the first focusing X-ray telescope in the hard X-ray (3-79 keV) band. Among the phenomena that can be studied in this energy band, some require high time resolution and stability: rotation-powered and accreting millisecond pulsars, fast variability from black holes and neutron stars, X-ray bursts, and more. Moreover, a good alignment of the timestamps of X-ray photons to UTC is key for multi-instrument studies of fast astrophysical processes. In this Paper, we describe the timing calibration of the NuSTAR mission. In particular, we present a method to correct the temperature-dependent frequency response of the on-board temperature-compensated crystal oscillator. Together with measurements of the spacecraft clock offsets obtained during downlinks passes, this allows a precise characterization of the behavior of the oscillator. The calibrated NuSTAR event timestamps for a typical observation are shown to be accurate to a precision of ~65 microsec.

astro-ph.IM↗

arXiv2021

StrayCats: A catalog of NuSTAR Stray Light Observations

We present StrayCats: a catalog of NuSTAR stray light observations of X-ray sources. Stray light observations arise for sources 1--4$^{\circ}$ away from the telescope pointing direction. At this off-axis angle, X-rays pass through a gap between optics and aperture stop and so do not interact with the X-ray optics but, instead, directly illuminate the NuSTAR focal plane. We have systematically identified and examined over 1400 potential observations resulting in a catalog of 436 telescope fields and 78 stray light sources that have been identified. The sources identified include historically known persistently bright X-ray sources, X-ray binaries in outburst, pulsars, and Type I X-ray bursters. In this paper we present an overview of the catalog and how we identified the StrayCats sources and the analysis techniques required to produce high level science products. Finally, we present a few brief examples of the science quality of these unique data.

astro-ph.HE↗

arXiv2020

NuSTAR low energy effective area correction due to thermal blanket tear

A rip in the MLI at the exit aperture of OMA, the NuSTAR optic aligned with detector focal plane module FPMA, has resulted in an increased photon flux through OMA that has manifested itself as a low energy excess. Overall, the MLI coverage has decreased by 10%, but there is an additional time-varying component, which occasionally causes the opening to increase by up to 20%. We address the problem with a calibration update, and in this paper, we describe the attributes of the problem, the implications it has on data analysis, and the solution.

astro-ph.IM↗

arXiv2020

Evidence of Particle Acceleration in the Superbubble 30 Doradus C with NuSTAR

We present evidence of diffuse, non-thermal X-ray emission from the superbubble 30 Doradus C (30 Dor C) using hard X-ray images and spectra from NuSTAR observations. For this analysis, we utilize data from a 200 ks targeted observation of 30 Dor C as well as 2.8 Ms of serendipitous off-axis observations from the monitoring of nearby SN 1987A. The complete shell of 30 Dor C is detected up to 20 keV, and the young supernova remnant MCSNR J0536-6913 in the southeast of 30 Dor C is not detected above 8 keV. Additionally, six point sources identified in previous Chandra and XMM-Newton investigations have hard X-ray emission coincident with their locations. Joint spectral fits to the NuSTAR and XMM-Newton spectra across the 30 Dor C shell confirm the non-thermal nature of the diffuse emission. Given the best-fit rolloff frequencies of the X-ray spectra, we find maximum electron energies of 70-110 TeV (assuming a B-field strength of 4$μ$G), suggesting 30 Dor C is accelerating particles. Particles are either accelerated via diffusive shock acceleration at locations where the shocks have not stalled behind the H$α$ shell, or cosmic-rays are accelerated through repeated acceleration of low-energy particles via turbulence and magnetohydrodynamic waves in the bubble's interior.

astro-ph.HE↗

arXiv2019

Observing the transient pulsations of SMC X-1 with NuSTAR

We report on NuSTAR observations of transient pulsations in the neutron star X-ray binary SMC X-1. The transition from non-pulsing to pulsing states was not accompanied by a large change in flux. Instead, both pulsing and non-pulsing states were observed in a single observation during the low-flux super-orbital state. During the high-state, we measure a pulse period of $P = 0.70117(9)\,\mathrm{s}$ at $T_{ref} = 56145\,\mathrm{MJD}$. Spectral analysis during non-pulsing and pulsing states reveals that the observations can be consistently modeled by an absorbed power law with a phenomenological cutoff resembling a Fermi-Dirac distribution, or by a partially obscured cutoff power law. The shapes of the underlying continua show little variability between epochs, while the covering fraction and column density vary between super-orbital states. The strength of pulsations also varies, leading us to infer that the absence and reemergence of pulsations are related to changing obscuration, such as by a warped accretion disk. SMC X-1 is accreting near or above its Eddington limit, reaching an unabsorbed X-ray luminosity of $L_{\rm X}({\rm 2-10~keV}) \approx 5 \times 10^{38}\, {\rm erg}\, {\rm s}^{-1}$. This suggests that SMC X-1 may be a useful local analog to ultraluminous X-ray pulsars (ULXPs), which likewise exhibit strong variability in their pulsed fractions, as well as flux variability on similar timescales. In particular, the gradual pulse turn-on which has been observed in M82 X-2 is similar to the behavior we observe in SMC X-1. Thus we propose that pulse fraction variability of ULXPs may also be due to variable obscuration.

astro-ph.HE↗

arXiv2018

Reflection Spectra of the Black Hole Binary Candidate MAXI J1535-571 in the Hard State Observed by NuSTAR

We report on a NuSTAR observation of the recently discovered bright black hole candidate MAXI J1535-571. NuSTAR observed the source on MJD 58003 (five days after the outburst was reported). The spectrum is characteristic of a black hole binary in the hard state. We observe clear disk reflection features, including a broad Fe K$α$ line and a Compton hump peaking around 30 keV. Detailed spectral modeling reveals narrow Fe K$α$ line complex centered around 6.5 keV on top of the strong relativistically broadened Fe K$α$ line. The narrow component is consistent with distant reflection from moderately ionized material. The spectral continuum is well described by a combination of cool thermal disk photons and a Comptonized plasma with the electron temperature $kT_{\rm e}=19.7\pm{0.4}$ keV. An adequate fit can be achieved for the disk reflection features with a self-consistent relativistic reflection model that assumes a lamp-post geometry for the coronal illuminating source. The spectral fitting measures a black hole spin $a>0.84$, inner disk radius $R_{\rm in}<2.01~r_{\rm ISCO}$, and a lamp-post height $h=7.2^{+0.8}_{-2.0} r_{\rm g}$ (statistical errors, 90% confidence), indicating no significant disk truncation and a compact corona. Although the distance and mass of this source are not currently known, this suggests the source was likely in the brighter phases of the hard state during this NuSTAR observation.

astro-ph.HE↗

arXiv2017

First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events

We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the \textit{Nuclear Spectroscopic Telescope Array} (\textit{NuSTAR}) satellite. While \textit{NuSTAR} was designed as an astrophysics mission, it can observe the Sun above 2~keV with unprecedented sensitivity due to its pioneering use of focusing optics. \textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun transient brightenings on time scales of 100 s and set upper limits on emission in two energy bands. We set 2.5--4~keV limits on brightenings with time scales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10--20~keV limits on brightenings with time scales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the \textit{NuSTAR} sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background.

astro-ph.SR↗

arXiv2017

Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During its 2016 Outburst

We present a spectral and timing study of the NuSTAR and Swift observations of the black hole candidate IGR J17091-3624 in the hard state during its outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra taken in three epochs. Fitting with relativistic reflection models reveals that the accretion disk is truncated during all epochs with $R_{\rm in}>10~r_{\rm g}$, with the data favoring a low disk inclination of $\sim 30^{\circ}-40^{\circ}$. The steepening of the continuum spectra between epochs is accompanied by a decrease in the high energy cut-off: the electron temperature $kT_{\rm e}$ drops from $\sim 64$ keV to $\sim 26$ keV, changing systematically with the source flux. We detect type-C QPOs in the power spectra with frequency varying between 0.131 Hz and 0.327 Hz. In addition, a secondary peak is found in the power spectra centered at about 2.3 times the QPO frequency during all three epochs. The nature of this secondary frequency is uncertain, however a non-harmonic origin is favored. We investigate the evolution of the timing and spectral properties during the rising phase of the outburst and discuss their physical implications.

astro-ph.HE↗

arXiv2017

Observational Artifacts of NuSTAR: Ghost Rays and Stray Light

The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June 2012, flies two conical approximation Wolter-I mirrors at the end of a 10.15m mast. The optics are coated with multilayers of Pt/C and W/Si that operate from 3--80 keV. Since the optical path is not shrouded, aperture stops are used to limit the field of view from background and sources outside the field of view. However, there is still a sliver of sky (~1.0--4.0 degrees) where photons may bypass the optics altogether and fall directly on the detector array. We term these photons Stray-light. Additionally, there are also photons that do not undergo the focused double reflections in the optics and we term these Ghost Rays. We present detailed analysis and characterization of these two components and discuss how they impact observations. Finally, we discuss how they could have been prevented and should be in future observatories.

astro-ph.IM↗

arXiv2017

Sagittarius A* High Energy X-ray Flare Properties During NuSTAR Monitoring of the Galactic Center from 2012 to 2015

Understanding the origin of the flaring activity from the Galactic center supermassive black hole, Sagittarius A*, is a major scientific goal of the NuSTAR Galactic plane survey campaign. We report on the data obtained between July 2012 and April 2015, including 27 observations on Sgr A* with a total exposure of ~ 1 Ms. We found a total of ten X-ray flares detected in the NuSTAR observation window, with luminosities in the range of $L_{3-79~keV}$~$(0.2-4.0) \times 10^{35}~erg~s^{-1}$. With this largest hard X-ray Sgr A* flare dataset to date, we studied the flare spectral properties. Seven flares are detected above 5σ significance, showing a range of photon indices (Γ ~ 2.0-2.8) with typical uncertainties of +/-0.5 (90% confidence level). We found no significant spectral hardening for brighter flares as indicated by a smaller sample. The accumulation of all the flare spectra in 1-79 keV can be well fit with an absorbed power-law model with Γ=2.2+/-0.1, and does not require the existence of a spectral break. The lack of variation in X-ray spectral index with luminosity would point to a single mechanism for the flares and is consistent with the synchrotron scenario. Lastly, we present the quiescent state spectrum of Sgr A*, and derived an upper limit on the quiescent luminosity of Sgr A* above 10 keV to be $L_{Xq, 10-79 keV}$ < $(2.9{\pm}0.2) \times 10^{34}~erg~s^{-1}$.

astro-ph.HE↗

arXiv2017

Measurement of the absolute Crab flux with NuSTAR

We present results from a NuSTAR observation of the Crab made at a large off-axis angle of 1.5\degree. At these angles X-rays do not pass through the optics, but rather illuminate the detectors directly due to incomplete baffling. Due to the simplicity of the instrument response in this configuration and the good absolute calibration of the detectors, we are able to measure the absolute intrinsic flux of the Crab to better than 4%. We find the spectral parameters of the powerlaw to be Gamma=2.106 +/- 0.006, N=9.71 +/-0.16, in agreement with the values measured 42 years ago by Toor & Seward (1974). This suggests that the observed variability of the Crab is not part of a long term trend, but instead results from fluctuations around a steady mean. The NuSTAR observation also enabled improved measurement of the detector absorption parameters without the added complications of the mirror response.

astro-ph.HE↗

arXiv2017

The NuSTAR Hard X-ray Survey of the Norma Arm Region

We present a catalog of hard X-ray sources in a square-degree region surveyed by NuSTAR in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and typical and maximum exposure depths of 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of $5\times10^{-14}$ and $4\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected and ten are detected with low significance; eight of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multi-wavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of $\approx$10-20 keV, consistent with the Galactic Ridge X-ray emission spectrum but lower than temperatures of CVs near the Galactic Center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic Center. The NuSTAR log$N$-log$S$ distribution in the 10-20 keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with $kT\approx15$~keV, as observed for the CV candidates.

astro-ph.HE↗