arXiv subjects

Noa Keshet

Publications and source records attributed to Noa Keshet.

6 recordsLinked to original sources

arXiv2026

Stratified Wind Geometries of GRS 1915+105: Density Profiles and Radii

The low-mass X-ray binary GRS 1915+105 exhibits dramatic spectral and flux variability, often accompanied by highly ionized outflows. In this work, we analyze four archival Chandra/HETG observations spanning over a decade, which vary greatly in their ionizing spectrum from soft (low inner disk temperature) to hard (high inner disk temperature). We measure the wind properties, using the insightful tool of the absorption measure distribution (AMD, a distribution of column density with ionization). While the soft epoch reveals a steep, monotonically rising AMD, hard epochs feature a distinct turnover at high ionization parameter ($\log\xi>3.3$). This is the first time we observe a negative AMD slope in an outflow, which we interpret as a localized density inversion, which could point to radiation-pressure compression zones. We utilize the AMD shape and total column density measurements to calculate the density profile and distance of the wind from the central source. For the hard states, we obtain densities of $10^{12}-10^{13}$ cm$^{-3}$ at radii of $10^{10}-10^{11}$ cm. Such measurements are usually not directly possible in X-ray spectroscopy. Here we constrain the inner radius and density of the outflow to a factor of two. The high-$\xi$ values up to $\log\xi\sim6.5$ with no turnover in the soft state imply a smaller radius of a few $10^{8}$cm, which is in tension with the slow velocities observed. We demonstrate that this is a generic tension between compact launch radii and sub-escape velocities of high-column, high-ionization outflows observed in stellar-mass black hole binaries.

astro-ph.HE↗

arXiv2025

The dynamic central environment of NGC 3516 revealed by XRISM

We present a detailed, time-resolved analysis of the Fe K band of the Seyfert 1.5 galaxy NGC 3516 observed with XRISM. The 249 ks observation spanning $\sim$310 ks in elapsed time reveals an exceptionally rich and time-variable absorption spectrum. Six distinct absorption components are detected across multiple ionization states, spanning more than an order of magnitude in ionization parameter and a wide range of systemic velocities, from a potential inflow ($+4300~\rm km~s^{-1}$) to a mildly relativistic ultra-fast outflow ($-9800~\rm km~s^{-1}$). Despite their diversity, the components exhibit relatively small broadening ($\lesssim$$400~\rm km~s^{-1}$), implying comparable internal dynamics within a medium of a complex structure. Time-resolved spectroscopy reveals pronounced variability in three highly ionized absorbers, with Fe XXV$-$Fe XXVI features that appear and disappear on timescales of tens of kiloseconds. This behavior likely reflects a combination of geometrical transits of clumpy gas and ionization-state changes driven by continuum variability. An additional temporary absorption feature in the red wing of the Fe K$\alpha$ line, consistent with Fe XXV absorption, indicates a possible transient ultra-fast inflow at $\sim$$15\,000~\rm km~s^{-1}$ ($\sim$5% $c$). Finally, the continuum light curve exhibits a tentative $\sim$40 ks oscillatory pattern, accompanied by correlated shifts of a weak, narrow Fe K$\alpha$ emission feature, suggesting dynamic coupling between the continuum and the line-emitting region. Together, these results reveal that the nuclear environment of NGC 3516 is dominated by rapidly evolving, multi-phase gas flows, where accretion, ejection, and ionization processes are tightly coupled on sub-parsec scales.

astro-ph.GA↗

arXiv2025

A Sharper View of the X-ray Spectrum of MCG--6-30-15 with XRISM, XMM-Newton and NuSTAR

We present a time-averaged spectral analysis of the 2024 XRISM observation of the narrow-line Seyfert-1 galaxy MCG--6-30-15, taken contemporaneously with XMM-Newton and NuSTAR. Our analysis leverages a unique combination of broadband and high-resolution X-ray spectroscopy to definitively isolate and characterize both broad and narrow emission and absorption features in this source. The best-fitting model for the joint spectral analysis is very well described by reflection from the inner accretion disk illuminated by a compact corona, modified by multi-zone ionized absorption from an outflowing wind along the line of sight. The XRISM/Resolve data confirm that a strong, relativistically-broadened Fe K$\alpha$ emission line is required in order to obtain an adequate model fit. The Resolve data additionally verify the presence of a $v_{\rm out} \sim 2300$ km/s component of this outflowing wind, find tentative evidence for a $v_{\rm out} \sim 20,000$ km/s wind component, and indicate that the reflection from distant, neutral material may originate in a non-uniform structure rather than the traditional torus of AGN unification schemes. Though a rapid prograde black hole spin is statistically preferred by the best-fitting model, consistent with previous results, the AGN flux variability over the course of the observation complicates the interpretation of the time-averaged spectra. This insight, clarified by the combination of high signal-to-noise and high spectral resolution in the joint dataset, emphasizes the importance of time-resolved, high-resolution spectral analysis in unambiguously measuring the physical properties of variable AGN.

astro-ph.HE↗

arXiv2025

Elemental Abundances in X-ray Binary Outflows

Line resolved X-ray spectra of outflows from X-ray binaries are interesting since they provide quantifiable measures of the accreted material on to the compact object (black hole or neutron star), which can not be observed directly in the accretion disk. One such measurement that has been largely overlooked is that of the elemental abundances, which potentially provide insights into the origin of the ejected material. Using the Chandra/HETG grating spectrometer we measure and present elemental abundances in four low-mass X-ray binaries. We compare two measurement methods. One is by fitting line series of individual ions and reconstructing the absorption measure distribution (AMD), and the other is a global fit with one or two individual ionization components. All outflows feature a steep AMD strongly favoring high ionization degrees. The present abundances are consistent with previous works suggesting the abundances in the outflows are non-solar. We find a tentative trend of increasing abundances with atomic number, which fits some core-collapse supernova models, but no exact match to a specific one.

astro-ph.HE↗

arXiv2024

Supernova Archaeology with X-Ray Binary Winds -- The Case of GRO J1655-40

Supernovae are responsible for the elemental enrichment of the galaxy and some are postulated to leave behind a black hole. In a stellar binary system the supernova pollutes its companion, and the black hole can accrete back its own debris and emit X-rays. In this sequence of events, which is only poorly understood, winds are ejected, and observed through X-ray absorption lines. Measuring abundances of elements in the wind can lead to inferences about the historical explosion and possibly identify the long-gone progenitor of the compact object. Here, we re-analyze the uniquely rich X-ray spectrum of the 2005 outburst of GRO J1655-40. We reconstruct the absorption measure distribution (AMD) of the wind, and find that it increases sharply with ionization from H-like O up to H-like Ca, and then flattens out. The AMD is then used to measure relative abundances of 18 different elements. The present abundances are in partial agreement with a previous work with discrepancies mostly for low-Z elements. The overabundance of odd-Z elements hints at a high-metallicity, high-mass ($\simeq25\,M_\odot$) progenitor. Interestingly, the abundances are different from those measured in the companion atmosphere, indicating that the wind entrains lingering ambient supernova debris. This can be expected since the current total stellar mass of the binary ($<10\,M_\odot$) is much less than the progenitor mass.

astro-ph.HE↗

arXiv2022

Ionization Distributions in Outflows of Active Galaxies: Universal Trends and Prospect of Future XRISM Observations

The physics behind the ionization structure of outflows from black holes is yet to be fully understood. Using archival observations with the Chandra\HETG gratings over the past two decades, we measured an absorption measure distribution for a sample of outflows in nine active galaxies (AGNs), namely the dependence of outflow column density, $N_H$, on the ionization parameter, $\xi$. The slope of $\log(N_H)$ vs. $\log\xi$ is found to be between 0.00 and 0.72. We find an anti-correlation between the log of total column density of the outflow and the log of AGN luminosity, and none with the black hole mass and accretion efficiency. A major improvement in the diagnostics of AGN outflows will potentially occur with the launch of the XRISM/Resolve spectrometer. We study the ability of Resolve to reveal the outflow ionization structure by constructing the absorption measure distribution from simulated Resolve spectra, utilizing its superior resolution and effective area. Resolve constrains the column density as well as HETG, but with much shorter observations.

astro-ph.HE↗