Observer's Line Of Sight Research Articles

Young radio-loud gamma-ray pulsar light curve fitting

Context. Since the launch of the Fermi gamma-ray telescope, several hundred radio-loud gamma-ray pulsars have been detected, many belonging to millisecond pulsars but some belonging to the young pulsar population with spin periods longer than 30 ms. Aims. Observing simultaneously pulsed radio and gamma-ray emission from these stars helps to constrain the geometry and radiation mechanisms within their magnetosphere and to localize the multiple photon production sites. In this paper we fit the time-aligned gamma-ray light curves of young radio-loud gamma-ray pulsars. We assume a dipole force-free magnetosphere where radio photons emanate from high altitudes above the polar caps and gamma rays originate from outside the light cylinder, within the striped wind current sheet. Methods. We computed a full atlas of radio and gamma-ray pulse profiles depending on the magnetic axis obliquity and line-of-sight inclination with respect to the neutron star rotation axis. By applying a χ2 fitting technique, we were able to pin down accurately the magnetosphere geometry. Further constraints were obtained from radio polarization measurement following the rotating vector model, including aberration and retardation effects. Results. We find a good agreement between our model and the time-aligned single- or double-peaked gamma-ray pulsar observations. We deduce the magnetic inclination angle and the observer line of sight with respect to the rotation axis within a small error bar. The distinction between radio-loud or radio-quiet gamma-ray pulsars or only radio pulsars can entirely be related to the geometry of the associated emitting regions. Conclusions. The high-altitude polar cap model combined with the striped wind represents a minimalistic approach able to reproduce a wealth of gamma-ray pulse profiles for young radio pulsars. Based on self-consistent force-free simulations, it gives a full geometrical picture of the emission properties without resorting to detailed knowledge of the individual particle dynamics and energetics.

Periodic Fast Radio Bursts from Luminous X-ray Binaries

Abstract The discovery of periodicity in the arrival times of the fast radio bursts (FRBs) poses a challenge to the oft-studied magnetar scenarios. However, models that postulate that FRBs result from magnetized shocks or magnetic reconnection in a relativistic outflow are not specific to magnetar engines; instead, they require only the impulsive injection of relativistic energy into a dense magnetized medium. Motivated thus, we outline a new scenario in which FRBs are powered by short-lived relativistic outflows (“flares”) from accreting black holes or neutron stars, which propagate into the cavity of the pre-existing (“quiescent”) jet. In order to reproduce FRB luminosities and rates, we are driven to consider binaries of stellar-mass compact objects undergoing super-Eddington mass transfer, similar to ultraluminous X-ray (ULX) sources. Indeed, the host galaxies of FRBs, and their spatial offsets within their hosts, show broad similarities with ULXs. Periodicity on timescales of days to years could be attributed to precession (e.g., Lens-Thirring) of the polar accretion funnel, along which the FRB emission is geometrically and relativistically beamed, which sweeps across the observer line of sight. Accounting for the most luminous FRBs via accretion power may require a population of binaries undergoing brief-lived phases of unstable (dynamical-timescale) mass transfer. This will lead to secular evolution in the properties of some repeating FRBs on timescales of months to years, followed by a transient optical/IR counterpart akin to a luminous red nova, or a more luminous accretion-powered optical/X-ray transient. We encourage targeted FRB searches of known ULX sources.

A Bayesian approach to matching thermonuclear X-ray burst observations with models

ABSTRACT We present a new method of matching observations of Type-I (thermonuclear) X-ray bursts with models, comparing the predictions of a semi-analytic ignition model with X-ray observations of the accretion-powered millisecond pulsar SAX J1808.4–3658 in outburst. We used a Bayesian analysis approach to marginalize over the parameters of interest and determine parameters such as fuel composition, distance/anisotropy factors, neutron star mass, and neutron star radius. Our study includes a treatment of the system inclination effects, inferring that the rotation axis of the system is inclined $\left(69^{+4}_{-2}\right)^\circ$ from the observers line of sight, assuming a flat disc model. This method can be applied to any accreting source that exhibits Type-I X-ray bursts. We find a hydrogen mass fraction of $0.57^{+0.13}_{-0.14}$ and CNO metallicity of $0.013^{+0.006}_{-0.004}$ for the accreted fuel is required by the model to match the observed burst energies, for a distance to the source of $3.3^{+0.3}_{-0.2}\, \mathrm{kpc}$. We infer a neutron star mass of $1.5^{+0.6}_{-0.3}\, \mathrm{M}_{\odot }$ and radius of $11.8^{+1.3}_{-0.9}\, \mathrm{km}$ for a surface gravity of $1.9^{+0.7}_{-0.4}\times 10^{14}\, \mathrm{cm}\, \mathrm{s}^{-2}$ for SAX J1808.4–3658.

On Optimal Pursuit Trajectories for Visibility-Based Target-Tracking Game

In this paper, we address a class of visibility-based pursuit-evasion game in which a mobile observer tries to maintain a line-of-sight (LOS) with a mobile target in an environment containing obstacles. The observer knows the current position of the target as long as the target is in the observer's LOS. At first, we address this problem in an environment containing a single corner. We formulate the game as an optimal control problem of maximizing the time for which the observer can keep the reachability set of the target in its field-of-view. Using Pontryagin's principle, we show that the primitives for optimal motion of the observer are straight lines ( $ST$ ) and spiral-like curves ( $C$ ). Next, we present the synthesis of the optimal trajectories from any given initial position of the observer. We show that the optimal path of the observer belongs to the class $\lbrace ST,C-ST,ST-C-ST\rbrace$ . Given any initial position of the target, we present a partition of the workspace around a corner based on the optimal control policy of the observer.

THE GLOBAL IMPLICATIONS OF THE HARD EXCESS. II. ANALYSIS OF THE LOCAL POPULATION OF RADIO-QUIET AGNs

ABSTRACT Active galactic nuclei (AGNs) show evidence for reprocessing gas, outflowing from the accreting black hole. The combined effects of absorption and scattering from the circumnuclear material likely explain the “hard excess” of X-ray emission above 20 keV, compared with the extrapolation of spectra from lower X-ray energies. In a recent Suzaku study, we established that the ubiquitous hard excess in hard, X-ray-selected, radio-quiet type 1 AGNs is consistent with a reprocessing of the X-ray continuum in an ensemble of clouds, located tens to hundreds of gravitational radii from the nuclear black hole. Here we add hard X-ray-selected, type 2 AGNs to extend our original study and show that the gross X-ray spectral properties of the entire local population of radio-quiet AGNs may be described by a simple unified scheme. We find a broad, continuous distribution of spectral hardness ratio and Fe Kα equivalent width across all AGN types, which can be reproduced by varying the observer's sightline through a single, simple model cloud ensemble, provided that the radiative transfer through the model cloud distribution includes not only photoelectric absorption but also three-dimensional (3D) Compton scattering. Variation in other parameters of the cloud distribution, such as column density or ionization, should be expected between AGNs, but such variation is not required to explain the gross X-ray spectral properties.

DETERMINING NEUTRON STAR PROPERTIES BY FITTING OBLATE-STAR WAVEFORM MODELS TO X-RAY BURST OSCILLATIONS

We describe sophisticated new Bayesian analysis methods that make it possible to estimate quickly the masses and radii of rapidly rotating, oblate neutron stars by fitting oblate-star waveform models to energy-resolved observations of the X-ray oscillations produced by a hot spot on such stars. We conclude that models that take the oblate shape of the star into account should be used for stars with large radii and rotation rates $>300$ Hz. We find that a 25% variation of the temperature of the hot spot with latitude does not significantly bias estimates of the mass $M$ and equatorial radius $R_{\rm eq}$ derived by fitting a model that assumes a uniform-temperature spot. Our results show that fits of oblate-star waveform models to waveform data can simultaneously determine $M$ and $R_{\rm eq}$ with uncertainties $\lesssim\,$7% if (1) the star's rotation rate is $\gtrsim\,$$600$ Hz; (2) the spot center and observer's sightline are both within $30^\circ$ of the star's rotational equator; (3) the oscillations have a fractional rms amplitude $\gtrsim\,$10%; and (4) $\gtrsim$$10^7$ counts are collected from the star. This is a realistic fractional amplitude, and this many counts could be obtained from a single star by the accepted NICER and proposed LOFT and AXTAR space missions by combining data from many X-ray bursts. These uncertainties are small enough to improve substantially our understanding of cold, ultradense matter.

SIGNATURES OF PHOTON-AXION CONVERSION IN THE THERMAL SPECTRA AND POLARIZATION OF NEUTRON STARS

Conversion of photons into axions under the presence of a strong magnetic field can dim the radiation from magnetized astrophysical objects. Here we perform a detailed calculation aimed at quantifying the signatures of photon-axion conversion in the spectra, light curves, and polarization of neutron stars (NSs). We take into account the energy and angle-dependence of the conversion probability and the surface thermal emission from NSs. The latter is computed from magnetized atmosphere models that include the effect of photon polarization mode conversion due to vacuum polarization. The resulting spectral models, inclusive of the general-relativistic effects of gravitational redshift and light deflection, allow us to make realistic predictions for the effects of photon to axion conversion on observed NS spectra, light curves, and polarization signals. We identify unique signatures of the conversion, such as an increase of the effective area of a hot spot as it rotates away from the observer line of sight. For a star emitting from the entire surface, the conversion produces apparent radii that are either larger or smaller (depending on axion mass and coupling strength) than the limits set by NS equations of state. For an emission region that is observed phase-on, photon-axion conversion results in an inversion of the plane of polarization with respect to the no-conversion case. While the quantitative details of the features that we identify depend on NS properties (magnetic field strength, temperature) and axion parameters, the spectral and polarization signatures induced by photon-axion conversion are distinctive enough to make NSs very interesting and promising probes of axion physics.

PSR J0007+7303 IN THE CTA1 SUPERNOVA REMNANT: NEW GAMMA-RAY RESULTS FROM TWO YEARS OFFERMILARGE AREA TELESCOPE OBSERVATIONS

One of the main results of the Fermi Gamma-Ray Space Telescope is the discovery of {\gamma}-ray selected pulsars. The high magnetic field pulsar, PSR J0007+7303 in CTA1, was the first ever to be discovered through its {\gamma}-ray pulsations. Based on analysis of 2 years of LAT survey data, we report on the discovery of {\gamma}-ray emission in the off-pulse phase interval at the ~ 6{\sigma} level. The flux from this emission in the energy range E \geq 100 MeV is F_100 = (1.73\pm0.40)\times10^(-8) photons/cm^2/s and is best fitted by a power law with a photon index of {\Gamma} = 2.54\pm0.14. The pulsed {\gamma}-ray flux in the same energy range is F_100 = (3.95\pm0.07)\times10^(-7) photons/cm^2/s and is best fitted by an exponentially-cutoff power-law spectrum with a photon index of {\Gamma} = 1.41 \pm 0.23 and a cutoff energy E_c = 4.04 \pm 0.20 GeV. We find no flux variability neither at the 2009 May glitch nor in the long term behavior. We model the {\gamma}-ray light curve with two high-altitude emission models, the outer gap and slot gap, and find that the model that best fits the data depends strongly on the assumed origin of the off-pulse emission. Both models favor a large angle between the magnetic axis and observer line of sight, consistent with the nondetection of radio emission being a geometrical effect. Finally we discuss how the LAT results bear on the understanding of the cooling of this neutron star.

Earthquake deformation field characteristics associated with the 2010 Yushu M s7.1 earthquake

The spatial distribution and characterization of a heavily damaged area can be determined by studying surface ruptures of seismogenic faults. If the distribution of surface ruptures can be obtained shortly after they occur, then areas heavily damaged by an earthquake can be readily identified. The information can then be used as a guide for earthquake relief programs. In this paper, an intensity offset-tracking method applied to an ALOS PALSAR image is used to map the Yushu earthquake rupture and to identify the faults activated by the earthquake. Azimuthal displacement analysis indicates that the surface rupture is about 55 km long, running from the epicenter to the southeast, trending N310°W, with a relative displacement of ∼1 m characterized by sinistral slip. The result of range displacement observations indicates that the north wall of the fault is dominated by decreases (i.e., uplift in line of sight observations), whereas in the south wall of the fault, the range displacement is dominated by increases (drops in line of sight observations). Given the position from which the images were recorded, this means that the north wall moves westward, and the south wall move eastward, i.e., left-lateral slip motion across the fault. Finally, an earthquake disaster assessment using computer-assisted image analysis software shows that buildings near the fault rupture have been destroyed most heavily; therefore, the shape of the heavily damage belt is controlled partially by the fault rupture’s geometry and the damage degree relates to the magnitude of displacement field.

The magnetic field and the location of the TeV emitter in Cygnus X-1 and LS 5039

Context. Cygnus X-1 and LS 5039 are two X-ray binaries observed at TeV energies. Both sources are compact systems, contain jet-like (radio) structures, and harbor very luminous O sta rs. A TeV signal has been found around the superior conjunction of the compact object in both objects, when the highest gamma-ray opacities are expected. Aims. We investigate the implications of finding TeV emission from Cygnus X-1 and LS 5039 around the superior conjunction, since this can give information on the system magnetic field and the location of the TeV emitter. Methods. Using the very high-energy spectra and fluxes observed aroun d the superior conjunction in Cygnus X-1 and LS 5039, we compute the absorbed luminosity that is caused by pair creation in the stellar photon field for di fferent emitter positions with respect to the star and the observer line of sight. The role of the magnetic field and electromagnetic cascading are discussed. For the case of ineffi cient electromagnetic cascading, the expected secondary synchrotron fluxes are compared with the observed ones at X-ra y energies. Results. We find that, in Cygnus X-1 and LS 5039, either the magnetic fiel d in the star surroundings is much smaller than the one expected for O stars or the TeV emitter is located at a distance > 10 12 cm from the compact object. Conclusions. Our results strongly suggest that the TeV emitters in Cygnus X-1 and LS 5039 are located at the borders of the binary system and well above the orbital plane. This would not agree with those models for which the emitter is well inside the system, like the innermost-jet region (Cygnus X-1 and LS 5039; microquasar scenario), or the region between the pulsar and the primary star (LS 5039; standard pulsar scenario).

Further Analysis of Absorption Features in the X‐Ray Spectrum of the Isolated Neutron Star 1E 1207.4−5209

Observation of the isolated neutron star 1E 1207.4-5209 shows three distinct absorption features, regularly spaced at 0.7, 1.4, and 2.1 keV, which vary in phase with the star's rotation. In this paper, we attempt to clarify the understanding of these absorption lines by using the quantum theory of the cyclotron radiation under the quadrupole approximation, given in our previous papers. We found that these lines are more likely to arise from the electron-cyclotron absorption, rather than from proton-cyclotron. We explain why the observed fundamental and the second-harmonic cyclotron features with significantly different absorption coefficients have nearly the same strength. Assuming that the absorption features arise from an approximately uniform narrow column at the bottom of the pole region of neutron star, we complete a simplified model calculation to fit the observational data; thus, the included angle between the spinning axis and the observer's sight line, as well as that between the spinning and the magnetic axes of the neutron star, are determined by this method. The geometrical configuration inferred from the deduced directions of these axes is helpful to better understand the physics and some other relevant observation facts of star 1E 1207.4-5209.

Radio Compact Structure in OQ 208

In light of the newly detected proper motions in two lobes of the gigahertz-peaked spectrum source OQ 208, we introduced the Doppler effect into the fit to the observed spectral turnovers. It shows that two models containing the same beaming parameters but having quite different absorption mechanisms can fit two lobes equally well. In one model, only FFA (free-free absorption) is needed. In the other model, both the SSA (synchrotron self-absorption) in two lobes and an extra FFA toward the southwest lobe are required. Our analysis cannot unambiguously distinguish between two models. Although the FFA process is invoked in both models, the geometry of the absorbing gas could be quite different. However, the beaming effects in both models are eventually the same, with an intrinsically symmetrical advance speed of ≈0.33c for the two-sided lobes and an inclination angle of ≈33° for the radio jet emission to the observer's line of sight.

Comparing geometrical and delay radio emission heights in pulsars

We use a set of carefully selected, published average multifrequency polarimetric observations for six bright cone-dominated pulsars and devise a method to combine the multifrequency polarization position angle (PPA) sweep traverses. We demonstrate that the PPA traverse is in excellent agreement with the rotating vector model over this broad frequency range, confirming that radio emission emanates from perfectly dipolar field lines. Correcting for the effect of retardation we firmly establish the steepest gradient point in the combined PPA traverse to be the fiducial phase in these pulsars. We use this combined curve and inputs from earlier studies to determine the geometrical angles of the neutron star, namely a - the angle between the rotation and the dipole magnetic axis and β - the angle between the magnetic axis and the observers line of sight. Using these estimates of α and β we derive the geometrical emission heights (r geo ). Further, using the relativistic beaming model based on effects of aberration and retardation, we find the delay emission heights r BCW delay suggested by Blaskiewicz et al. (1991). We find in general r BCW delay < r geo , which can be explained by a broad emission region operating in pulsars or/and the signature of the magnetic field sweepback effect as suggested by Dyks et al. (2003). For pulsars with central core emission in our sample, we find the peak of the central core component to lag behind the steepest gradient of the PPA traverse at several frequencies. Significant frequency evolution of the core width is observed over this frequency range. The above facts strongly suggest: (a) the peak core emission does not lie on the fiducial plane containing the dipole magnetic axis and the rotation axis, and (b) the core emission does not originate from the polar cap surface.

The Transition between Nonorthogonal Polarization Modes in PSR B2016+28 at 1404 MHz

Polarization observations of the radio emission from PSR B2016+28 at 1404 MHz reveal properties that are consistent with two, very different, interpretations of the pulsar's viewing geometry. The pulsar's average polarization properties show a rapid change in position angle (PA) near the pulse center, suggesting that the observer's sightline nearly intersects the star's magnetic pole. But single pulse, polarization observations of the pulsar show nearly orthogonal modes of polarization following relatively flat and parallel PA trajectories across the pulse, suggesting that the sightline is far from the pole. Additionally, PA histograms reveal a connecting bridge, of unknown origin, joining the modal PA trajectories over much of the pulse and following the rapid PA change shown in the average data. The nonorthogonality of polarization modes is incorporated in a statistical model of radio polarization to account for the deviations from mode orthogonality that are observed in the pulsar. The model is used to interpret the rapid PA change and modal connecting bridge as a longitudinally-resolved transition between modes of nonorthogonal polarization. Thus, the modal PA trajectories are argued to reflect the pulsar's true viewing geometry. This interpretation is consistent with the pulsar's morphological classification, preserves the Radhakrishnan & Cooke model of pulsar radio emission, and avoids the complication that the modal connecting bridge might be produced by some other emission mechanism. The statistical model's ability to simulate the rich variety of polarization properties observed in the emission lends additional support to the model's applicability and its underlying assumption that the polarization modes occur simultaneously.

Multi-site observations of the association between aurora and plasma convection in the cusp/polar cap during a southeastward(&amp;lt;i&amp;gt;B&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;&amp;lt;/i&amp;gt; &amp;lt;u&amp;gt;~&amp;lt;/u&amp;gt; |&amp;lt;i&amp;gt;B&amp;lt;sub&amp;gt;z&amp;lt;/sub&amp;gt;&amp;lt;/i&amp;gt;|) IMF orientation

Abstract. In a case study we demonstrate the spatiotemporal structure of aurora and plasma convection in the cusp/polar cap when the interplanetary magnetic field (IMF) Bz &lt; 0 and By ~ | Bz | (clock angle in GSM Y - Z plane: ~ 135°). This IMF orientation elicited a response different from that corresponding to strongly northward and southward IMF. Our study of this "intermediate state" is based on a combination of ground observations of optical auroral emissions and ionospheric plasma convection. Utilizing all-sky cameras at NyAlesund, Svalbard and Heiss Island (Russian arctic), we are able to monitor the high-latitude auroral activity within the ~10:00–15:00 MLT sector. Information on plasma convection is obtained from the SuperDARN radars, with emphasis placed on line of sight observations from the radar situated in Hankasalmi, Finland (Cutlass). A central feature of the auroral observations in the cusp/polar cap region is a ~ 30-min long sequence of four brightening events, some of which consists of latitudinally and longitudinally separated forms, which are found to be associated with pulsed ionospheric flows in merging and lobe convection cells. The auroral/convection events may be separated into different forms/cells and phases, reflecting a spatiotem-poral evolution of the reconnection process on the dayside magnetopause. The initial phase consists of a brightening in the postnoon sector (~ 12:00–14:00 MLT) at ~ 73° MLAT, accompanied by a pulse of enhanced westward convection in the postnoon merging cell. Thereafter, the event evolution comprises two phenomena which occur almost simultaneously: (1) westward expansion of the auroral brightening (equatorward boundary intensification) across noon, into the ~ 10:00–12:00 MLT sector, where the plasma convection subsequently turns almost due north, in the convection throat, and where classical poleward moving auroral forms (PMAFs) are observed; and (2) auroral brightening at slightly higher latitudes (~ 75° MLAT) in the postnoon lobe cell, with expansion towards noon, giving rise to a clear cusp bifurcation. The fading phase of PMAFs is accompanied by a "patch" of enhanced (~ 1 km/s) poleward-directed merging cell convection at high latitudes (75–82° MLAT), e.g. more than 500 km poleward of the cusp equatorward boundary. The major aurora/convection events are recurring at ~ 5–10 min intervals. Key words. Magnetospheric physics (auroral phenomena; magnetopause, cusp, and boundary layers; plasma convection)

Afterglow Emission from Naked Gamma-Ray Bursts

We calculate the afterglow emission for gamma-ray bursts (GRBs) going off in an extremely low density medium, referred to as naked bursts. Our results also apply to the case where the external medium density falls off sharply at some distance from the burst. The observed afterglow flux in this case originates at high latitudes, i.e., where the angle between the fluid velocity and the observer line of sight is greater than Γ-1. The observed peak frequency of the spectrum for naked bursts decreases with observer time as t-1, and the flux at the peak of the spectrum falls off as t-2. The 2-10 keV X-ray flux from a naked burst of average fluence should be observable by the Swift satellite for time duration of about 103 longer than the burst variability timescale. The high-latitude emission contributes to the early X-ray afterglow flux for any GRB, not just naked bursts, and can be separated from the shocked interstellar medium emission by their different spectral and temporal properties. Measurements of the high-latitude emission could be used to map the angular structure of GRB-producing shells.

Damped Lyα Absorbers as Tracers of the Evolution of the Mass Spectrum of Interstellar Matter in the Galaxy Population

A generally accepted approach to the interpretation of the damped Lyα absorption systems seen in QSO spectra is to attribute them to cases in which the observer's sight line passes through high column density gaseous disks of galaxies, i.e., spiral galaxies or the progenitors of spirals. Here we consider an alternative natural possibility, consistent with available observational data, namely that the absorption is simply associated with neutral gas in giant hydrogen clouds that could be associated with any type of gaseous galaxy or protogalaxy. At high redshift such galaxies could be the progenitors of different types of galaxies (e.g., ellipticals, spirals, etc.) that are observed at the present epoch. We show that the observational data, which include low to moderate redshift data recently obtained with HST and moderate to high-redshift data compiled using large ground-based telescopes over the last decade, coupled with some reasonable assumptions about the properties of giant hydrogen clouds in galaxies, can be used to form the two-dimensional distribution for the number of damped Lyα systems in redshift and column density, d2/dz dNH I, over the redshift interval 0.1 < z < 3.5. This can be further extended to redshift z = 0 using information provided by 21 cm observations of nearby galaxies. By combining this result with the assumption that damped Lyα absorbers behave like the giant hydrogen clouds in our Galaxy and neighboring galaxies, we show that the mass spectrum of clouds responsible for the damped Lyα absorption systems can be derived. This mass spectrum can be used to study the evolution of the giant hydrogen cloud population associated with galaxies over cosmic time. An interesting feature of the mass spectrum is that its steepness increases systematically as the redshift decreases over the interval 3 < z < 0.75. This effect is likely to be related to intensive star formation processes, which lead to the destruction of high-mass clouds and the formation of numerous low-mass clouds.

Some Intrinsic Properties of Parsec-scale Radio Jets

AbstractA simple model for estimating the intrinsic flow direction and speed in the parsec-scale jets associated with extragalactic radio sources is presented. In this model, radio source brightness asymmetries are attributed to the apparent amplification caused when a relativistic jet of radiating material is somewhat aligned with the observers line of sight. The knots of emission commonly seen in parsec-scale radio jets are interpreted as shocks in the relativistic fluid.