Shami Chatterjee

We have discovered PSR J2108+5055, an extremely luminous but sporadic mode-changing rotating radio transient (RRAT), with a period of 0.495 s using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Fast Radio Burst (FRB) instrument. We show that the brightest pulse from PSR J2108+5055 has a flux density of 3.1 ± 1.0 kJy, corresponding to a spectral luminosity of (7.5 ± 2.3) × 10 ²³ erg s ⁻¹ Hz ⁻¹ , assuming a dispersion-measure-derived distance of 1.6 ± 0.3 kpc. We place an upper limit on the duty cycle of PSR J2108+5055 at 0.002 and measure a surface magnetic field of 5.8 × 10 ¹¹ G, a characteristic age of 1.2 × 10 ⁷ yr, and a spindown luminosity of 2.2 × 10 ³² erg s ⁻¹ . Using a simple model based on the Green Bank North Celestial Cap (GBNCC) survey, we place an upper limit on the number of PSR J2108+5055–like sources with similar peak flux densities in the GBNCC survey area of ∼80 RRATs. We show that if PSR J2108+5055 were placed at the edge of the Local Group of galaxies, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) would detect its brightest pulse. The second brightest pulse has a peak flux density of 1.5 ± 0.7 kJy, and FAST would detect such a pulse out to ∼1 Mpc, covering the inner Local Group, including Andromeda.

Pulsar timing is a valuable source of high-precision astrophysical measurements that can be used to probe gravitational physics, including by detecting gravitational waves. An important factor limiting the precision of these measurements is pulse jitter; i.e., intrinsic, short-timescale variation in the amplitude and shape of pulses from a given pulsar. Because conventional pulse time-of-arrival (TOA) measurement relies on template matching, which assumes the average pulse shape is stable, such variation gives rise to jitter noise in TOA measurements. Here, we introduce a generalization of the template matching technique, making use of principal component analysis, which can account for variations in pulse shape. We compare this technique to other proposals for mitigating jitter noise in pulsar timing, paying particular attention to the possibility of corrections absorbing other astrophysical signals of interest, and we demonstrate its effectiveness using simulated data.

Quantifying the maximum energy of fast radio bursts (FRBs) can provide stringent constraints on their emission mechanisms and progenitor models. However, the most energetic bursts are rare, requiring a large sample of FRBs to detect them. In this work, we use the largest available such sample, 2998 one-off FRBs from the Second CHIME/FRB Catalog, to obtain a lower limit on the maximum energy ( E iso max ) of FRBs, assuming isotropic energy distribution from FRB sources. In the absence of known redshifts ( z ) for most sources, we present a framework that uses the dispersion measures (DMs) and fluences of these FRBs, together with the probability distribution of z given DM, to derive the lower limit on E iso max . We generate simulated FRB samples assuming different parameter values for a log-normal DM _host distribution and a Schechter function form of the FRB energy function to estimate how many outliers—FRBs with large DM contributions from the host galaxy or intervening galaxy halos—could artificially inflate this limit. After accounting for outliers, the lower limit on E iso max from Catalog 2 FRBs ranges between 1.1 × 10 ⁴¹ and 1.9 × 10 ⁴² erg, with best estimate 1.5 × 10 ⁴² erg. This limit is consistent with estimates of the Schechter function cutoff energy ( E iso cutoff ) derived from much smaller FRB samples. Moreover, inferred energies of hundreds of FRBs appear collectively limited around ∼10 ⁴² erg, suggesting a physical limit on the energy reservoir of FRB sources. The corresponding isotropic-equivalent FRB source energy is consistent with the total energy available in a magnetar’s external dipole magnetic field, supporting magnetars as FRB progenitors.

We quantify pulsar timing array (PTA) sensitivity to anisotropy in the gravitational-wave (GW) background using the cross-correlation-based Fisher information matrix in the pixel and spherical harmonic bases. We use a set of simulations to empirically determine scaling relations of a PTA’s sensitivity to anisotropy with the number of pulsars N _psr in the array, the error δt on the times of arrival, the frequency f _GW of the GWs, and the angular scale ΔΩ of the anisotropy. The sensitivity scales approximately as N psr 0.8 , δt ^−0.08 , and ΔΩ ^1.6 − ΔΩ ^2.1 (depending on the ranges of ℓ and m under consideration). In addition, we use realistic simulations to project the NANOGrav PTA sensitivity to a 30 yr baseline and quantify the growth in sensitivity at several time slices. Except at the lowest frequencies, we find negligible effect on sensitivity through increasing the observation duration only. Finally, we introduce a multi-resolution pixel basis motivated by the large dependence of the sensitivity on sky location, and demonstrate the operation of the basis through a set of injections and recoveries.

In the published article, we presented inferences about the population of supermassive black hole (SMBH) binaries emitting at nanohertz gravitational-wave (GW) frequencies based on the 15 yr dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Here, we report two bugs in the astrophysical analysis software used for the published article. While parameter posteriors of our inference can be seen to have slight differences on close inspection, none of the conclusions from the original publication have changed, including the finding that the measured GW background is dominated by the most massive, high-mass-ratio SMBH binaries. Our corrected results show slightly less evidence for environmental interactions with SMBH binaries and indicate that the population of SMBH binaries contributing to the GW background occurs at slightly higher mass ratios and redshifts than indicated in the published article. Correcting these errors does not change the central conclusion that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency GW spectrum. These errors did not affect any other analyses of the NANOGrav 15 yr dataset, including the evidence for a GW background presented in another of our published articles.

The average pulse shape of a pulsar is typically stable over decadal timescales, enabling estimation of pulse times of arrival to better than a small fraction of the pulse width using matched filtering techniques. However, in North American Nanohertz Observatory for Gravitational Waves (NANOGrav) observations of PSR J1713+0747, three discrete timing events that depart from the prevailing timing model have been seen in the last 20 yr. All three correspond to morphological changes in pulse shape. Using principal component analysis, we analyze the pulse profiles of nine NANOGrav pulsars, including seven with profiles from the 15 yr data set and two with additional profiles from the forthcoming 20 yr data set. We recover the three known pulse shape change events in PSR J1713+0747 and another previously known event in PSR J1643−1224. We implement a ranking metric for candidate events and address four highly ranked candidates in this nine-pulsar sample. We also recover known slow pulse shape variations in PSR J1643−1224, PSR J1903+0327, and PSR B1937+21 and report an unexpected recurrence after ∼10 yr of one such variation in PSR B1937+21.

The NANOGrav 15 yr (NG15) data set provides evidence for a gravitational-wave background (GWB) signal at nHz frequencies, which is expected to originate either from a cosmic population of inspiraling supermassive black hole binaries or new particle physics in the early Universe. A firm identification of the source of the NG15 signal requires an accurate reconstruction of its frequency spectrum. In this Letter, we provide such a spectral characterization of the NG15 signal based on a piecewise power-law (PPL) ansatz that strikes a balance between existing alternatives in the literature. Our PPL reconstruction is more flexible than the standard constant power-law model, which describes the GWB spectrum in terms of only two parameters: an amplitude A and a spectral index γ . Concurrently, it better approximates physically realistic GWB spectra—especially those of cosmological origin—than the free spectral model, since the latter allows for arbitrary variations in the GWB amplitude from one frequency bin to the next. Our PPL reconstruction of the NG15 signal relies on individual PPL models with a fixed number of internal nodes (i.e., constant power law, broken power law, doubly broken power law, etc.), which are ultimately combined in a Bayesian model average. The data products resulting from our analysis provide the basis for fast refits of spectral GWB models.

Pulsar timing arrays (PTAs) have recently entered the detection era, quickly moving beyond the goal of simply improving sensitivity at the lowest frequencies for the sake of observing the stochastic gravitational wave background (GWB), and focusing on its accurate spectral characterization. While all PTA collaborations around the world use Fourier-domain Gaussian processes to model the GWB and intrinsic long time-correlated (red) noise, techniques to model the time-correlated radio-frequency-dependent (chromatic) processes have varied from collaboration to collaboration. Here we test a new class of models for PTA data, Gaussian processes based on time-domain kernels that model the statistics of the chromatic processes starting from the covariance matrix. As we will show, these models can be effectively equivalent to Fourier-domain models in mitigating chromatic noise. This work presents a method for Bayesian model selection across the various choices of kernel as well as deterministic chromatic models for nonstationary chromatic events and the solar wind. As PTAs turn toward high frequency (>1 yr ⁻¹ ) sensitivity, the size of the basis used to model these processes will need to increase, and these time-domain models present some computational efficiencies compared to Fourier-domain models.