arXiv subjects

Renee Ludlam

Publications and source records attributed to Renee Ludlam.

13 recordsLinked to original sources

arXiv2026

Decoding Cygnus X-2: The Critical Role of Reflection in IXPE Data

We present a spectro-polarimetric re-analysis of the first IXPE observation of Cygnus X-2 which we determine to be mainly in the normal branch, from quasi-simultaneous observations with NuSTAR, NICER, and INTEGRAL. We measure the hard X-ray polarization angle and find it to be consistent with the previously measured position angle of the radio jet. Leveraging NuSTAR's detection of both the relativistic Fe K emission line and the Compton hump, we constrain the flux contribution of the reflected emission from the inner accretion disk to be 10% of the total X-ray flux in the IXPE energy band. Unlike previous studies that modeled only the Fe K emission line, we fit the full-band reflection spectrum using a fully relativistic disk model. There is strong degeneracy between the Comptonized and reflection components. Given that the Comptonized component is not expected to be highly polarized, a polarization degree of approximately 20% for the reflection component could explain the X-ray polarization data from IXPE. We also discuss the disk inclination angle inferred from our spectro-polarimetric modeling, as well as other possible explanations for the data.

astro-ph.HE↗

arXiv2025

Complex Ionization and Velocity Structures in GX 340+0 X-ray Binary Revealed by XRISM

We present the first high-resolution XRISM spectrum of the neutron star low-mass X-ray binary GX 340+0, revealing unprecedented detail in its emission and absorption features. The spectrum reveals a rich and complex Fe XXV He$\alpha$ line profile and a P-Cygni profile from Ca XX. We use the state-of-the-art spectral synthesis code Cloudy to model the emission and absorption features in detail. Our analysis reveals multi-ionization and multi-velocity structures, where the combination of broad ($\sim$ 800 km/s) and narrow ($\sim$ 360 km/s) line components, along with rest-frame and blueshifted emission and absorption lines, accounts for the observed line profile complexity. We identify a modest $\sim$ 2735 km/s accretion disk wind exhibiting both absorption and emission features. We also detect a relativistic reflection feature in the spectrum, which we model using relxillNS - specifically designed to characterize X-ray reprocessing in accretion disks around neutron stars. Furthermore, we examine the detailed physics of the Fe XXV He$\alpha$ complex, focusing on the forbidden-to-resonance line ratio under the influence of continuum pumping and optical depth effects.

astro-ph.HE↗

arXiv2025

Radio Continuum Studies of Ultra-Compact and Short Orbital Period X-Ray Binaries

We present the radio continuum counterparts to the enigmatic ultra-compact X-ray binaries (UCXBs); a black hole or neutron star accreting from a hydrogen-deficient white dwarf donor star, with short orbital periods ($<$ 80 minutes). For the sample of UCXBs hosted by globular clusters (GCs), we search for whether certain GC properties are more likely to enhance UCXB formation. We determine that GCs which host UCXBs are drawn from a distinct population in terms of cluster concentration, core radius and half-light radius, but are similar to other well-studied GCs in metallicity and cluster mass. In particular, UCXB-hosting GCs tend to be on average more compact, with a higher concentration than other GCs, with significantly higher encounter rates. We investigate whether a correlation exists between radio luminosity and orbital period, using new and archival observations. We determine that there is not a clear connection between the two observable quantities.

astro-ph.HE↗

arXiv2025

XRISM insights for interstellar Sulfur

The X-ray Imaging Spectroscopy Mission (XRISM) provides the best spectral resolution with which to study Sulfur (S) K-shell photoabsorption features from the interstellar medium (ISM). For the first time, we demonstrate the high-signal detection of interstellar atomic SII K-beta absorption in the spectrum of X-ray binaries (XRBs) 4U 1630-472 and GX 340+0. The persistence of this feature across multiple instruments, targets, and flux states implies that it is interstellar in nature. We measure the SII Kbeta line centroid at 2470.8 +/- 1.1 eV after including systematic uncertainties. We also find that the most recently published high resolution SII absorption template requires a systematic energy scale shift of +7-8 eV, which is comparable to the level of disagreement among various atomic modeling procedures. The XRISM 300 ks observation of GX 340+0 provides unprecedented signal-to-noise in the S K region, and we find evidence of residual absorption from solid S in the spectra of GX 340+0. Absorption templates from three Fe-S compounds, troilite (FeS), pyrrhotite (Fe_7S_8) and pyrite (FeS_2), provide equally good fits to the residuals. Even though we are not able to distinguish among these three compounds, they provide equal estimates for the abundance of S locked in dust grains. Having accounted for both the gaseous and solid S in the GX 340+0 sightline provides us with a direct measurement of S depletion, which is 40% +/- 15%. Our depletion measurement provides an upper limit to the fraction of interstellar Fe bound in Fe-S compounds of < 25%, which is consistent with prior studies of Fe-S compounds via Fe L-shell absorption. Both XRBs in this study are at a distance of approximately 11 kpc and on the opposite side of the Galactic disk, suggesting that this value could represent the average S depletion of the Milky Way when integrated across all phases of the ISM.

astro-ph.GA↗

arXiv2024

Physics of 1 keV line in X-ray binaries

X-ray binaries (XRBs) often exhibit spectral residuals in the 0.5 to 2 keV range, known as the ``1 keV residual/1 keV feature", with variable centroid and intensity across different systems. Yet a comprehensive scientific explanation of the variability of the 1 keV feature has remained largely elusive. In this paper, we explain for the first time the origin and variability of the 1 keV feature in XRBs using the spectral synthesis code \textsc{Cloudy}. We constructed line blends for the emission and absorption lines and study the variability of these blends with ionization parameters, temperature, and column density. We conducted a sample study involving five XRBs including two ultraluminous X-ray sources (ULXs): NGC 247 ULX-1, NGC 1313 X-1, a binary X-ray pulsar : Hercules X-1, and two typical low-mass X-ray binaries (LMXBs): Cygnus X-2, and Serpens X-1. Our analysis establishes a self-consistent framework explaining the variability of the 1 keV spectral feature, attributing its diversity to differences in spectral energy distribution, ionization parameter, temperature, column density, and disk reflection properties. This framework provides a comprehensive explanation for the observed 1 keV feature across these diverse XRB systems, offering insights into the underlying physical mechanisms at play.

astro-ph.HE↗

arXiv2024

Return to the forgotten ULX: a broadband NICER+NuSTAR study of NGC 4190 ULX-1

We observed the nearby and relatively understudied ultraluminous X-ray source (ULX) NGC 4190 ULX-1 jointly with NICER and NuSTAR to investigate its broadband spectrum, timing properties, and spectral variation over time. We found NGC 4190 ULX-1 to have a hard spectrum characterized by two thermal components (with temperatures ~0.25keV and ~1.6keV) and a high-energy excess typical of the ULX population, although the spectrum turns over at an unusually low energy. While no pulsations were detected, (with pulsed fraction 3-sigma upper limits of 16% for NICER and 35% for NuSTAR), the source shows significant stochastic variability and the covariance spectrum indicates the presence of a high-energy cut-off power-law component, potentially indicative of an accretion column. Additionally, when fitting archival XMM-Newton data with a similar model, we find that the luminosity-temperature evolution of the hot thermal component follows the behavior of a super-Eddington slim disk though the expected spectral broadening for such a disk is not seen, suggesting that the inner accretion disk may be truncated by a magnetic field. Therefore, despite the lack of detected pulsations, there is tantalizing evidence for NGC 4190 ULX-1 being a candidate neutron star accretor, although further broadband observations will be required to confirm this behavior.

astro-ph.HE↗

arXiv2024

Investigating the Ultra-Compact X-ray Binary Candidate SLX 1735-269 with NICER and NuSTAR

We present two simultaneous NICER and NuSTAR observations of the ultra-compact X-ray binary (UCXB) candidate SLX 1735-269 while the source was in two different spectral states. Using various reflection modeling techniques, we find that xillverCO, a model used for fitting X-ray spectra of UCXBs with high carbon and oxygen abundances is an improvement over relxill or relxillns, which instead contains solar-like chemical abundances. This provides indirect evidence in support of the source being ultra-compact. We also use this reflection model to get a preliminary measurement of the inclination of the system, $i = 57^{+23}_{-7}$ degrees. This is consistent with our timing analysis, where a lack of eclipses indicates an inclination of $i<80^{\circ}$. The timing analysis is otherwise inconclusive, and we can not confidently measure the orbital period of the system.

astro-ph.HE↗

arXiv2023

Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 0614+091

We present the first joint NuSTAR and NICER observations of the ultra-compact X-ray binary (UCXB) 4U 0614+091. This source shows quasi-periodic flux variations on the timescale of ~days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.

astro-ph.HE↗

arXiv2022

Burst-Disk Interaction in 4U 1636-536 as observed by NICER

We present the detection of 51 thermonuclear X-ray bursts observed from 4U 1636-536 by the Neutron Star Interior Composition Explorer (NICER) over the course of a three year monitoring campaign. We performed time resolved spectroscopy for 40 of these bursts and showed the existence of a strong soft excess in all the burst spectra. The excess emission can be characterized by the use of a scaling factor (f_a method) to the persistent emission of the source, which is attributed to the increased mass accretion rate on to the neutron star due to Poynting-Robertson drag. The soft excess emission can also be characterized by the use of a model taking into account the reflection of the burst emission off of the accretion disk. We also present time resolved spectral analysis of 5 X-ray bursts simultaneously observed by NICER and AstroSat, which confirm the main results with even greater precision. Finally, we present evidence for Compton cooling using 7 X-ray bursts observed contemporaneously with \nustar, by means of a correlated decrease in the hard X-ray lightcurve of 4U 1636-536 as the bursts start.

astro-ph.HE↗

arXiv2021

Relativistic X-ray Reflection Models for Accreting Neutron Stars

We present new reflection models specifically tailored to model the X-ray radiation reprocessed in accretion disks around neutron stars, in which the primary continuum is characterized by a single temperature blackbody spectrum, emitted either at the surface of the star, or at the boundary layer. These models differ significantly from those with a standard power-law continuum, typically observed in most accreting black holes. We show comparisons with earlier reflection models, and test their performance in the NuSTAR observation of the neutron star 4U 1705-44. Simulations of upcoming missions such as XRISM-Resolve and Athena X-IFU are shown to highly the diagnostic potential of these models for high-resolution X-ray reflection spectroscopy. These new reflection models xillverNS, and their relativistic counterpart relxillNS, are made publicly available to the community as an additional flavor in the relxill suite of reflection models.

astro-ph.HE↗

arXiv2020

Relativistic reflection and reverberation in GX 339-4 with NICER and NuSTAR

We analyze seven NICER and NuSTAR epochs of the black hole X-ray binary GX 339-4 in the hard state during its two most recent hard-only outbursts in 2017 and 2019. These observations cover the 1-100 keV unabsorbed luminosities between 0.3% and 2.1% of the Eddington limit. With NICER's negligible pile-up, high count rate and unprecedented time resolution, we perform a spectral-timing analysis and spectral modeling using relativistic and distant reflection models. Our spectral fitting shows that as the inner disk radius moves inwards, the thermal disk emission increases in flux and temperature, the disk becomes more highly ionized and the reflection fraction increases. This coincides with the inner disk increasing its radiative efficiency around ~1% Eddington. We see a hint of hysteresis effect at ~0.3% of Eddington: the inner radius is significantly truncated during the rise ($>49R_{g}$), while only a mild truncation ($\sim5R_g$) is found during the decay. At higher frequencies ($2-7$~Hz) in the highest luminosity epoch, a soft lag is present, whose energy dependence reveals a thermal reverberation lag, with an amplitude similar to previous findings for this source. We also discuss the plausibility of the hysteresis effect and the debate of the disk truncation problem in the hard state.

astro-ph.HE↗

arXiv2020

A NICER View of Spectral and Profile Evolution for Three X-ray Emitting Millisecond Pulsars

We present two years of Neutron star Interior Composition Explorer (NICER) X-ray observations of three energetic rotation-powered millisecond pulsars (MSPs): PSRs B1937+21, B1821-24, and J0218+4232. We fit Gaussians and Lorentzians to the pulse profiles for different energy sub-bands of the soft X-ray regime to measure the energy dependence of pulse separation and width. We find that the separation between pulse components of PSR J0218+4232 decreases with increasing energy at $\gt 3σ$ confidence. The 95% upper limit on pulse separation evolution for PSRs B1937+21 and B1821-24 is less than 2 milliperiods per keV. Our phase-resolved spectral results provide updated constraints on the non-thermal X-ray emission of these three pulsars. The photon indices of the modeled X-ray emission spectra for each pulse component of PSR B1937+21 are inconsistent with each other at the 90% confidence level, suggesting different emission origins for each pulse. We find that the PSR B1821-24 and PSR J0218+4232 emission spectra are invariant with phase at the 90% confidence level. We describe the implications of our profile and spectral results in the context of equatorial current sheet emission models for these three MSPs with non-thermal, magnetospheric X-ray emission.

astro-ph.HE↗

arXiv2018

A NICER look at the Aql X-1 hard state

We report on a spectral-timing analysis of the neutron star low-mass X-ray binary Aql~X-1 with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. Aql~X-1 was observed with NICER during a dim outburst in 2017 July, collecting approximately $50$ ks of good exposure. The spectral and timing properties of the source correspond to that of an (hard) extreme island state in the atoll classification. We find that the fractional amplitude of the low frequency ($<0.3$ Hz) band-limited noise shows a dramatic turnover as a function of energy: it peaks at 0.5 keV with nearly 5\% rms, drops to $12\%$ rms at 2 keV, and rises to $15\%$ rms at 10 keV. Through the analysis of covariance spectra, we demonstrate that band-limited noise exists in both the soft thermal emission and the power-law emission. Additionally, we measure hard time lags, indicating the thermal emission at $0.5$ keV leads the power-law emission at 10 keV on a timescale of $\sim100$ ms at $0.3$ Hz to $\sim10$ ms at $3$ Hz. Our results demonstrate that the thermal emission in the hard state is intrinsically variable, and driving the modulation of the higher energy power-law. Interpreting the thermal spectrum as disk emission, we find our results are consistent with the disk propagation model proposed for accretion onto black holes.

astro-ph.HE↗