arXiv subjects

Roger W. Romani

Publications and source records attributed to Roger W. Romani.

At least 91 records · Page 5Linked to original sources

arXiv2022

The X-ray Polarization View of Mrk~421 in an Average Flux State as Observed by the Imaging X-ray Polarimetry Explorer

Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multi-wavelength polarization properties of blazars. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2-8 keV) to probe the properties of the jet synchrotron emission in high-frequency-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree $Π_{\rm x}=15\pm$2\% and electric-vector position angle $Ψ_{\rm x}=35^\circ\pm4^\circ$) from the jet of the HSP Mrk~421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of $Π_{\rm x}$ compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock as the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.

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arXiv2022

The X-ray polarimetry view of the accreting pulsar Cen X-3

Cen X-3 is the first X-ray pulsar discovered 50 years ago. Radiation from such objects is expected to be highly polarized due to birefringence of plasma and vacuum associated with propagation of photons in presence of the strong magnetic field. Here we present results of the observations of Cen X-3 performed with the Imaging X-ray Polarimetry Explorer. The source exhibited significant flux variability and was observed in two states different by a factor of ~20 in flux. In the low-luminosity state no significant polarization was found either in pulse phase-averaged (with the 3$σ$ upper limit of 12%) or phase-resolved data (the 3$σ$ upper limits are 20-30%). In the bright state the polarization degree of 5.8$\pm$0.3% and polarization angle of $49.6°\pm1.5°$ with significance of about 20$σ$ was measured from the spectro-polarimetric analysis of the phase-averaged data. The phase-resolved analysis showed a significant anti-correlation between the flux and the polarization degree as well as strong variations of the polarization angle. The fit with the rotating vector model indicates a position angle of the pulsar spin axis of about 49$°$ and a magnetic obliquity of 17$°$. The detected relatively low polarization can be explained if the upper layers of the neutron star surface are overheated by the accreted matter and the conversion of the polarization modes occurs within the transition region between the upper hot layer and a cooler underlying atmosphere. A fraction of polarization signal can also be produced by reflection of radiation from the neutron star surface and the accretion curtain.

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arXiv2022

X-ray polarization detection of Cassiopeia A with IXPE

We report on a $\sim 5σ$ detection of polarized 3-6 keV X-ray emission from the supernova remnant Cassiopeia A with the Imaging X-ray Polarimetry Explorer (IXPE). The overall polarization degree of $1.8 \pm 0.3$% is detected by summing over a large region, assuming circular symmetry for the polarization vectors. The measurements imply an average polarization degree for the synchrotron component of $\sim 2.5$%, and close to 5% for the X-ray synchrotron-domimated forward-shock region. These numbers are based on an assessment of the thermal and non-thermal radiation contributions, for which we used a detailed spatial-spectral model based on Chandra X-ray data. A pixel-by-pixel search for polarization provides a few tentative detections from discrete regions at the $\sim 3σ$ confidence level. Given the number of pixels, the significance is {insufficient} to claim a detection for individual pixels, but implies considerable turbulence on scales smaller than the angular resolution. Cas A's X-ray continuum emission is dominated by synchrotron radiation from regions within $\lesssim 10^{17}$ cm of the forward- and reverse shocks. We find that i) the measured polarization angle corresponds to a radially-oriented magnetic field, similar to what has been inferred from radio observations; ii) the X-ray polarization degree is lower than in the radio band ($\sim 5$%). Since shock compression should impose a tangential magnetic field structure, the IXPE results imply that magnetic-fields are reoriented within $\sim 10^{17}$cm of the shock. If the magnetic-field alignment is due to locally enhanced acceleration near quasi-parallel shocks, the preferred X-ray polarization angle suggests a size of $3\times 10^{16}$ cm for cells with radial magnetic fields.

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arXiv2022

Simultaneous space and phase resolved X-ray polarimetry of the Crab Pulsar and Nebula

The Crab pulsar and its nebula are among the most studied astrophysical systems, and constitute one of the most promising environments where high energy processes and particle acceleration can be investigated. They are the only objects for which previous X-ray polarisation has been reported. We present here the first Imaging X-ray Polarimetry Explorer (IXPE) observation of the Crab pulsar and nebula. The total pulsar pulsed emission in the [2-8] keV energy range is unpolarised. Significant polarisation up to 15% is detected only in the core of the main peak. The nebula has a total space integrated polarised degree of 20% and polarisation angle of 145deg. The polarised maps show a large variation in the local polarisation, and regions with polarised degree up to 45-50%. The polarisation pattern suggests a predominantly toroidal magnetic field.

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arXiv2022

Limits on X-ray Polarization at the Core of Centaurus A as Observed with the Imaging X-ray Polarimetry Explorer

We present measurements of the polarization of X-rays in the $2-8 \thinspace \mathrm{keV}$ band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at $99 \%$ percent confidence (MDP$_{99}$) of $6.5 \%$ using a weighted, spectral model-independent calculation in the $2-8 \thinspace \mathrm{keV}$ band. The polarization angle $ψ$ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy ($0.3-400 \thinspace \mathrm{keV}$) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption ($N_H \sim 10^{23} \thinspace \mathrm{cm}^{-2}$) and a Fe K$α$ emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below $2 \thinspace \mathrm{keV}$. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe K$α$ emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.

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arXiv2022

PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star

We describe Keck-telescope spectrophotometry and imaging of the companion of the ``black widow" pulsar PSR~J0952$-$0607, the fastest known spinning neutron star (NS) in the disk of the Milky Way. The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated ``day" half of the orbit. The model fits indicate system inclination $i=59.8\pm 1.9^\circ$ and a pulsar mass $M_{NS} = 2.35\pm 0.17 M_\odot$, the largest well-measured mass found to date. Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit. If the NS started at a typical pulsar birth mass, nearly $1 M_\odot$ has been accreted; this may be connected with the especially low intrinsic dipole surface field, estimated at $6\times 10^7$G. Joined with reanalysis of other black widow and redback pulsars, we find that the minimum value for the maximum NS mass is $M_{\rm max} > 2.19 M_\odot$$(2.09 M_\odot)$ at $1σ$$(3σ)$ confidence. This is $\sim 0.15 M_\odot$ heavier than the lower limit on $M_{\rm max}$ implied by the white-dwarf--pulsar binaries measured via radio Shapiro-delay techniques.

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arXiv2022

Polarized x-rays from a magnetar

We report on the first detection of linearly polarized x-ray emission from an ultra-magnetized neutron star with the Imaging X-ray Polarimetry Explorer (IXPE). The IXPE 35 observations of the anomalous x-ray pulsar 4U 0142+61 reveal a linear polarization degree of $(12\pm 1)\%$ throughout the IXPE 2--8 keV band. We detect a substantial variation of the polarization with energy: the degree is $(14\pm 1)\%$ at 2--4 keV and $(41\pm 7)\%$ at 5.5--8 keV, while it drops below the instrumental sensitivity around 4--5 keV, where the polarization angle swings by $\sim 90^\circ$. The IXPE observations give us completely new information about the properties of the neutron star surface and magnetosphere and lend further support to the presence of the quantum mechanical effect of vacuum birefringence.

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arXiv2022

Testing High-Energy Emission Models for Blazars with X-ray Polarimetry

Both leptonic and hadronic emission processes may contribute to blazar jet emission; which dominates in blazars's high energy emission component remains an open question. Some intermediate synchrotron peaked blazars transition from their low to high energy emission components in the X-ray band making them excellent laboratories to probe both components simultaneously, and good targets for the newly launched Imaging X-ray Polarimetry Explorer. We characterize the spectral energy distributions for three such blazars: CGRaBS~J0211+1051, TXS~0506+056, and S5~0716+714, predicting their X-ray polarization behavior by fitting a multizone polarized leptonic jet model. We find that a significant detection of electron synchrotron dominated polarization is possible with a 300~ks observation for S5~0716+714 and CGRaBS~J0211+1051 in their flaring states, while even 500~ks observations are unlikely to measure synchrotron self-Compton polarization. Importantly, non-leptonic emission processes like proton synchrotron are marginally detectable for our brightest ISP, S5~0716+714, during a flaring state. Improved {\it IXPE} data reduction methods or next generation telescopes like {\it eXTP} are needed to confidently measure SSC polarization.

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arXiv2022

The Long Filament of PSR J2030+4415

New X-ray and optical observations shed light on the remarkable X-ray filament of the Gamma-ray pulsar PSR~J2030+4415. Images of the associated H$α$ bow shock's evolution over the past decade compared with its velocity structure provide an improved kinematic distance of $\sim$0.5kpc. These velocities also imply that the pulsar spin axis lies $\sim 15^\circ$ from the proper motion axis which is close to the plane of the sky. The multi-bubble shock structure indicates that the bow shock stand-off was compressed to a small value $\sim 20-30$y ago when the pulsar broke through the bow shock to its present bubble. This compression allowed multi-TeV pulsar $e^\pm$ to escape to the external ISM, lighting up an external magnetic field structure as the `filament'. The narrow filament indicates excellent initial confinement and the full $15^\prime$ ($2.2$~pc=7~lt-y) projected length of the filament indicates rapid $e^\pm$ propagation to its end. Spectral variation along the filament suggests that the injected particle energy evolved during the break-through event.

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arXiv2021

The Imaging X-Ray Polarimetry Explorer (IXPE): Pre-Launch

Scheduled to launch in late 2021, the Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Small Explorer Mission in collaboration with the Italian Space Agency (ASI). The mission will open a new window of investigation - imaging X-ray polarimetry. The observatory features 3 identical telescopes each consisting of a mirror module assembly with a polarization-sensitive imaging X-ray detector at the focus. A coilable boom, deployed on orbit, provides the necessary 4-m focal length. The observatory utilizes a 3-axis-stabilized spacecraft which provides services such as power, attitude determination and control, commanding, and telemetry to the ground. During its 2-year baseline mission, IXPE will conduct precise polarimetry for samples of multiple categories of X-ray sources, with follow-on observations of selected targets.

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arXiv2021

XMM-Newton Observes the Intrabinary Shock of PSR J1959+2048

In a multi-orbit (100 ks) $\mathrm{\it XMM-Newton}$ exposure of the original black widow pulsar, PSR J1959+2048, we measure the strong orbital modulation caused by intrabinary shock (IBS) emission. The IBS light curve peak appears asymmetric, which we attribute to sweep-back effects in the companion wind. We also see evidence for an X-ray eclipse by the companion and its wind. Together with the IBS fit, this supports an edge-on $i\sim 90^\circ$ view of the system and a modest $\sim 1.8M_\odot$ mass for the recycled pulsar. Our IBS fit parameters imply a wind flux that, if persistent, would evaporated the companion within a few Gyr.

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arXiv2021

PSR J1810+1744: Companion Darkening and a Precise High Neutron Star Mass

Keck-telescope spectrophotometry of the companion of PSR J1810+1744 shows a flat, but asymmetric light-curve maximum and a deep, narrow minimum. The maximum indicates strong gravity darkening near the L_1 point, along with a heated pole and surface winds. The minimum indicates a low underlying temperature and substantial limb darkening. The gravity darkening is a consequence of extreme pulsar heating and the near-filling of the Roche lobe. Light-curve modeling gives a binary inclination i=65.7+/-0.4deg. With the Keck-measured radial-velocity amplitude K_c=462.3+/-2.2km/s, this gives an accurate neutron star mass M_NS=2.13+/-0.04M_o, with important implications for the dense-matter equation of state. A classic direct-heating model, ignoring the L_1 gravitational darkening, would predict an unphysical M_NS>3M_o. A few other ``spider" pulsar binaries have similar large heating and fill factor; thus, they should be checked for such effects.

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arXiv2020

PSR J1709-4429's Proper Motion and its Relationship to SNR G343.1-2.3

We have obtained a deep 670 ks CXO ACIS image of the remarkable pulsar wind nebula (PWN) of PSR J1709-4429, in 4 epochs during 2018-2019. Comparison with an archival 2004 data set provides a pulsar proper motion mu = 13 \pm 3 mas/yr at a PA of 86 \pm 9 degree (1 sigma 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 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.

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arXiv2020

On the vanishing orbital X-ray variability of the eclipsing binary millisecond pulsar 47 Tuc W

Redback millisecond pulsars (MSPs) typically show pronounced orbital variability in their X-ray emission due to our changing view of the intrabinary shock (IBS) between the pulsar wind and stellar wind from the companion. Some redbacks ("transitional" MSPs) have shown dramatic changes in their multiwavelength properties, indicating a transition from a radio pulsar state to an accretion-powered state. The redback MSP 47 Tuc W showed clear X-ray orbital variability in the Chandra ACIS-S observations in 2002, which were not detectable in the longer Chandra HRC-S observations in 2005-06, suggesting that it might have undergone a state transition. However, the Chandra observations of 47 Tuc in 2014-15 show similar X-ray orbital variability as in 2002. We explain the different X-ray light-curves from these epochs in terms of two components of the X-ray spectrum (soft X-rays from the pulsar, vs. harder X-rays from the IBS), and different sensitivities of the X-ray instruments observing in each epoch. However, when we use our best-fit spectra with HRC response files to model the HRC light-curve, we expect a more significant and shorter dip than that observed in the 2005-06 Chandra data. This suggests an intrinsic change in the IBS of the system. We use the ICARUS stellar modelling software, including calculations of heating by an IBS, to model the X-ray, optical, and UV light-curves of 47 Tuc W. Our best-fitting parameters point towards a high-inclination system (i~60 deg), which is primarily heated by the pulsar radiation, with an IBS dominated by the companion wind momentum.

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arXiv2020

X-ray constraints on the spectral energy distribution of the $z=5.18$ blazar SDSS J013127.34-032100.1

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 $N_{\rm H}=1.1\times10^{21}\rm \ cm^{-2}$, or a broken power law with $Γ_l=0.5$, $Γ_h=1.8$, and a break energy $E_b=0.7$ keV for an expected absorbing column density of $N_{\rm H}=3.6\times 10^{20}\rm \ cm^{-2}$, 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 $\ge$7 and $\ge$6 (90%) for the low- and high-$N_{\rm H}$ SEDs, respectively. The corresponding beaming implies $\ge$130 (low $N_{\rm H}$) or $\ge$100 (high $N_{\rm H}$) high-spin supermassive black holes similar to J0131 exist at similar redshifts.

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arXiv2020

Heated Poles on the Companion of Redback PSR J2339$-$0533

We analyze photometry and spectra of the "redback" millisecond pulsar binary J2339$-$0533. These observations include new measurements from Keck and GROND, as well as archival measurements from the OISTER, WIYN, SOAR, and HET telescopes. The parameters derived from GROND, our primary photometric data, describe well the rest of the datasets, 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 $i = 69.3^\circ\pm 2.3^\circ$, together with the center-of-mass velocity (from the radial-velocity fits) $K_{\rm C} = 347.0\pm 3.7\,$ $\mathrm{km\,s}^{-1}$, give a fairly typical neutron star mass of $1.47\pm 0.09\,M_\odot$.

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arXiv2020

PSR J2030+4415's Remarkable Bow Shock, PWN and Filament

We report on new X-ray and optical observations of PSR J2030+4415, a Gamma-ray Pulsar with an H$α$ bow shock. These data reveal the velocity structure of the bow shock apex and resolve unusual X-ray structure in its interior. In addition the system displays a very long, thin filament, extending at least $5^\prime$ at $\sim 130^\circ$ to the pulsar motion vector. Careful astrometry, compared with a short archival exposure, detects the pulsar proper motion at 85 mas yr$^{-1}$. With the H$α$ velocity structure this allows us to estimate the distance as $0.75$ kpc.

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arXiv2020

Orbital Modulation of Gamma Rays from PSR~J2339$-$0533

We report on orbital modulation of the 100-600 MeV gamma-ray emission of the $P_{\rm B}=4.6$ hr millisecond pulsar binary PSR J2339$-$0533 using 11 yr of Fermi Large Area Telescope data. The modulation has high significance (chance probability $p\approx 10^{-7}$), 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.

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