arXiv subjects

Mesut Çalışkan

Publications and source records attributed to Mesut Çalışkan.

16 recordsLinked to original sources

arXiv2026

A compact time-frequency representation for gravitational-wave data analysis

Time-frequency (wavelet) domain analyses are seeing greater use for gravitational-wave data analysis due to the advantages they have in handling non-stationary noise. A popular choice is the Wilson-Daubechies-Meyer (WDM) wavelet transform, which uses a window function that is very compact in frequency, but more spread out in time. In this work, we consider an alternative window function that is built from a sum of phase-shifted Gaussians. This "Gaussian" window is more symmetric in time-frequency, and consequently more compact. We examine the properties of this window function and the implications it has for gravitational-wave analyses. We calculate the window's time-frequency variance product analytically and verify it numerically. For the symmetric case, where the window has the same form in time and frequency, the construction comes within $\sim2.4\%$ of saturating the Heisenberg-Gabor uncertainty limit. We conjecture that this Gaussian window achieves the minimum time-frequency area of any WD wavelet window.

gr-qc↗

arXiv2026

A Novel kinetic Sunyaev-Zel'dovich Estimator for Electron-Electron Correlations

Recent advancements in small-scale observations of the cosmic microwave background (CMB) have provided a unique opportunity to characterize the distribution of baryons in the outskirts of galaxies via stacking-based analyses of the kinetic Sunyaev-Zel'dovich (kSZ) effect. Such measurements, mathematically equivalent to probing the galaxy-electron cross-correlation, have revealed that gas is more extended than dark matter and that the strength of baryonic feedback may vary with halo mass and redshift. However, because these analyses are conditioned on galaxy positions, deriving a host-independent description of the baryon distribution depends on uncertain galaxy-halo modeling on small scales. In this work, we present a novel kSZ$\times$galaxy four-point estimator that directly probes the full ionized electron field, extending beyond the gas traced by luminous galaxies. This method exploits large-scale velocity reconstruction from galaxy surveys to characterize the electron distribution unbiased by small-scale galaxy clustering. We forecast that the proposed signal can be measured with a signal-to-noise ratio of $\sim8$ ($\sim31$) for a configuration corresponding to Atacama Cosmology Telescope DR6 (Simons Observatory) CMB data combined with spectroscopic galaxy samples from DESI. This approach will enable the first tomographic measurements of the electron auto-power spectrum, providing new constraints on feedback-driven redistribution of baryons and its role in shaping cosmic structure.

astro-ph.CO↗

arXiv2025

Reconstructing patchy helium reionization using the cosmic microwave background and large-scale structure

The intergalactic helium became fully ionized by the end of cosmic noon ($z\sim2$). Similarly to the reionization of hydrogen, helium reionization is expected to be patchy, driven by luminous quasars that ionize the intergalactic gas in their surrounding environment. Probing the morphology of ionized electrons during this epoch can provide crucial information about early structure formation, including the clustering and luminosities of quasars, the accretion rates, variability, and lifetimes of active galactic nuclei, as well as the growth and evolution of supermassive black holes. In this study, we present how measurements of the cosmic microwave background (CMB) can be used to reconstruct the optical-depth fluctuations resulting from patchy helium reionization. As helium reionization occurred at lower redshifts, upcoming probes of large-scale structure surveys will present a significant opportunity to enhance the prospects of probing this epoch by their combined analysis with the CMB. Using a joint information-matrix analysis of hydrogen and helium reionization, we show that near-future galaxy and CMB surveys will have enough statistical power to detect optical-depth fluctuations due to doubly-ionized helium, providing a way of measuring the redshift and duration of helium reionization to high significance. We also show that modeling uncertainties in helium reionization can impact the measurement precision of parameters characterizing hydrogen reionization.

astro-ph.CO↗

arXiv2025

A Fog Over the Cosmological SGWB: Unresolved Massive Black Hole Binaries in the LISA Band

Disentangling the rich astrophysical structure of the stochastic gravitational-wave background (SGWB) from its cosmological component is essential for the Laser Interferometer Space Antenna (LISA) to access the physics of the early Universe beyond the reach of any other probe. In this work, we develop an analytical framework to compute the astrophysical contribution to the SGWB arising from an unresolved ensemble of inspiraling and merging black hole binaries. Accounting for various resolvability thresholds, we leverage this framework to predict the amplitude, spectral shape, and detection signal-to-noise ratio of the unresolved background from massive black hole binaries (MBHBs), capturing the possible diversity of its SGWB imprint across a range of astrophysically motivated populations. Through a joint analysis of astrophysical and primordial contributions to the SGWB, we determine the minimum detectable amplitude of the cosmological background across a range of spectral shapes. We demonstrate that, even under optimistic subtraction thresholds, unresolved MBHBs can degrade the detectability of a cosmological signal by multiple orders of magnitude, depending on the spectral shape of the primordial component. Ultimately, the MBHB-induced astrophysical SGWB acts both as a veil and a lens: it imposes a fundamental limit on cosmological sensitivity, yet simultaneously reveals the hidden population of massive black holes beyond the reach of individual detections. Accurate modeling of this late-Universe background is therefore a prerequisite for robust component separation and for realizing LISA's full scientific potential.

astro-ph.CO↗

arXiv2025

Patchy Helium and Hydrogen Reionization from the Kinetic Sunyaev-Zel'dovich Effect and Galaxies

Upcoming cosmic microwave background (CMB) experiments will measure temperature fluctuations on small angular scales with unprecedented precision, enabling improved measurements of the kinetic Sunyaev-Zel'dovich (kSZ) effect. This secondary anisotropy has emerged as a valuable probe of the distribution of ionized electrons in the post-recombination Universe. Although the sensitivity of the kSZ effect has recently been utilized to study the high-redshift epoch of hydrogen (H) reionization, its redshift-integrated nature -- combined with anticipated improvements in measurement precision -- suggests that accounting for the later epoch of helium (He) reionization will become increasingly important in the near future. Joint characterization of the epochs will allow for a more coherent understanding of early-star and -quasar formation, as these sources drive the ionization of H and He in the intergalactic medium. In this paper, we extend the kSZ higher-order statistic introduced by Smith \& Ferraro (2017) to forecast the ability of upcoming CMB surveys to probe the morphology of both H and He reionization. Moreover, given that upcoming large-scale structure surveys will trace density fluctuations at redshifts overlapping with the epoch of He reionization, we propose a novel cross-correlation between the kSZ higher-order statistic and galaxy survey measurements. Using a joint information-matrix analysis of H and He reionization, we show that next-generation CMB and galaxy surveys will have sufficient statistical power to characterize the patchy morphology of H reionization and set constraints on the redshift evolution of its He counterpart.

astro-ph.CO↗

arXiv2025

Multi-messenger Gravitational Lensing

We introduce the rapidly emerging field of multi-messenger gravitational lensing - the discovery and science of gravitationally lensed phenomena in the distant universe through the combination of multiple messengers. This is framed by gravitational lensing phenomenology that has grown since the first discoveries in the 20th century, messengers that span 30 orders of magnitude in energy from high energy neutrinos to gravitational waves, and powerful "survey facilities" that are capable of continually scanning the sky for transient and variable sources. Within this context, the main focus is on discoveries and science that are feasible in the next 5-10 years with current and imminent technology including the LIGO-Virgo-KAGRA network of gravitational wave detectors, the Vera C. Rubin Observatory, and contemporaneous gamma/X-ray satellites and radio surveys. The scientific impact of even one multi-messenger gravitational lensing discovery will be transformational and reach across fundamental physics, cosmology and astrophysics. We describe these scientific opportunities and the key challenges along the path to achieving them. This article is the introduction to the Theme Issue of the Philosophical Transactions of The Royal Society A on the topic of Multi-messenger Gravitational Lensing, and describes the consensus that emerged at the associated Theo Murphy Discussion Meeting in March 2024.

astro-ph.HE↗

arXiv2024

Linear polarization of the stochastic gravitational-wave background with pulsar timing arrays

Pulsar-timing collaborations have recently reported evidence for the detection of an isotropic stochastic gravitational-wave background consistent with one sourced by a population of inspiralling supermassive black hole binaries. However, a certain degree of anisotropy and polarization may be present. Thus, the characterization of the energy density and polarization of the background at different angular scales is important. In this paper, we describe the signatures of linear polarization in the stochastic gravitational-wave background on the timing residuals obtained with pulsar-timing arrays. We expand the linear polarization map in terms of spin-weighted spherical harmonics and recast it into the $E$-mode (parity even) and $B$-mode (parity odd) basis. We provide expressions for the minimum-variance estimators for the coefficients of that expansion and evaluate the smallest detectable signal as a function of the signal-to-noise ratio with which the isotropic GW signal is detected and the number of pulsars in the survey. We evaluate the covariance between the estimators for the spherical-harmonic coefficients of the linear polarization $E$-modes and those for the intensity anisotropy. We also show that there is no covariance between the spherical-harmonic coefficients for the $B$-modes of the linear polarization and those for the circular polarization, even though both have the same parity. Our approach results in simple, elegant, and easily evaluated expressions for the overlap reduction functions for linear polarization.

astro-ph.CO↗

arXiv2024

Probing wave-optics effects and low-mass dark matter halos with lensing of gravitational waves from massive black holes

The Laser Interferometer Space Antenna (LISA) will detect gravitational waves (GWs) emitted by massive black hole binaries (MBHBs) in the low-frequency ($\sim$mHz) band. Low-mass lenses, such as low-mass dark matter halos or subhalos, have sizes comparable to the wavelength of these GWs. Encounters with these lenses produce wave-optics (WO) effects that alter waveform phase and amplitude. Thus, a single event with observable WO effects can be used to probe the lens properties. In this paper, we first compute the probability of observing WO effects in a model-agnostic way. We perform information-matrix analyses over approximately 1000 MBHBs with total mass, mass ratio, and redshift spanning the ranges relevant to LISA. We then calculate lensing rates using three semi-analytical models of MBHB populations. In both cases, we use a waveform model that includes merger, ringdown, and higher-order modes. We use two lens population models: the theory-based Press-Schechter halo mass function and an observation-based model derived from Sloan Digital Sky Survey. We find that the probability of detecting WO effects can be as large as $\sim 3\%$, $\sim1.5\%$, and $\sim 1 \%$ at $1σ$, $3σ$, and $5σ$ confidence levels, respectively. The most optimistic MBHB population model yields $\sim 8$, $\sim 4$, and $\sim 3$ events with detectable WO effects at the same confidence levels, while the rates drop to $\sim 0.01$ in the more pessimistic scenarios. The most likely lens masses probed by LISA are in the range $(10^3, 10^8)\, M_{\odot}$, and the most probable redshifts are in the range $(0.3, 1.7)$. Therefore, LISA observations of WO effects can probe low-mass DM halos, complementing strong lensing and other observations.

astro-ph.CO↗

arXiv2024

Dissecting the Stochastic Gravitational Wave Background with Astrometry

Astrometry, the precise measurement of star motions, offers an alternative avenue to investigate low-frequency gravitational waves through the spatial deflection of photons, complementing pulsar timing arrays reliant on timing residuals. Upcoming data from Gaia and Roman can not only cross-check pulsar timing array findings but also explore the uncharted frequency range bridging pulsar timing arrays and LISA. We present an analytical framework to evaluate the feasibility of detecting a gravitational wave background, considering measurement noise and the intrinsic variability of the stochastic background. Furthermore, we highlight astrometry's crucial role in uncovering key properties of the gravitational wave background, such as spectral index and chirality, employing information-matrix analysis. Finally, we simulate the emergence of quadrupolar correlations, commonly referred to as the generalized Hellings-Downs curves.

gr-qc↗

arXiv2023

Subtracting Compact Binary Foregrounds to Search for Subdominant Gravitational-Wave Backgrounds in Next-Generation Ground-Based Observatories

Stochastic gravitational-wave backgrounds (SGWBs) derive from the superposition of numerous individually unresolved gravitational-wave (GW) signals. Detecting SGWBs provides us with invaluable information about astrophysics, cosmology, and fundamental physics. In this paper, we study SGWBs from binary black-hole (BBH) and binary neutron-star (BNS) coalescences in a network of next-generation ground-based GW observatories (Cosmic Explorer and Einstein Telescope) and determine how well they can be measured; this then limits how well we can observe other subdominant astrophysical and cosmological SGWBs. We simulate all-Universe populations of BBHs and BNSs and calculate the corresponding SGWBs, which consist of a superposition of (i) undetected signals, and (ii) the residual background from imperfect removal of resolved sources. The sum of the two components sets the sensitivity for observing other SGWBs. Our results show that, even with next-generation observatories, the residual background is large and limits the sensitivity to other SGWBs. The main contributions to the residual background arise from uncertainties in inferring the coalescence phase and luminosity distance of the detected signals. Alternative approaches to signal subtraction would need to be explored to minimize the BBH and BNS foreground in order to observe SGWBs from other subdominant astrophysical and cosmological sources.

gr-qc↗

arXiv2023

Observability of lensing of gravitational waves from massive black hole binaries with LISA

The gravitational waves emitted by massive black hole binaries in the LISA band can be lensed. Wave-optics effects in the lensed signal are crucial when the Schwarzschild radius of the lens is smaller than the wavelength of the radiation. These frequency-dependent effects can enable us to infer the lens parameters, possibly with a single detection alone. In this work, we assess the observability of wave-optics effects with LISA by performing an information-matrix analysis using analytical solutions for both point-mass and singular isothermal sphere lenses. We use gravitational-waveform models that include the merger, ringdown, higher harmonics, and aligned spins to study how waveform models and source parameters affect the measurement errors in the lens parameters. We find that previous work underestimated the observability of wave-optics effects and that LISA can detect lensed signals with higher impact parameters and lower lens masses.

astro-ph.CO↗

arXiv2023

Lensing or luck? False alarm probabilities for gravitational lensing of gravitational waves

Strong gravitational lensing of gravitational waves (GWs) has been forecasted to become detectable in the upcoming observing runs. However, definitively distinguishing pairs of lensed sources from random associations is a challenging problem. We investigate the degree to which unlensed events mimic lensed ones because of the overlap of parameters due to a combination of random coincidence and errors in parameter estimation. We construct a mock catalog of lensed and unlensed events. We find that the false alarm probability (FAP) based on coincidental overlaps of the chirp mass, sky location, and coalescence phase are approximately $9\%$, $1\%$, and $10\%$ per pair, respectively. Combining all three, the overall FAP per pair is $\sim10^{-4}$. As the number of events, $N$, in the GW catalogs increases, the number of random pairs of events increases as $\sim N^2$. Meanwhile, the number of lensed events will increase linearly with $N$, implying that for sufficiently high $N$, the false alarms will always dominate over the true lensing events. This issue can be compensated for by placing higher thresholds on the lensing candidates (e.g., selecting a higher signal-to-noise ratio (SNR) threshold), which will lead to better parameter estimation and, thus, lower FAP per pair -- at the cost of dramatically decreasing the size of the lensing sample (by $\sim 1/\mbox{SNR}^3$). We show that with our simple overlap criteria for current detectors at design sensitivity, the false alarms will dominate for realistic lensing rates ($\lesssim10^{-3}$) even when selecting the highest SNR pairs. These results highlight the necessity to design alternative identification criteria beyond simple waveform and sky location overlap. Future GW detectors Cosmic Explorer and Einstein Telescope may provide sufficient improvement in parameter estimation, allowing for the conclusive detection of strong lensing of GWs.

astro-ph.CO↗

arXiv2022

The impact of confusion noise on golden binary neutron-star events in next-generation terrestrial observatories

Next-generation terrestrial gravitational-wave observatories will detect $\mathcal{O}(10^{5})$ signals from compact binary coalescences every year. These signals can last for several hours in the detectors' sensitivity band and they will be affected by multiple unresolved sources contributing to a confusion-noise background in the data. Using an information-matrix formalism, we estimate the impact of the confusion noise power spectral density in broadening the parameter estimates of a GW170817-like event. If our estimate of the confusion noise power spectral density is neglected, we find that masses, spins, and distance are biased in about half of our simulations under realistic circumstances. The sky localization, while still precise, can be biased in up to $80\%$ of our simulations, potentially posing a problem in follow-up searches for electromagnetic counterparts.

gr-qc↗

arXiv2022

Compact Binary Foreground Subtraction in Next-Generation Ground-Based Observatories

The stochastic gravitational-wave backgrounds (SGWBs) for current detectors are dominated by binary black-hole (BBH) and binary neutron-star (BNS) coalescences. The sensitivity of current networks of gravitational-wave (GW) detectors allows only a small fraction of BBHs and BNSs to be resolved and subtracted, but previous work indicated that the situation should significantly improve with next-generation (XG) observatories. We revisit these conclusions by taking into account waveform-modeling uncertainties, updated astrophysical models, and (crucially) the full set of parameters that must be estimated to remove the resolved sources. Compared to previous studies, we find that the residual background from BBHs and BNSs is large even with XG detector networks. New data analysis methods will thus be required to observe the SGWB from cosmic supernovae or contributions from early-Universe phenomena like cosmic strings, stiff post-inflation fluids, or axion inflation.

gr-qc↗

arXiv2020

Tungsten Thermionic Emission as a Gauge for Low Pressures of Cesium Vapor

Heated metal filaments under electric fields and low pressures of alkali metal gaseject electrons by thermionic emission as a function of the pressure of the gas and thetemperature of the filament. To explore this process in a program to develop large-area alkali metal photocathodes, we have designed and built a gauge following thestudies of Taylor and Langmuir. We present proof-of-principle measurements ofthe thermionic emission of a tungsten filament in cesium vapor. We describe a secondgeneration design that corrects flaws in the first gauge.

physics.ins-det↗

arXiv2020

Localizing merging black holes with sub-arcsecond precision using gravitational-wave lensing

The current gravitational-wave localization methods rely mainly on sources with electromagnetic counterparts. Unfortunately, a binary black hole does not emit light. Due to this, it is generally not possible to localize these objects precisely. However, strongly lensed gravitational waves, which are forecasted in this decade, could allow us to localize the binary by locating its lensed host galaxy. Identifying the correct host galaxy is challenging because there are hundreds to thousands of other lensed galaxies within the sky area spanned by the gravitational-wave observation. However, we can constrain the lensing galaxy's physical properties through both gravitational-wave and electromagnetic observations. We show that these simultaneous constraints allow one to localize quadruply lensed waves to one or at most a few galaxies with the LIGO/Virgo/Kagra network in typical scenarios. Once we identify the host, we can localize the binary to two sub-arcsec regions within the host galaxy. Moreover, we demonstrate how to use the system to measure the Hubble constant as a proof-of-principle application.

astro-ph.HE↗