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1. On the overlap reduction function of pulsar timing array searches for gravitational waves in modified gravity

   https://doi.org/10.1088/1361-6382/ad9881  

   Cordes, Nina; Mitridate, Andrea; Schmitz, Kai; Schröder, Tobias; Wassner, Kim (December 2024, Classical and Quantum Gravity)

   Abstract Pulsar timing array (PTA) searches for gravitational waves (GWs) aim to detect a characteristic correlation pattern in the timing residuals of galactic millisecond pulsars. This pattern is described by the PTA overlap reduction function (ORF) ${\mathrm{\Gamma }}_{ab}\left({\xi }_{ab}\right)$, which is known as the Hellings–Downs (HD) curve in general relativity (GR). In theories of modified gravity, the HD curve often receives corrections. Assuming, e.g. a subluminal GW phase velocity, one finds a drastically enhanced ORF in the limit of small angular separations between pulsaraand pulsarbin the sky, ${\xi }_{ab}\to 0$. In particular, working in harmonic space and performing an approximate resummation of all multipole contributions, the auto correlation coefficientΓ_aaseems to diverge. In this paper, we confirm that this divergence is unphysical and provide an exact and analytical expression forΓ_aain dependence of the pulsar distanceL_aand the GW phase velocity $v_{\mathrm{ph}}$. In the GR limit and assuming a large pulsar distance, our expression reduces to ${\mathrm{\Gamma }}_{aa}=1$. In the case of subluminal phase velocity, we show that the regularization of the naive divergent result is a finite-distance effect, meaning thatΓ_aascales linearly withfL_a, wherefis the GW frequency. For superluminal phase velocity (subluminal group velocity), which is relevant in the case of massive gravity, we correct an earlier analytical result forΓ_ab. Our results pave the way for fitting modified-gravity theories with nonstandard phase velocity to PTA data, which requires a proper understanding of the auto correlation coefficientΓ_aa. 

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2. Strongly Lensed Supermassive Black Hole Binaries as Nanohertz Gravitational-wave Sources

   https://doi.org/10.3847/1538-4357/ace16f  

   Khusid, Nicole M.; Mingarelli, Chiara M. F.; Natarajan, Priyamvada; Casey-Clyde, J. Andrew; Barnacka, Anna (September 2023, The Astrophysical Journal)

   Abstract Supermassive black hole binary systems (SMBHBs) should be the most powerful sources of gravitational waves (GWs) in the universe. Once pulsar timing arrays (PTAs) detect the stochastic GW background from their cosmic merger history, searching for individually resolvable binaries will take on new importance. Since these individual SMBHBs are expected to be rare, here we explore how strong gravitational lensing can act as a tool for increasing their detection prospects by magnifying fainter sources and bringing them into view. Unlike for electromagnetic waves, when the geometric optics limit is nearly always valid, for GWs the wave-diffraction-interference effects can become important when the wavelength of the GWs is larger than the Schwarzchild radius of the lens, i.e., $M_{\mathrm{lens}}\sim {10}^8{\left(\frac{f}{\mathrm{mHz}}\right)}^{-1}{M}_{\odot }$. For the GW frequency range explored in this work, the geometric optics limit holds. We investigate GW signals from SMBHBs that might be detectable with current and future PTAs under the assumption that quasars serve as bright beacons that signal a recent merger. Using the black hole mass function derived from quasars and a physically motivated magnification distribution, we expect to detect a few strongly lensed binary systems out toz≈ 2. Additionally, for a range of fixed magnifications 2 ≤μ≤ 100, strong lensing adds up to ∼30 more detectable binaries for PTAs. Finally, we investigate the possibility of observing both time-delayed electromagnetic signals and GW signals from these strongly lensed binary systems—that will provide us with unprecedented multi-messenger insights into their orbital evolution. 

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3. Electron impact excitation of allowed and forbidden transitions between fine-structure levels in Ti III

   https://doi.org/10.1088/1361-6595/ad95b8  

   Wang, Yang; Cao, Qiu-Yao; Zhu, Xi-Ming; Du, Chun-Meng; Bartschat, Klaus (November 2024, Plasma Sources Science and Technology)

   Abstract Introduction: We present an extensive theoretical investigation of the electron impact excitation of doubly-ionized titanium (Ti III) to meet the needs of spectral analysis and plasma modeling. OBJECTIVES: The main objective of this work is to extend the currently scarce database of both structure and collision data for Ti III. METHODS: The calculation was performed in the close-coupling approximation using theB-splineR-matrix method. The multi-configuration Hartree–Fock method in combination withB-spline configuration interaction expansions and the non-orthogonal orbitals technique is employed for accurate descriptions of the target wave functions and adequate accounts of the various interactions between the target states. Relativistic effects are treated at the semi-relativistic Breit-Pauli approximation level. RESULTS: The present close-coupling expansion includes 138 fine-structure levels of Ti III belonging to the $3d^2$, $4s^2$, $4s4p$, $3d4l$( $l=0-3$), $3d5l$( $l=0-3$), $3d6s$, and $3d6p$configurations. Comprehensive sets of radiative and electron collisional data are reported for all of the possible transitions between the 138 fine-structure levels. Thermally averaged collision strengths are determined using a Maxwellian distribution for a wide range of temperatures from ${10}^2$K to ${10}^5$K. The accuracy of the calculated radiative parameters is validated by comparing with available values from the NIST database and previous literature. CONCLUSION: Given the lack of sufficient currently available experimental and theoretical data, the electron impact excitation cross sections of the Ti III fine-structure levels presented here are systematic, extensive, and internally consistent, thus making them suitable for many modeling applications. 

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4. The NANOGrav 15 yr Data Set: Chromatic Gaussian Process Noise Models for Six Pulsars

   https://doi.org/10.3847/1538-4357/ad5291  

   Larsen, Bjorn; Mingarelli, Chiara_M_F; Hazboun, Jeffrey_S; Chalumeau, Aurélien; Good, Deborah_C; Simon, Joseph; Agazie, Gabriella; Anumarlapudi, Akash; Archibald, Anne_M; Arzoumanian, Zaven; et al (August 2024, The Astrophysical Journal)

   Abstract Pulsar timing arrays (PTAs) are designed to detect low-frequency gravitational waves (GWs). GWs induce achromatic signals in PTA data, meaning that the timing delays do not depend on radio frequency. However, pulse arrival times are also affected by radio-frequency-dependent “chromatic” noise from sources such as dispersion measure (DM) and scattering delay variations. Furthermore, the characterization of GW signals may be influenced by the choice of chromatic noise model for each pulsar. To better understand this effect, we assess if and how different chromatic noise models affect the achromatic noise properties in each pulsar. The models we compare include existing DM models used by the North American Nanohertz Observatory for Gravitational waves (NANOGrav) and noise models used for the European PTA Data Release 2 (EPTA DR2). We perform this comparison using a subsample of six pulsars from the NANOGrav 15 yr data set, selecting the same six pulsars as from the EPTA DR2 six-pulsar data set. We find that the choice of chromatic noise model noticeably affects the achromatic noise properties of several pulsars. This is most dramatic for PSR J1713+0747, where the amplitude of its achromatic red noise lowers from ${\log }_{10}{A}_{\mathrm{RN}}=-{14.1}_{-0.1}^{+0.1}$to $-{14.7}_{-0.5}^{+0.3}$, and the spectral index broadens from ${\gamma }_{\mathrm{RN}}={2.6}_{-0.4}^{+0.5}$to ${\gamma }_{\mathrm{RN}}={3.5}_{-0.9}^{+1.2}$. We also compare each pulsar's noise properties with those inferred from the EPTA DR2, using the same models. From the discrepancies, we identify potential areas where the noise models could be improved. These results highlight the potential for custom chromatic noise models to improve PTA sensitivity to GWs. 

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5. Sub-1 V/cm E-FISH-based picosecond electric field measurements in atmospheric pressure air

   https://doi.org/10.1088/1361-6595/adae32  

   LaCombe, Grayson; Wang, Jianan; Frederickson, Kraig; Simeni_Simeni, Marien (February 2025, Plasma Sources Science and Technology)

   Abstract We report on the development of a highly sensitive electric field induced second harmonic generation diagnostic setup capable of measuring electric field strengths as low as 1 $\mathrm{V}{\mathrm{cm}}^{-1}$at the picosecond time scale under atmospheric pressure conditions. This unprecedented sensitivity is achieved through passive homodyne detection, which utilizes stray signals generated by an optical component in the beam path. Our detection limit of 0.3–0.5 $\mathrm{V}{\mathrm{cm}}^{-1}$represents an improvement of over 2–3 orders of magnitude compared to previous reports (100–1000 $\mathrm{V}{\mathrm{cm}}^{-1}$) in the literature. Additionally, we demonstrate sensitivity to the polarity of the electric field. Experimental results are corroborated by simulations of the 400 ps time-resolved homodyne process, offering deeper insights into the enhanced detection capabilities and the system’s ability to resolve the field sign. 

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