arXiv subjects

Roger W. Romani

Publications and source records attributed to Roger W. Romani.

At least 109 records · Page 6Linked to original sources

arXiv2020

Atmospheric Circulation on Black Widow Companions

We present a model for atmospheric wind circulation in binary millisecond pulsar (MSP) companions, showing how the optical light curve (LC) and radial velocities (RV) are sensitive to the wind flow, causing LC orbital phase shifts and asymmetries, as observed for several `spider' MSP. Velocity widths of spectral lines offer additional opportunities for measuring surface wind speed. As examples, we fit optical data for a black widow pulsar J1959+2048 and a redback pulsar J2215+5135; the wind heating models (WH) are statistically strongly preferred over direct heating (DH) for both objects, although the latter is even better fit with a heated spot. In general, WH effects tend to increase the inferred orbital inclination $i$ and decrease the inferred companion center-of-mass radial velocity amplitude $K_\mathrm{c}$; both effects decrease the inferred neutron star mass. Even with such a decrease, we find large masses for the two neutron stars: $2.18\pm0.09M_\odot$ and $2.28^{+0.10}_{-0.09}M_\odot$, respectively (for the modest surface speeds fit from the bulk heat flow; supersonic photospheric winds can slightly change these values). These are among the highest masses known, and our improved modeling increases confidence that the results are important for understanding the dense matter equation of state.

astro-ph.HE↗

arXiv2019

Multiband Optical Light Curves of Black-Widow Pulsars

We collect new and archival optical observations of nine "black-widow" millisecond pulsar binaries. New measurements include direct imaging with the Keck, Gemini-S, MDM, and LCO 2~m telescopes. This is supplemented by synthesized colors from Keck long-slit spectra. Four black-widow optical companions are presented here for the first time. Together these data provide multicolor photometry covering a large fraction of the orbital phase. We fit these light curves with a direct (photon) heating model using a version of the ICARUS light-curve modeling code. The fits provide distance and fill-factor estimates, inclinations, and heating powers. We compare the heating powers with the observed GeV luminosities, noting that the ratio is sensitive to pulsar distance and to the gamma-ray beaming. We make a specific correction for "outer-gap" model beams, but even then some sources are substantially discrepant, suggesting imperfect beaming corrections and/or errors in the fit distance. The fits prefer large metal abundance for half of the targets, a reasonable result for these wind-stripped secondaries. The companion radii indicate substantial Roche-lobe filling, $f_c \approx 0.7-1$ except for PSR J0952$-$0607, which with $f_c< 0.5$ has a companion density $ρ\approx 10\,{\rm g\,cm^{-3}}$, suggesting unusual evolution. We note that the direct-heating fits imply large heating powers and rather small inclinations, and we speculate that unmodeled effects can introduce such bias.

astro-ph.HE↗

arXiv2019

The Synchrotron Emission Pattern of IntraBinary Shocks

We model millisecond pulsars winds colliding with radiatively-driven companion winds in black widow and redback systems. For the redbacks, the geometry of this intrabinary shock (IBS) is quite sensitive to the expected equatorial concentration in the pulsar outflow. We thus analytically extend IBS thin-shock models to $\sim \sin^{2n}θ$ pulsar winds. We compute the synchrotron emission from such shocks, including the build-up and cooling of the particle population as it accelerates along the IBS. For reasonable parameters, this IBS flux dominates the binary emission in the X-ray band. The modeling shows subtle variation in spectrum across the IBS peak, accessible to sensitive X-ray studies. As example applications, we fit archival CXO/XMM data from the black widow pulsar J1959+2048 and the redback PSR J2339-0533, finding that the model reproduces well the orbital light curve profiles and energy spectra. The results show a very hard injected electron spectrum, indicating likely dominance by reconnection. The light curve fitting is sensitive to the geometric parameters, including the very important orbital inclination $i$. Coupled with optical fits of the companion star, such IBS X-ray light curve modeling can strongly constrain the binary geometry and the energetics of the MSP wind.

astro-ph.HE↗

arXiv2019

Astro2020 Science White Paper: Using X-Ray Polarimetry to Probe the Physics of Black Holes and Neutron Stars

This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results for bright objects, a follow-up mission could be one order of magnitude more sensitive and would be able to use a broader bandpass to perform physics type experiments for representative samples of sources.

astro-ph.HE↗

arXiv2018

Signatures of Intra-binary shock emission in the black widow pulsar binary PSR J2241-5236

We report on high-energy properties of the black widow pulsar PSR J2241$-$5236 in the X-ray and the Fermi-LAT (GeV gamma-ray) bands. In the LAT band, the phase-averaged gamma-ray light curve shows orbital modulation below $\sim$1 GeV with a chance probability ($p$) monotonically decreasing with time to $p\sim 10^{-5}$. The peak of the light curve is near the superior conjunction of the pulsar (binary phase $ϕ_{\rm B}\approx 0.25$). We attribute the modulation to the intra-binary shock (IBS) emission and search for IBS signatures in the archival X-ray data. We find that the X-ray spectral fit requires a non-thermal component, which implies a possible IBS origin of the X-rays. We discuss our observations in the context of IBS scenarios.

astro-ph.HE↗

arXiv2018

The Polarization Behavior of Relativistic Synchrotron Jets

We describe a geometric model for synchrotron radiation from blazar jets, involving multiple emission zones with turbulent magnetic fields and a transient core with a helical B field. Including the effects of jet divergence, particle cooling and the Relativistic PA rotation (RPAR) to the observer frame, we find polarization behavior consistent with recent data from monitoring campaigns. We predict that under some circumstances multi-$π$ rotation phases should exhibit relativistically-induced steps in rate $d{\rm PA}/dt$ and modulation in polarization $Π$ that can be helpful in pinning down the jet $Γ$ and $θ_{\rm obs}$. Also, RPAR enhances waveband differences that will be particularly interesting for comparing radio, optical and, soon, X-ray PA and $Π$ variations.

astro-ph.HE↗

arXiv2018

PSR J0636+5128: A Heated Companion in a Tight Orbit

We present an analysis of archival Gemini g', r', K and Keck H, K_s imaging of this nearby short period binary (P_B=95.8min) 2.87ms pulsar. The heated companion is clearly detected. Direct pulsar heating provides an acceptable model at the revised >700pc parallax distance. The relatively shallow light curve modulation prefers an inclination i<40deg; this high latitude view provides a likely explanation for the lack of radio signatures of wind dispersion or eclipse. It also explains the low minimum companion mass and, possibly, the faintness of the source in X- and gamma-rays.

astro-ph.HE↗

arXiv2018

SED constraints on the highest-$z$ blazar jet: QSO J0906+6930

We report on Gemini, NuSTAR and 8-year Fermi observations of the most distant blazar QSO~J0906$+$6930 ($z=5.48$). We construct a broadband spectral energy distribution (SED) and model the SED using a synchro-Compton model. The measurements find a $\sim 4 \times 10^9 M_\odot$ mass for the black hole and a spectral break at $\sim$4 keV in the combined fit of the new NuSTAR and archival Chandra data. The SED fitting constrains the bulk Doppler factor $δ$ of the jet to $9^{+2.5}_{-3}$ for QSO~J0906$+$6930. Similar, but weaker $δ$ constraints are derived from SED modeling of the three other claimed $z>5$ blazars. Together, these extrapolate to $\sim620$ similar sources, fully 20\% of the optically bright, high mass AGN expected at $5<z<5.5$. This has interesting implications for the early growth of massive black holes.

astro-ph.HE↗

arXiv2017

Mid-band gravitational wave detection with precision atomic sensors

We assess the science reach and technical feasibility of a satellite mission based on precision atomic sensors configured to detect gravitational radiation. Conceptual advances in the past three years indicate that a two-satellite constellation with science payloads consisting of atomic sensors based on laser cooled atomic Sr can achieve scientifically interesting gravitational wave strain sensitivities in a frequency band between the LISA and LIGO detectors, roughly 30 mHz to 10 Hz. The discovery potential of the proposed instrument ranges from from observation of new astrophysical sources (e.g. black hole and neutron star binaries) to searches for cosmological sources of stochastic gravitational radiation and searches for dark matter.

astro-ph.IM↗

arXiv2017

The Asymmetric Bowshock/PWN of PSR J2124$-$3358

We describe new measurements of the remarkable H$α$/UV/X-ray bowshock and pulsar wind nebula of the isolated millisecond pulsar PSR J2124$-$3358. {\it CXO} imaging shows a one-sided jet structure with a softer equatorial outflow. KOALA IFU spectroscopy shows that non-radiative emission dominates the bow shock and that the H$α$ nebula is asymmetric about the pulsar velocity with an elongation into the plane of the sky. We extend analytic models of the contact discontinuity to accommodate such shapes and compare these to the data. Using {\it HST} UV detections of the pulsar and bow shock, radio timing distance and proper motion measurements and the {\it CXO}-detected projected spin axis we model the 3-D PWN momentum flux distribution. The integrated momentum flux depends on the ionization of the ambient ISM, but for an expected ambient WNM we infer $I = 2.4 \times 10^{45} {\rm g\,cm^2}$. This implies $M_{NS} = 1.6-2.1 M_\odot$, depending on the equation of state, which in turn suggests that the MSP gained significant mass during recycling and then lost its companion. However, this conclusion is at present tentative, since lower ionization allows $\sim 30\%$ lower masses and uncertainty in the parallax allows up to 50% error.

astro-ph.HE↗

arXiv2017

High-Energy Variability of PSR J1311-3430

We have studied the variability of the black-widow type binary millisecond pulsar PSR J1311$-$3430 from optical to gamma-ray energies. We confirm evidence for orbital modulation in the weak off-pulse $\ge$200-MeV emission, with a peak at $ϕ_B\approx 0.8$, following pulsar inferior conjunction. The peak has a relatively hard spectrum, extending above $\sim 1$\,GeV. XMM-Newton and Swift UV observations also show that this source's strong X-ray flaring activity is associated with optical/UV flares. With a duty cycle $\sim 7-19$%, this flaring is quite prominent with an apparent power-law intensity distribution. Flares are present at all orbital phases, with a slight preference for $ϕ_B=0.5-0.7$. We explore possible connections of these variabilities with the intrabinary shock and magnetic activity on the low mass secondary.

astro-ph.HE↗

arXiv2017

B-ducted Heating of Black Widow Companions

The companions of evaporating binary pulsars (black widows and related systems) show optical emission suggesting strong heating. In a number of cases large observed temperatures and asymmetries are inconsistent with direct radiative heating for the observed pulsar spindown power and expected distance. Here we describe a heating model in which the pulsar wind sets up an intrabinary shock (IBS) against the companion wind and magnetic field, and a portion of the shock particles duct along this field to the companion magnetic poles. We show that a variety of heating patterns, and improved fits to the observed light curves, can be obtained at expected pulsar distances and luminosities, at the expense of a handful of model parameters. We test this `IBS-B' model against three well observed binaries and comment on the implications for system masses.

astro-ph.HE↗

arXiv2017

Constraining the Orbit of Supermassive Black Hole Binary 0402+379

The radio galaxy 0402+379 is believed to host a supermassive black hole binary (SMBHB). The two compact core sources are separated by a projected distance of 7.3 pc, making it the most (spatially) compact resolved SMBHB known. We present new multi-frequency VLBI observations of 0402+379 at 5, 8, 15 and 22 GHz, and combine with previous observations spanning 12 years. A strong frequency dependent core shift is evident, which we use to infer magnetic fields near the jet base. After correcting for these shifts we detect significant relative motion of the two cores at $β=v/c=0.0054 \pm 0.0003$ at $PA= -34.4^\circ$. With some assumptions about the orbit, we use this measurement to constrain the orbital period $P\approx 3 \times 10^4$ y and SMBHB mass $M \approx 15 \times 10^9\ M_\odot$. While additional observations are needed to confirm this motion and obtain a precise orbit, this is apparently the first black hole system resolved as a visual binary.

astro-ph.GA↗

arXiv2017

Light curve and SED modeling of the gamma-ray binary 1FGL J1018.6$-$5856: constraints on the orbital geometry and relativistic flow

We present broadband spectral energy distributions (SEDs) and light curves of the gamma-ray binary 1FGL J1018.6$-$5856 measured in the X-ray and the gamma-ray bands. We find that the orbital modulation in the low-energy gamma-ray band is similar to that in the X-ray band, suggesting a common spectral component. However, above a GeV the orbital light curve changes significantly. We suggest that the GeV band contains significant flux from a pulsar magnetosphere, while the X-ray to TeV light curves are dominated by synchrotron and Compton emission from an intrabinary shock (IBS). We find that a simple one-zone model is inadequate to explain the IBS emission, but that beamed Synchrotron-self Compton radiation from adiabatically accelerated plasma in the shocked pulsar wind can reproduce the complex multiband light curves, including the variable X-ray spike coincident with the gamma-ray maximum. The model requires inclination $\sim$50$^\circ$ and orbital eccentricity $\sim$0.35, consistent with the limited constraints from existing optical observations. This picture motivates searches for pulsations from the energetic young pulsar powering the wind shock.

astro-ph.HE↗

arXiv2016

Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54

We report on Chandra X-ray Observatory (CXO) observations of the pulsar wind nebula (PWN) associated with PSR B0355+54 (eight observations with a 395 ks total exposure, performed over an 8 month period). We investigated the spatial and spectral properties of the emission coincident with the pulsar, compact nebula (CN), and extended tail. We find that the CN morphology can be interpreted in a way that suggests a small angle between the pulsar spin axis and our line-of-sight, as inferred from the radio data. On larger scales, emission from the 7' (2 pc) tail is clearly seen. We also found hints of two faint extensions nearly orthogonal to the direction of the pulsar's proper motion. The spectrum extracted at the pulsar position can be described with an absorbed power-law + blackbody model. The nonthermal component can be attributed to magnetospheric emission, while the thermal component can be attributed to emission from either a hot spot (e.g., a polar cap) or the entire neutron star surface. Surprisingly, the spectrum of the tail shows only a slight hint of cooling with increasing distance from the pulsar. This implies either a low magnetic field with fast flow speed, or particle re-acceleration within the tail. We estimate physical properties of the PWN and compare the morphologies of the CN and the extended tail with those of other bow shock PWNe observed with long CXO exposures.

astro-ph.HE↗

arXiv2016

Intra-Binary Shock Heating of Black Widow Companions

The low mass companions of evaporating binary pulsars (black widows and their ilk) are strongly heated on the side facing the pulsar. However in high-quality photometric and spectroscopic data the heating pattern does not match that expected for direct pulsar illumination. Here we explore heating mediated by an intra-binary shock (IBS). We develop a simple analytic model and implement it in the popular `ICARUS' light curve code. The model is parameterized by the wind momentum ratio beta and velocity v_Rel v_orb and assumes that the reprocessed pulsar wind emits prompt particles or radiation to heat the companion surface. We illustrate an interesting range of light curve asymmetries controlled by these parameters. The code also computes the IBS synchrotron emission pattern, and thus can model black widow X-ray light curves. As a test we apply the results to the high quality asymmetric optical light curves of PSR J2215+5135; the resulting fit gives a substantial improvement upon direct heating models and produces an X-ray light curve consistent with that seen. The IBS model parameters imply that, at the present loss rate, the companion evaporation has a characteristic timescale tau_evap ~150My. Still, the model is not fully satisfactory, indicating additional unmodeled physical effects.

astro-ph.HE↗

arXiv2016

Binary Black Holes, Gas Sloshing, and Cold Fronts in the X-ray Halo Hosting 4C+37.11

We analyzed deep $Chandra$ ACIS-I exposures of the cluster-scale X-ray halo surrounding the radio source 4C+37.11. This remarkable system hosts the closest resolved pair of super-massive black hole and an exceptionally luminous elliptical galaxy, the likely product of a series of past mergers. We characterize the halo with $r_{500} = 0.95$ Mpc, $M_{500} = (2.5 \pm 0.2) \times 10^{14} \ M_{\rm{\odot}}$, $ kT = 4.6\pm 0.2$ keV, and a gas mass of $M_{\rm g,500} = (2.2 \pm 0.1) \times 10^{13} M_\odot$. The gas mass fraction within $r_{500}$ is $f_{\rm g} = 0.09 \pm 0.01$. The entropy profile shows large non-gravitational heating in the central regions. We see several surface brightness jumps, associated with substantial temperature and density changes, but approximate pressure equilibrium, implying that these are sloshing structures driven by a recent merger. A residual intensity image shows core spiral structure closely matching that seen for the Perseus cluster, although at $z=0.055$ the spiral pattern is less distinct. We infer the most recent merger occurred $1-2$ Gyr ago and that the event that brought the two observed super-massive black holes to the system core is even older. Under that interpretation, this black hole binary pair has, unusually, remained at pc-scale separation for more than 2 Gyr.

astro-ph.CO↗

arXiv2015

A binary counterpart for 2FGL J2039.6-5620

We have identified an optical/X-ray binary with orbital period P_b=5.47h as the likely counterpart of the Fermi source 2FGL J2039.6-5620. GROND, SOAR and DES observations provide an accurate orbital period and allow us to compare with the light curve of an archival XMM exposure. Like many short-period optical X-ray binaries associated with LAT sources this may be a interacting (black widow/redback) millisecond pulsar binary. The X-ray light curve is consistent with the emission associated with an intrabinary shock. The optical light curve shows evidence of companion heating, but has a peculiar asymmetric double peak. The nature of this optical structure is not yet clear; additional optical studies and, especially, detection of an orbital modulation in a gamma-ray pulsar are needed to elucidate the nature of this peculiar source.

astro-ph.HE↗