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We present the first Chandra X-ray Observatory (CXO) catalog of "pulsar X-ray filaments," or "misaligned outflows." These are linear, synchrotron-radiating features powered by ultrarelativistic electrons and positrons that escape from bow shock pulsars. The filaments are misaligned with the (large) pulsar velocity, distinguishing them from the pulsar wind nebula (PWN) trail, which is also often visible in CXO ACIS images. Spectral fits and morphological properties are extracted for five secure filaments and three candidates using a uniform method. We present a search of archival CXO data for linear diffuse features; the known examples are recovered and a few additional weak candidates are identified. We also report on a snapshot CXO ACIS survey of pulsars with properties similar to the filament producers, finding no new filaments but some diffuse emission, including one PWN trail. Finally, we provide an updated model for the pulsar properties required to create filaments in light of these new observations. [ABSTRACT FROM AUTHOR]

In so-called spider pulsars, the X-ray band is dominated by intrabinary shock (IBS) synchrotron emission. While the double-peaked X-ray light curves from these shocks have been well characterized in several spider systems (both black widows and redbacks), the polarization of this emission is yet to be studied. Motivated by the new polarization capability of the Imaging X-ray Polarization Explorer and the confirmation of highly ordered magnetic fields in pulsar wind nebulae, we model the IBS polarization by employing two potential magnetic field configurations: toroidal magnetic fields imposed by the preshock pulsar wind, and tangential shock-generated fields that follow the postshock flow. We find that if IBSs host ordered magnetic fields, the synchrotron X-rays from spider binaries can display a high degree of polarization (≳50%), while the variation in polarization angle provides a good probe of the binary geometry and the magnetic field structure. Our results encourage observational polarization studies of spider pulsars that can distinguish the proposed magnetic models and that constrain the unique properties of these systems better. [ABSTRACT FROM AUTHOR]

We describe IXPE polarization observations of the pulsar wind nebula (PWN) MSH 15−52, the "Cosmic Hand." We find X-ray polarization across the PWN, with B -field vectors generally aligned with filamentary X-ray structures. High-significance polarization is seen in arcs surrounding the pulsar and toward the end of the "jet," with polarization degree PD > 70%, thus approaching the maximum allowed synchrotron value. In contrast, the base of the jet has lower polarization, indicating a complex magnetic field at significant angle to the jet axis. We also detect significant polarization from PSR B1509−58 itself. Although only the central pulse phase bin of the pulse has high individual significance, flanking bins provide lower-significance detections and, in conjunction with the X-ray image and radio polarization, can be used to constrain rotating vector model solutions for the pulsar geometry. [ABSTRACT FROM AUTHOR]

X-ray polarization from the Imaging X-ray Polarimetry Explorer (IXPE) provides an important new probe of the geometry of the pulsar emission zone and of particle acceleration in the surrounding pulsar wind nebula. However, with IXPE's modest ∼20″–30″ spatial resolution, separation of the pulsar signal from the nebula is a challenge. Conventional analysis defines an "off" phase window as pure nebular emission and subtracts its polarization to isolate the phase-varying pulsar (on-off fitting). We present a more sensitive scheme that uses external measurements of the nebula structure and the pulsar's light curve to isolate their contributions to the phase- and spatially-varying polarization via least-squares regression (simultaneous fitting). Tests with simulation data show ∼30% improvement in pulse phase polarization uncertainties, decreased background systematics, and substantially improved nebular polarization maps. Applying simultaneous fitting to early IXPE Crab data extracts additional phase bins with significant polarization. These bins show interesting departures from the well-known optical polarization sweeps, although additional exposure will be needed for precise model confrontation. [ABSTRACT FROM AUTHOR]

Recent progress with pulsar timing array experiments, especially from the NANOGrav collaboration, indicates that we are on the cusp of detecting significant signals from the inspiral of supermassive black hole (BH) binaries. While recent analysis has focused on nearby galaxies as possible sources of the loudest signals, we show that mergers in identified clusters at z ∼ 1 can have larger strain amplitudes. We make an estimate comparing the nearby Two Micron All Sky Survey redshift survey (2MASS) galaxy sample with the more distant Massive and Distant Clusters of WISE Survey (MaDCoWs) cluster sample, showing that the latter might be expected to contribute more, and louder, gravitational-wave events. Thus, the first individual source detections may well be from ultramassive BHs in clusters at z ∼ 1, rather than nearby galaxies. [ABSTRACT FROM AUTHOR]

The optical study of the heated substellar companions of black widow (BW) millisecond pulsars (MSPs) provides unique information on the MSP particle and radiation output and on the neutron star mass. Here we present an analysis of optical photometry and spectroscopy of a set of relatively bright BWs, many newly discovered in association with Fermi γ -ray sources. Interpreting the optical data requires sophisticated models of the companion heating. We provide a uniform analysis, selecting the preferred heating model and reporting on the companion masses and radii, the pulsar heating power, and neutron star mass. The substellar companions are substantially degenerate, with average densities 15–30× Solar, but are inflated above their zero temperature radii. We find evidence that the most extreme recycled BW pulsars have both large >0.8 M ⊙ accreted mass and low <108G magnetic fields. Examining a set of heavy BWs, we infer that neutron star masses larger than 2.19 M ⊙ (1 σ confidence) or 2.08 M ⊙ (3 σ confidence) are required; these bounds exclude all but the stiffest equations of state in standard tabulations. [ABSTRACT FROM AUTHOR]

Pulsar H α bow shocks provide rare opportunities to constrain the energetics and kinematics of the relativistic pulsar wind. We have acquired optical imaging and integral field unit spectroscopy of the bow shock of the millisecond pulsar PSR J1959+2048, measuring the shock symmetry axis at a position angle = 213.2 ± 0.°2 and showing that this slow nonradiative shock has a broad-to-narrow line component ratio I b / I n = 4. The data show that the pulsar’s velocity lies 2.°2 out of the plane of the sky. Coupled with a new fit for its timing proper motion, giving ÎĽ tot = 30.05 mas yrâˆ'1 and a Very Long Baseline Array (VLBA) interferometric parallax measurement giving d = 2.57 âˆ' 0.77 + 1.84 kpc (90% range), we have unusually complete information on the pulsar kinematics. The bow shock constraints on the wind momentum flux imply that, at the best-fit parallax distance, the pulsar moment of inertia must be very large and/or the H α efficiency at its modest shock velocity must be very high. [ABSTRACT FROM AUTHOR]

We have obtained a deep (670 ks) CXO ACIS image of the remarkable pulsar wind nebula (PWN) of PSR J1709−4429, in four epochs during 2018–2019. Comparison with an archival 2004 data set provides a pulsar proper motion μ = 13 ± 3 mas yr−1 at a PA of 86° ± 9° (1σ combined statistical and systematic uncertainties), precluding birth near the center of SNR G343.1−2.3. At the pulsar's characteristic age of 17 kyr, the association can be preserved through a combination of progenitor wind, birth kick, and PWN outflow. Associated TeV emission may, however, indicate an explosion in an earlier supernova. Inter-epoch comparison of the X-ray images shows that the PWN is dynamic, but we are unable to conclusively measure flow speeds from blob motion. The pulsar has generated a radio/X-ray wind bubble, and we argue that the PWN's long narrow jets are swept back by shocked pulsar wind venting from this cavity. These jets may trace the polar magnetic field lines of the PWN flow, an interesting challenge for numerical modeling. [ABSTRACT FROM AUTHOR]

We report on X-ray measurements constraining the spectral energy distribution (SED) of the high-redshift z = 5.18 blazar SDSS J013127.34−032100.1 with new XMM-Newton and NuSTAR exposures. The blazar's X-ray spectrum is well fit by a power law with Γ = 1.9 and , or a broken power law with , , and a break energy for an expected absorbing column density of , supported by spectral fitting of a nearby bright source. No additional spectral break is found at higher X-ray energies (1–30 keV). We supplement the X-ray data with lower-energy radio-to-optical measurements and Fermi-LAT gamma-ray upper limits, construct broadband SEDs of the source, and model the SEDs using a synchro-Compton scenario. This modeling constrains the bulk Doppler factor of the jets to ≥7 and ≥6 (90%) for the low- and high-NH SEDs, respectively. The corresponding beaming implies ≥130 (low NH) or (high NH) high-spin supermassive black holes similar to J0131 exist at similar redshifts. [ABSTRACT FROM AUTHOR]

We analyze the photometry and spectra of the "redback" millisecond pulsar binary J2339−0533. These observations include new measurements from Keck and the Gamma-Ray Burst Optical/Near-infrared Detector (GROND), as well as archival measurements from the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER), WIYN, Southern Astrophysical Research (SOAR), and Hobby–Eberly Telescope (HET) telescopes. The parameters derived from GROND, our primary photometric data, describe well the rest of the data sets, raising our confidence in our fitted binary properties. Our fit requires hot spots (likely magnetic poles) on the surface of the companion star, and we see evidence that these spots move over the 8 yr span of our photometry. The derived binary inclination i = 69.°3 ± 2.°3, together with the center-of-mass velocity (from the radial-velocity fits) KC = 347.0 ± 3.7 , give a fairly typical neutron star mass of 1.47 ± 0.09 M⊙. [ABSTRACT FROM AUTHOR]

We report on orbital modulation of the 100–600 MeV gamma-ray emission of the PB = 4.6 hr millisecond pulsar binary PSR J2339−0533 using 11 yr of Fermi Large Area Telescope data. The modulation has high significance (chance probability p ≈ 10−7), is approximately sinusoidal, peaks near pulsar superior conjunction, and is detected only in the low-energy 100–600 MeV band. The modulation is confined to the on-pulse interval, suggesting that the variation is in the 2.9 ms pulsed signal itself. This contrasts with the few other known systems exhibiting GeV orbital modulations, as these are unpulsed and generally associated with beamed emission from an intrabinary shock. The origin of the modulated pulsed signal is not yet clear, although we describe several scenarios, including Compton upscattering of photons from the heated companion. This would require high coherence in the striped pulsar wind. [ABSTRACT FROM AUTHOR]

We have obtained WIYN and SOAR i′ images of BL Lacertae objects and used these to detect or constrain the flux of the host galaxy. Under common standard candle assumptions, these data provide estimates of, or lower bounds on, the redshift. Our targets are a set of flat-spectrum radio counterparts of high flux Fermi Large Area Telescope sources, with sensitive spectral observations showing them to be continuum-dominated BL Lac objects. In this sample, 5 of 11 BL Lac objects yielded significant host detections, with standard candle redshifts z = 0.13-0.58. Our estimates and lower bounds are generally in agreement with other redshifts estimates, although our z = 0.374 estimate for J0543–5532 implies a significantly sub-luminous host. [ABSTRACT FROM AUTHOR]

We report on a survey for Hα bow shock emission around nearby γ-detected energetic pulsars. This survey adds three Balmer-dominated neutron star bow shocks to the six previously confirmed examples. In addition to the shock around Fermi pulsar PSR J1741–2054, we now report Hα structures around two additional γ-ray pulsars, PSR J2030+4415 and PSR J1509–5850. These are the first known examples of Hα nebulae with pre-ionization halos. With new measurements, we show that a simple analytic model can account for the angular size and flux of the bow shocks' apices. The latter, in particular, provides a new pulsar probe and indicates large moments of inertia and smaller distances than previously assumed in several cases. In particular, we show that the re-measured PSR J0437–4715 shock flux implies I = (1.7 ± 0.2) × 1045/(fH I sin i) g cm2. We also derive a distance d ≈ 0.72 kpc for the γ-ray only pulsar PSR J2030+4415 and revised distances for PSRs J1959+2048 (1.4 kpc) and J2555+6535 (∼1 kpc), smaller than the conventional DM-estimated values. Finally, we report upper limits for 94 additional LAT pulsars. An estimate of the survey sensitivity indicates that for a warm neutral medium filling factor ϕWNM ∼ 0.3 there should be a total of approximately nine Hα bow shocks in our LAT-targeted survey; given that seven such objects are now known, a much larger ϕWNM seems problematic. [ABSTRACT FROM AUTHOR]

4C+37.11, at z = 0.055, shows two compact radio nuclei, imaged by very long baseline interferometry at 7 mas separation, making it the closest known resolved supermassive black hole binary (SMBHB). An important question is whether this unique object is young and was caught on the way to a gravitational in-spiral and merger, or has “stalled” at 7 pc. We describe new radio/optical/X-ray observations of the massive host and its surrounding X-ray halo. These data reveal X-ray/optical channels following the radio outflow and large scale edges in the X-ray halo. These structures are promising targets for further study which should elucidate their relationship to the unique SMBHB core. [ABSTRACT FROM AUTHOR]

We report the probable identification of the X-ray counterpart to the γ-ray pulsar PSR J2021+4026 using imaging with the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer and timing analysis with the Fermi satellite. Given the statistical and systematic errors, the positions determined by both satellites are coincident. The X-ray source position is R.A. 20h21m30s.733, decl. +40°26'46".04 (J2000) with an estimated uncertainty of 1".3 combined statistical and systematic error. Moreover, both the X-ray to γ-ray and the X-ray to optical flux ratios are sensible assuming a neutron star origin for the X-ray flux. The X-ray source has no cataloged infrared-to-visible counterpart and, through new observations, we set upper limits to its optical emission of i' > 23.0 mag and r' > 25.2 mag. The source exhibits an X-ray spectrum with most likely both a power law and a thermal component. We also report on the X-ray and visible light properties of the 43 other sources detected in our Chandra observation. [ABSTRACT FROM AUTHOR]

The pulsar wind nebula associated with PSR J1826-1334, HESS J1825-137, is a bright very high energy (VHE) source with an angular extent of ~ 1° and spatially resolved spectroscopic TeV measurements. The gamma-ray spectral index is observed to soften with increasing distance from the pulsar, likely the result of cooling losses as electrons traverse the nebula. We describe analysis of X-ray data of the extended nebula, as well as three-dimensional time-dependent spectral energy distribution modeling, with emphasis on the spatial variations within HESS J1825-137. The multi-wavelength data place significant constraints on electron injection, transport, and cooling within the nebula. The large size and high nebular energy budget imply a relatively rapid initial pulsar spin period of 13 ± 7 ms and an age of 40 4 ± 9 kyr. The relative fluxes of each VHE zone can be explained by advective particle transport with a radially decreasing velocity profile with v(r) ∝ r-0.5. The evolution of the cooling break requires an evolving magnetic field which also decreases radially from the pulsar, B(r, t) ∝ r-0.7 Ė(t)1/2. Detection of 10 TeV flux ~80 pc from the pulsar requires rapid diffusion of high-energy particles with τesc ≈ 90 (R/10 pc)² (Ee/100 TeV)-1 year, contrary to the common assumption of toroidal magnetic fields with strong magnetic confinement. The model predicts a rather uniform Fermi Large Area Telescope (LAT) surface brightness out to ~1° from the pulsar, in good agreement with the recently discovered LAT source centered 0°.5 southwest of PSR J1826-1334 with extension 0°6 ± 0°1. [ABSTRACT FROM AUTHOR]

We have been using the 0.76 m Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory to optically monitor a sample of 157 blazars that are bright in gamma-rays being detected with high significance (⩾10σ) in one year by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. We attempt to observe each source on a three-day cadence with KAIT, subject to weather and seasonal visibility. The gamma-ray coverage is essentially continuous. KAIT observations extend over much of the five-year Fermi mission for several objects, and most have >100 optical measurements spanning the last three years. These blazars (flat-spectrum radio quasars and BL Lac objects) exhibit a wide range of flaring behavior. Using the discrete correlation function (DCF), here we search for temporal relationships between optical and gamma-ray light curves in the 40 brightest sources in hopes of placing constraints on blazar acceleration and emission zones. We find strong optical-gamma-ray correlation in many of these sources at time delays of ∼1 to ∼10 days, ranging between –40 and +30 days. A stacked average DCF of the 40 sources verifies this correlation trend, with a peak above 99% significance indicating a characteristic time delay consistent with 0 days. These findings strongly support the widely accepted leptonic models of blazar emission. However, we also find examples of apparently uncorrelated flares (optical flares with no gamma-ray counterpart and gamma-ray flares with no optical counterpart) that challenge simple, one-zone models of blazar emission. Moreover, we find that flat-spectrum radio quasars tend to have gamma-rays leading the optical, while intermediate- and high-synchrotron peak blazars with the most significant peaks have smaller lags/leads. It is clear that long-term monitoring at high cadence is necessary to reveal the underlying physical correlation. [ABSTRACT FROM AUTHOR]

We report on a new Chandra exposure of PSR J1809–2332, the recently discovered pulsar powering the bright EGRET source 3EG J1809–2328. By registration of field X-ray sources in an archival exposure, we measure a significant proper motion for the pulsar point source over an ≈11 yr baseline. The shift of 0.30 ± 0.″06 (at P.A. = 153.3 ± 18.4) supports an association with proposed supernova remnant (SNR) parent G7.5–1.7. Spectral analysis of diffuse emission in the region also supports the interpretation as a hard wind nebula trail pointing back toward the SNR. [ABSTRACT FROM AUTHOR]

Traditional pulsar polarization sweep analysis starts from the point dipole rotating vector model (RVM) approximation. If augmented by a measurement of the sweep phase shift, one obtains an estimate of the emission altitude (Blaskiewicz et al.). However, a more realistic treatment of field line sweepback and finite altitude effects shows that this estimate breaks down at modest altitude ∼0.1 RLC. Such radio emission altitudes turn out to be relevant to the young energetic and millisecond pulsars that dominate the γ-ray population. We quantify the breakdown height as a function of viewing geometry and provide simple fitting formulae that allow observers to correct RVM-based height estimates, preserving reasonable accuracy to R ∼ 0.3 RLC. We discuss briefly other observables that can check and improve height estimates. [ABSTRACT FROM AUTHOR]

We report on optical spectroscopy of 165 flat spectrum radio quasars (FSRQs) in the Fermi 1LAC sample, which have helped allow a nearly complete study of this population. Fermi FSRQs show significant evidence for non-thermal emission even in the optical; the degree depends on the γ-ray hardness. They also have smaller virial estimates of hole mass than the optical quasar sample. This appears to be largely due to a preferred (axial) view of the γ-ray FSRQ and non-isotropic (H/R ∼ 0.4) distribution of broad-line velocities. Even after correction for this bias, the Fermi FSRQs show higher mean Eddington ratios than the optical population. A comparison of optical spectral properties with Owens Valley Radio Observatory radio flare activity shows no strong correlation. [ABSTRACT FROM AUTHOR]