Showing papers by "Deepto Chakrabarty published in 2019"
University of Amsterdam1, Columbia University2, United States Naval Research Laboratory3, California Institute of Technology4, University of Toulouse5, Hoffmann-La Roche6, Goddard Space Flight Center7, Massachusetts Institute of Technology8, Haverford College9, University of Southampton10, Stony Brook University11, University of Alberta12
TL;DR: In this paper, the mass and equatorial radius of the millisecond pulsar PSR J0030+0451 were estimated based on a relativistic ray-tracing of thermal emission from hot regions of the pulsar surface.
Abstract: We report on Bayesian parameter estimation of the mass and equatorial radius of the millisecond pulsar PSR J0030+0451, conditional on pulse-profile modeling of Neutron Star Interior Composition Explorer X-ray spectral-timing event data. We perform relativistic ray-tracing of thermal emission from hot regions of the pulsar’s surface. We assume two distinct hot regions based on two clear pulsed components in the phase-folded pulse-profile data; we explore a number of forms (morphologies and topologies) for each hot region, inferring their parameters in addition to the stellar mass and radius. For the family of models considered, the evidence (prior predictive probability of the data) strongly favors a model that permits both hot regions to be located in the same rotational hemisphere. Models wherein both hot regions are assumed to be simply connected circular single-temperature spots, in particular those where the spots are assumed to be reflection-symmetric with respect to the stellar origin, are strongly disfavored. For the inferred configuration, one hot region subtends an angular extent of only a few degrees (in spherical coordinates with origin at the stellar center) and we are insensitive to other structural details; the second hot region is far more azimuthally extended in the form of a narrow arc, thus requiring a larger number of parameters to describe. The inferred mass M and equatorial radius R eq are, respectively, and , while the compactness is more tightly constrained; the credible interval bounds reported here are approximately the 16% and 84% quantiles in marginal posterior mass.
737 citations
University of Amsterdam1, Columbia University2, United States Naval Research Laboratory3, California Institute of Technology4, Hoffmann-La Roche5, University of Toulouse6, Goddard Space Flight Center7, Massachusetts Institute of Technology8, University of Southampton9, Haverford College10, Stony Brook University11, University of Alberta12
TL;DR: In this article, the mass and equatorial radius of the millisecond pulsar PSR J0030$+$0451 were estimated from the ICER X-ray spectral-timing event data.
Abstract: We report on Bayesian parameter estimation of the mass and equatorial radius of the millisecond pulsar PSR J0030$+$0451, conditional on pulse-profile modeling of Neutron Star Interior Composition Explorer (NICER) X-ray spectral-timing event data. We perform relativistic ray-tracing of thermal emission from hot regions of the pulsar's surface. We assume two distinct hot regions based on two clear pulsed components in the phase-folded pulse-profile data; we explore a number of forms (morphologies and topologies) for each hot region, inferring their parameters in addition to the stellar mass and radius. For the family of models considered, the evidence (prior predictive probability of the data) strongly favors a model that permits both hot regions to be located in the same rotational hemisphere. Models wherein both hot regions are assumed to be simply-connected circular single-temperature spots, in particular those where the spots are assumed to be reflection-symmetric with respect to the stellar origin, are strongly disfavored. For the inferred configuration, one hot region subtends an angular extent of only a few degrees (in spherical coordinates with origin at the stellar center) and we are insensitive to other structural details; the second hot region is far more azimuthally extended in the form of a narrow arc, thus requiring a larger number of parameters to describe. The inferred mass $M$ and equatorial radius $R_\mathrm{eq}$ are, respectively, $1.34_{-0.16}^{+0.15}$ M$_{\odot}$ and $12.71_{-1.19}^{+1.14}$ km, whilst the compactness $GM/R_\mathrm{eq}c^2 = 0.156_{-0.010}^{+0.008}$ is more tightly constrained; the credible interval bounds reported here are approximately the $16\%$ and $84\%$ quantiles in marginal posterior mass.
541 citations
University of Amsterdam1, Technische Universität Darmstadt2, University of Alberta3, Max Planck Society4, Hoffmann-La Roche5, University of Toulouse6, Goddard Space Flight Center7, Columbia University8, Massachusetts Institute of Technology9, University of Southampton10, Haverford College11, Stony Brook University12, California Institute of Technology13, United States Naval Research Laboratory14
TL;DR: In this paper, the mass and radius of the millisecond pulsar PSR J0030+0451 have been inferred via pulse-profile modeling of X-ray data obtained by NASA's Neutron Star Interior Composition Explorer (NICER) mission.
Abstract: Both the mass and radius of the millisecond pulsar PSR J0030+0451 have been inferred via pulse-profile modeling of X-ray data obtained by NASA's Neutron Star Interior Composition Explorer (NICER) mission. In this Letter we study the implications of the mass–radius inference reported for this source by Riley et al. for the dense matter equation of state (EoS), in the context of prior information from nuclear physics at low densities. Using a Bayesian framework we infer central densities and EoS properties for two choices of high-density extensions: a piecewise-polytropic model and a model based on assumptions of the speed of sound in dense matter. Around nuclear saturation density these extensions are matched to an EoS uncertainty band obtained from calculations based on chiral effective field theory interactions, which provide a realistic description of atomic nuclei as well as empirical nuclear matter properties within uncertainties. We further constrain EoS expectations with input from the current highest measured pulsar mass; together, these constraints offer a narrow Bayesian prior informed by theory as well as laboratory and astrophysical measurements. The NICER mass–radius likelihood function derived by Riley et al. using pulse-profile modeling is consistent with the highest-density region of this prior. The present relatively large uncertainties on mass and radius for PSR J0030+0451 offer, however, only a weak posterior information gain over the prior. We explore the sensitivity to the inferred geometry of the heated regions that give rise to the pulsed emission, and find a small increase in posterior gain for an alternative (but less preferred) model. Lastly, we investigate the hypothetical scenario of increasing the NICER exposure time for PSR J0030+0451.
203 citations
Columbia University1, Hoffmann-La Roche2, University of Toulouse3, United States Naval Research Laboratory4, Massachusetts Institute of Technology5, University of Southampton6, Haverford College7, University of Illinois at Urbana–Champaign8, California Institute of Technology9, University of Maryland, College Park10, Max Planck Society11, University of Arizona12, University of Amsterdam13, Harvard University14, Goddard Space Flight Center15, Kyoto University16
Abstract: We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437−4715, J0030+0451, J1231−1411, and J2124−3358, selected as targets for constraining the mass–radius relation of neutron stars and the dense matter equation of state (EoS) via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass–radius relation and the dense matter EoS. We find that the long-term timing and flux behavior and the Fourier-domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437−4715, for which multiple temperatures are required.
100 citations
TL;DR: In this paper, the authors examine previous attempts to constrain the magnetospheric configuration of PSR J0030+0451 and conclude that to the best of their knowledge, there is in fact no direct observational evidence that PSRJ0030 + 0451's magnetic field is a centered dipole.
Abstract: Recent modeling of Neutron Star Interior Composition Explorer observations of thermal X-ray pulsations from the surface of the isolated millisecond pulsar PSR J0030+0451 suggests that the hot emitting regions on the pulsar’s surface are far from antipodal, which is at odds with the classical assumption that the magnetic field in the pulsar magnetosphere is predominantly that of a centered dipole. Here, we review these results and examine previous attempts to constrain the magnetospheric configuration of PSR J0030+0451. To the best of our knowledge, there is in fact no direct observational evidence that PSR J0030+0451’s magnetic field is a centered dipole. Developing models of physically motivated, non-canonical magnetic field configurations and the currents that they can support poses a challenging task. However, such models may have profound implications for many aspects of pulsar research, including pulsar braking, estimates of birth velocities, and interpretations of multi-wavelength magnetospheric emission.
98 citations
TL;DR: In this paper, the authors describe the model of surface emission from a rapidly rotating neutron star that is applied to Neutron Star Interior Composition Explorer X-ray data of millisecond pulsars in order to statistically constrain the neutron star mass-radius relation and dense matter equation of state.
Abstract: We describe the model of surface emission from a rapidly rotating neutron star that is applied to Neutron Star Interior Composition Explorer X-ray data of millisecond pulsars in order to statistically constrain the neutron star mass-radius relation and dense matter equation of state. To ensure that the associated calculations are both accurate and precise, we conduct an extensive suite of verification tests between our numerical codes for both the Schwarzschild + Doppler and Oblate Schwarzschild approximations, and compare both approximations against exact numerical calculations. We find superb agreement between the code outputs, as well as in comparison against a set of analytical and semi-analytical calculations, which combined with their speed, demonstrates that the codes are well-suited for large-scale statistical sampling applications. A set of verified, high-precision reference synthetic pulse profiles is provided to the community to facilitate testing of other independently developed codes.
96 citations
University of Amsterdam1, Shanghai Astronomical Observatory2, University of Turku3, Royal Institute of Technology4, Tata Institute of Fundamental Research5, Columbia University6, University of Tübingen7, Leiden University8, Silesian University9, Middle East Technical University10, New York University Abu Dhabi11, INAF12, University of Pisa13, Michigan State University14, University of Southampton15, Princeton University16, Massachusetts Institute of Technology17, Technical University of Denmark18, University of Palermo19, International Space Science Institute20, University of East Anglia21, Technische Universität Darmstadt22, Pontifical Catholic University of Chile23, Clemson University24, Monash University, Clayton campus25, Netherlands Institute for Space Research26, George Washington University27, CERN28, University of Education, Winneba29, Xiamen University30, Nanjing University31, Xiangtan University32, Polytechnic University of Catalonia33, Goddard Space Flight Center34, Kapteyn Astronomical Institute35, Space Science Institute36, University of Alberta37, University of Washington38, University of Cagliari39, Max Planck Society40, University of Manchester41, National Tsing Hua University42, Frankfurt Institute for Advanced Studies43, Goethe University Frankfurt44, Spanish National Research Council45, University of Udine46, Peking University47, University College London48, Chinese Academy of Sciences49, Academy of Sciences of the Czech Republic50, Xinjiang Astronomical Observatory51
TL;DR: In this paper, the authors present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory.
Abstract: In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to be launched in the mid 2020s.
95 citations
TL;DR: In this article, the authors describe the model of surface emission from a rapidly rotating neutron star that is applied to Neutron Star Interior Composition Explorer X-ray data of millisecond pulsars in order to statistically constrain the neutron star mass-radius relation and dense matter equation of state.
Abstract: We describe the model of surface emission from a rapidly rotating neutron star that is applied to Neutron Star Interior Composition Explorer X-ray data of millisecond pulsars in order to statistically constrain the neutron star mass–radius relation and dense matter equation of state. To ensure that the associated calculations are both accurate and precise, we conduct an extensive suite of verification tests between our numerical codes for both the Schwarzschild + Doppler and Oblate Schwarzschild approximations, and compare both approximations against exact numerical calculations. We find superb agreement between the code outputs, as well as in comparisons against a set of analytical and semi-analytical calculations, which, combined with their speed, demonstrates that the codes are well suited for large-scale statistical sampling applications. A set of verified, high-precision reference synthetic pulse profiles is provided to the community to facilitate testing of other independently developed codes.
90 citations
Centre National D'Etudes Spatiales1, United States Naval Research Laboratory2, National Radio Astronomy Observatory3, George Mason University4, Goddard Space Flight Center5, Massachusetts Institute of Technology6, Haverford College7, Technical University of Denmark8, Marshall Space Flight Center9, University of Maryland, College Park10
TL;DR: In this article, the Neutron star Interior Composition Explorer observed several rotation-powered millisecond pulsars (MSPs) to search for or confirm the presence of X-ray pulsations.
Abstract: The Neutron star Interior Composition Explorer observed several rotation-powered millisecond pulsars (MSPs) to search for or confirm the presence of X-ray pulsations. When broad and sine-like, these pulsations may indicate thermal emission from hot polar caps at the magnetic poles on the neutron star surface. We report confident detections (≥4.7σ after background filtering) of X-ray pulsations for five of the seven pulsars in our target sample: PSR J0614−3329, PSR J0636+5129, PSR J0751+1807, PSR J1012+5307, and PSR J2241−5236, while PSR J1552+5437 and PSR J1744−1134 remain undetected. Of those, only PSR J0751+1807 and PSR J1012+5307 had pulsations previously detected at the 1.7σ and almost 3σ confidence levels, respectively, in XMM-Newton data. All detected sources exhibit broad sine-like pulses, which are indicative of surface thermal radiation. As such, these MSPs are promising targets for future X-ray observations aimed at constraining the neutron star mass–radius relation and the dense matter equation of state using detailed pulse profile modeling. Furthermore, we find that three of the detected MSPs exhibit a significant phase offset between their X-ray and radio pulses.
45 citations
TL;DR: In this article, a stable 131-second x-ray quasi-periodic oscillation from the ASASSN-14li event was observed, which implies that the periodicity originates from close to the event horizon and that the black hole is rapidly spinning.
Abstract: The tidal forces close to massive black holes can rip apart stars that come too close to them. As the resulting stellar debris spirals toward the black hole, the debris heats up and emits x-rays. We report observations of a stable 131-second x-ray quasi-periodic oscillation from the tidal disruption event ASASSN-14li. Assuming the black hole mass indicated by host galaxy scaling relations, these observations imply that the periodicity originates from close to the event horizon and that the black hole is rapidly spinning. Our findings demonstrate that tidal disruption events can generate quasi-periodic oscillations that encode information about the physical properties of their black holes.
41 citations
United States Naval Research Laboratory1, University of Toulouse2, University of Southampton3, Kyoto University4, Goddard Space Flight Center5, Columbia University6, Chesapeake Energy7, Massachusetts Institute of Technology8, George Mason University9, National Space Institute10, Marshall Space Flight Center11
TL;DR: In this article, the authors reported the discovery of three spin-down anti-glitches in the ultraluminous accreting X-ray pulsar NGC 300 ULX-1 in timing observations made with the Neutron Star Interior Composition Explorer (NICER).
Abstract: We report the discovery of three spin-down glitches (or `anti-glitches') in the ultraluminous accreting X-ray pulsar NGC 300 ULX-1 in timing observations made with the Neutron Star Interior Composition Explorer (NICER). We find evidence for three sudden spin-down events of magnitudes $\\Delta\
u = -23, -30,$ and $-43 \\,\\mu$Hz (fractional amplitudes $\\Delta\
u/\
u = -4.4, -5.5,$ and $-7.7 \\times 10^{-4}$). These are larger in magnitude (and opposite in sign) than any radio pulsar glitch. This may be caused by the prolonged rapid spin-up of the pulsar causing a sudden transfer of angular momentum between the superfluid and non-superfliud components of the star. We find no evidence for profile or spectral changes at the epochs of the glitches, supporting the conclusion that these are due to the same process as in normal pulsar glitches, but in reverse.
TL;DR: In this article, the authors examine previous attempts to constrain the magnetospheric configuration of PSR J0030+0451 and conclude that to the best of their knowledge, there is in fact no direct observational evidence that PSR's magnetic field is a centered dipole.
Abstract: Recent modeling of NICER observations of thermal X-ray pulsations from the surface of the isolated millisecond pulsar PSR J0030+0451 suggests that the hot emitting regions on the pulsar's surface are far from antipodal, which is at odds with the classical assumption that the magnetic field in the pulsar magnetosphere is predominantly that of a centered dipole. Here, we review these results and examine previous attempts to constrain the magnetospheric configuration of PSR J0030+0451. To the best of our knowledge, there is in fact no direct observational evidence that PSR J0030+0451's magnetic field is a centered dipole. Developing models of physically motivated, non-canonical magnetic field configurations and the currents that they can support poses a challenging task. However, such models may have profound implications for many aspects of pulsar research, including pulsar braking, estimates of birth velocities, and interpretations of multi-wavelength magnetospheric emission.
TL;DR: In this paper, the authors reported the detection of a bright helium-fueled Type I X-ray burst with a bolometric peak flux of (2.3 ± 0.1) × 10−7 erg s−1 cm−2.
Abstract: The Neutron Star Interior Composition Explorer (NICER) has extensively monitored the 2019 August outburst of the 401 Hz millisecond X-ray pulsar SAX J1808.4–3658. In this Letter, we report on the detection of a bright helium-fueled Type I X-ray burst. With a bolometric peak flux of (2.3 ± 0.1) × 10−7 erg s−1 cm−2, this was the brightest X-ray burst among all bursting sources observed with NICER to date. The burst shows a remarkable two-stage evolution in flux, emission lines at 1.0 and 6.7 keV, and burst oscillations at the known pulsar spin frequency, with ≈4% fractional sinusoidal amplitude. We interpret the burst flux evolution as the detection of the local Eddington limits associated with the hydrogen and helium layers of the neutron star envelope. The emission lines are likely associated with Fe, due to reprocessing of the burst emission in the accretion disk.
Technical University of Denmark1, Marshall Space Flight Center2, University of Cambridge3, Physical Research Laboratory4, Massachusetts Institute of Technology5, European Space Agency6, Georgia Institute of Technology7, California Institute of Technology8, University of Michigan9, University of Southampton10, Goddard Space Flight Center11, Hoffmann-La Roche12, University of Toulouse13, Universities Space Research Association14, Michigan State University15, United States Naval Research Laboratory16
TL;DR: In this article, a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017--2018 giant outburst is presented.
Abstract: We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017--2018 giant outburst. The 1.2--10~keV energy spectrum of the source can be approximated with an absorbed cut-off power law model. We detect strong, luminosity-dependent emission lines in the 6--7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex. An asymmetric broad line profile, peaking at 6.67 keV, is possibly due to Doppler effects and gravitational redshift. The 1.2--79 keV broadband spectrum from NuSTAR and NICER observations at the outburst peak is well described by an absorbed cut-off power law plus multiple Gaussian lines and a blackbody component. Physical reflection models are also tested to probe the broad iron line feature. Depending on the mass accretion rate, we found emission sites that are evolving from ~5000 km to a range closer to the surface of the neutron star. Our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultra-fast outflow expected at super-Eddington accretion rates. We do not detect any signatures of a cyclotron absorption line in the NICER or NuSTAR data.
Columbia University1, Hoffmann-La Roche2, University of Toulouse3, United States Naval Research Laboratory4, Massachusetts Institute of Technology5, University of Southampton6, Haverford College7, University of Illinois at Urbana–Champaign8, California Institute of Technology9, University of Maryland, College Park10, Max Planck Society11, University of Arizona12, University of Amsterdam13, Harvard University14, Goddard Space Flight Center15, Kyoto University16
TL;DR: In this article, a set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437-4715, J0030+0451, J1231-1411, and J2124-3358 were selected as targets for constraining the mass-radius relation of neutron stars and the dense matter equation of state via modeling of their pulsed thermal Xray emission.
Abstract: We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437-4715, J0030+0451, J1231-1411, and J2124-3358, selected as targets for constraining the mass-radius relation of neutron stars and the dense matter equation of state via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass-radius relation and the dense matter equation of state. We find that the long-term timing and flux behavior and the Fourier-domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437-4715, for which multiple temperatures are required.
TL;DR: In this article, the authors reported the detection of a bright helium-fueled Type I X-ray burst with a bolometric peak flux of $(2.3\pm0.1) times 10^{-7}$ erg/cm^2/s.
Abstract: The Neutron Star Interior Composition Explorer (NICER) has extensively monitored the August 2019 outburst of the 401 Hz millisecond X-ray pulsar SAX J1808.4-3658. In this Letter, we report on the detection of a bright helium-fueled Type I X-ray burst. With a bolometric peak flux of $(2.3\pm0.1)\times 10^{-7}$ erg/cm^2/s, this was the brightest X-ray burst among all bursting sources observed with NICER to date. The burst shows a remarkable two-stage evolution in flux, emission lines at $1.0$ keV and $6.7$ keV, and burst oscillations at the known pulsar spin frequency, with $\approx4$\% fractional sinusoidal amplitude. We interpret the burst flux evolution as the detection of the local Eddington limits associated with the hydrogen and helium layers of the neutron star envelope. The emission lines are likely associated with Fe, due to reprocessing of the burst emission in the accretion disk.
TL;DR: In this article, the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR during a spin-up state were analyzed.
Abstract: We present an analysis of the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR during a spin-up state. The pulsar, which experienced a torque reversal to spin-up in 2008, has a spin period of 7.7 s. Comparing the phase-averaged spectra obtained with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape changed between the two observations: the 3-10 keV flux increased by 5% while the 30-60 keV flux decreased significantly by 35%. Phase-averaged and phase-resolved spectral analysis shows that the continuum spectrum observed by NuSTAR is well described by an empirical NPEX continuum with an added broad Gaussian emission component around the spectral peak at 20 keV. Taken together with the observed Pdot value obtained from Fermi/GBM, we conclude that the spectral change between the Suzaku and NuSTAR observations was likely caused by an increase of the accretion rate. We also report the possible detection of asymmetry in the profile of the fundamental cyclotron line. Furthermore, we present a study of the energy-resolved pulse profiles using a new relativistic ray tracing code, where we perform a simultaneous fit to the pulse profiles assuming a two-column geometry with a mixed pencil- and fan-beam emission pattern. The resulting pulse profile decompositions enable us to obtain geometrical parameters of accretion columns (inclination, azimuthal and polar angles) and a fiducial set of beam patterns. This information is important to validate the theoretical predictions from radiation transfer in a strong magnetic field.
Harvard University1, The Aerospace Corporation2, Massachusetts Institute of Technology3, University of Michigan4, Pennsylvania State University5, Max Planck Society6, Netherlands Institute for Space Research7, University of Iowa8, Saint Mary's University9, University of Erlangen-Nuremberg10, California Institute of Technology11, University of Maryland, College Park12, Washington University in St. Louis13, Columbia University14, Goddard Space Flight Center15, Leibniz Association16, Johns Hopkins University17, University of Leicester18
TL;DR: Arcus provides high-resolution soft X-ray spectroscopy in the 12-50 A bandpass with unprecedented sensitivity, including spectral resolution < 2500 and effective area < 250 cm2 as mentioned in this paper.
Abstract: Arcus provides high-resolution soft X-ray spectroscopy in the 12-50 A bandpass with unprecedented sensitivity, including spectral resolution < 2500 and effective area < 250 cm2. The three top science goals for Arcus are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback, and (3) to explore how stars form and evolve. Arcus uses the same 12 m focal length grazing-incidence Silicon Pore X-ray Optics (SPOs) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. Combined with the high-heritage NGIS LEOStar-2 spacecraft and launched into 4:1 lunar resonant orbit, Arcus provides high sensitivity and high efficiency observing of a wide range of astrophysical sources.
Technical University of Denmark1, Goddard Space Flight Center2, Netherlands Institute for Space Research3, Monash University, Clayton campus4, Istanbul University5, University of Southampton6, Massachusetts Institute of Technology7, University of Toulouse8, Hoffmann-La Roche9, University of Maryland, College Park10, University of Michigan11, Marshall Space Flight Center12, Universities Space Research Association13
TL;DR: In this article, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass Xray binary 4U 1608-52.
Abstract: We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608-52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of $\approx3$ keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and re-burning in the cooling tail of the burst.
Chuo University1, University of Maryland, Baltimore County2, Goddard Space Flight Center3, Massachusetts Institute of Technology4, European Space Agency5, United States Naval Research Laboratory6, University of California, Berkeley7, Marshall Space Flight Center8, Saitama University9, Kyoto University10
TL;DR: In this paper, the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626−67 observed with Suzaku and Nuclear Spectroscopic Telescope Array (NuSTAR) during a spin-up state were analyzed.
Abstract: We present an analysis of the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626−67 observed with Suzaku and Nuclear Spectroscopic Telescope Array (NuSTAR) during a spin-up state. The pulsar, which experienced a torque reversal to spin-up in 2008, has a spin period of ~7.7 s. Comparing the phase-averaged spectra obtained with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape changed between the two observations: the 3–10 keV flux increased by ~5%, while the 30–60 keV flux decreased significantly by ~35%. Phase-averaged and phase-resolved spectral analysis shows that the continuum spectrum observed by NuSTAR is well described by an empirical negative and positive power law times exponential continuum with an added broad Gaussian emission component around the spectral peak at ~20 keV. Taken together with the observed Ṗ value obtained from the Fermi/gamma-ray burst monitor data, we conclude that the spectral change between the Suzaku and NuSTAR observations was likely caused by an increase in the accretion rate. We also report the possible detection of asymmetry in the profile of the fundamental cyclotron line. Furthermore, we present a study of the energy-resolved pulse profiles using a new relativistic ray tracing code, where we perform a simultaneous fit to the pulse profiles assuming a two-column geometry with a mixed pencil- and fan-beam emission pattern. The resulting pulse profile decompositions enable us to obtain geometrical parameters of accretion columns (inclination, azimuthal and polar angles) and a fiducial set of beam patterns. This information is important to validate the theoretical predictions from radiation transfer in a strong magnetic field.
TL;DR: In this article, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass Xray binary 4U 1608-52.
Abstract: We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608-52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of $\approx3$ keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and re-burning in the cooling tail of the burst.
Centre National D'Etudes Spatiales1, United States Naval Research Laboratory2, National Radio Astronomy Observatory3, George Mason University4, Goddard Space Flight Center5, Massachusetts Institute of Technology6, Haverford College7, Technical University of Denmark8, Marshall Space Flight Center9, University of Maryland, College Park10
TL;DR: In this article, the authors reported confident detections of X-ray pulsations for five of the seven pulsars in their target sample: PSRJ0614-3329, PSR J0636+5129, PSRsJ061+1807, PSrJ1012+5307, PSrsJ062+1744-1134, and PSr J2241-5236.
Abstract: NICER observed several rotation-powered millisecond pulsars to search for or confirm the presence of X-ray pulsations. When broad and sine-like, these pulsations may indicate thermal emission from hot polar caps at the magnetic poles on the neutron star surface. We report confident detections ($\ge4.7\sigma$ after background filtering) of X-ray pulsations for five of the seven pulsars in our target sample: PSR J0614-3329, PSR J0636+5129, PSR J0751+1807, PSR J1012+5307, and PSR J2241-5236, while PSR J1552+5437 and PSR J1744-1134 remain undetected. Of those, only PSR J0751+1807 and PSR J1012+5307 had pulsations previously detected at the 1.7$\sigma$ and almost 3$\sigma$ confidence levels, respectively, in XMM-Newton data. All detected sources exhibit broad sine-like pulses, which are indicative of surface thermal radiation. As such, these MSPs are promising targets for future X-ray observations aimed at constraining the neutron star mass-radius relation and the dense matter equation of state using detailed pulse profile modeling. Furthermore, we find that three of the detected millisecond pulsars exhibit a significant phase offset between their X-ray and radio pulses.
TL;DR: In this article, the authors present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst.
Abstract: In this paper we present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst. Using observations collected with the Neutron Star Interior Composition Explorer (NICER), we establish the pulsar spin frequency and orbital phase during its latest epoch. We find that the 2019 outburst shows a pronounced evolution in pulse phase over the course of the outburst. These phase shifts are found to correlate with the source flux, and are interpreted in terms of hot-spot drift on the stellar surface, driven by changes in the mass accretion rate. Additionally, we find that the long-term evolution of the pulsar spin frequency shows evidence for a modulation at the Earth's orbital period, enabling pulsar timing based astrometry of this accreting millisecond pulsar.
University of Michigan1, Michigan State University2, Goddard Space Flight Center3, INAF4, Curtin University5, Technical University of Denmark6, Centre National D'Etudes Spatiales7, University of Erlangen-Nuremberg8, California Institute of Technology9, University of Cambridge10, Massachusetts Institute of Technology11, Wayne State University12, University of Southampton13
TL;DR: In this paper, the authors report on X-ray and radio observations of the ultra-compact Xray binary 4U 1543-624 taken in August 2017 during an enhanced accretion episode.
Abstract: We report on X-ray and radio observations of the ultra-compact X-ray binary 4U 1543-624 taken in August 2017 during an enhanced accretion episode. We obtained NICER monitoring of the source over a $\sim10$ day period during which target-of-opportunity observations were also conducted with Swift, INTEGRAL, and ATCA. Emission lines were measured in the NICER X-ray spectrum at $\sim0.64$ keV and $\sim6.4$ keV that correspond to O and Fe, respectively. By modeling these line components, we are able to track changes in the accretion disk throughout this period. The innermost accretion flow appears to move inwards from hundreds of gravitational radii ($R_{g}=GM/c^{2}$) at the beginning of the outburst to $<8.7$ $R_{g}$ at peak intensity. We do not detect the source in radio, but are able to place a $3\sigma$ upper limit on the flux density at $27$ $\mu$Jy beam$^{-1}$. Comparing the radio and X-ray luminosities, we find that the source lies significantly away from the range typical of black holes in the ${L}_{{r}}$-${L}_{{x}}$ plane, suggesting a neutron star (NS) primary. This adds to the evidence that NSs do not follow a single track in the ${L}_{{r}}$-${L}_{{x}}$ plane, limiting its use in distinguishing between different classes of NSs based on radio and X-ray observations alone.
University of Michigan1, Michigan State University2, Goddard Space Flight Center3, INAF4, Curtin University5, Technical University of Denmark6, Centre National D'Etudes Spatiales7, California Institute of Technology8, University of Erlangen-Nuremberg9, University of Cambridge10, Massachusetts Institute of Technology11, Wayne State University12, University of Southampton13
TL;DR: In this paper, the authors report on X-ray and radio observations of the ultra-compact Xray binary 4U 1543-624 taken in August 2017 during an enhanced accretion episode.
Abstract: We report on X-ray and radio observations of the ultra-compact X-ray binary 4U 1543-624 taken in August 2017 during an enhanced accretion episode. We obtained NICER monitoring of the source over a $\sim10$ day period during which target-of-opportunity observations were also conducted with Swift, INTEGRAL, and ATCA. Emission lines were measured in the NICER X-ray spectrum at $\sim0.64$ keV and $\sim6.4$ keV that correspond to O and Fe, respectively. By modeling these line components, we are able to track changes in the accretion disk throughout this period. The innermost accretion flow appears to move inwards from hundreds of gravitational radii ($R_{g}=GM/c^{2}$) at the beginning of the outburst to $<8.7$ $R_{g}$ at peak intensity. We do not detect the source in radio, but are able to place a $3\sigma$ upper limit on the flux density at $27$ $\mu$Jy beam$^{-1}$. Comparing the radio and X-ray luminosities, we find that the source lies significantly away from the range typical of black holes in the ${L}_{{r}}$-${L}_{{x}}$ plane, suggesting a neutron star (NS) primary. This adds to the evidence that NSs do not follow a single track in the ${L}_{{r}}$-${L}_{{x}}$ plane, limiting its use in distinguishing between different classes of NSs based on radio and X-ray observations alone.
Goddard Space Flight Center1, University of Southampton2, Massachusetts Institute of Technology3, Technical University of Denmark4, University of Cambridge5, Hoffmann-La Roche6, University of Toulouse7, Netherlands Institute for Space Research8, University of Maryland, College Park9, University of Michigan10, Marshall Space Flight Center11, Universities Space Research Association12
TL;DR: In this article, the Neutron Star Interior Composition Explorer (NICER) was used to detect narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820−30.
Abstract: We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820−30. NICER observed 4U 1820−30 in 2017 August during a low-flux, hard spectral state, accumulating about 60 ks of exposure. Five thermonuclear X-ray bursts were detected, of which four showed clear signs of PRE. We extracted spectra during the PRE phases and fit each to a model that includes a Comptonized component to describe the accretion-driven emission, and a blackbody for the burst thermal radiation. The temperature and spherical emitting radius of the fitted blackbody are used to assess the strength of PRE in each burst. The two strongest PRE bursts (burst pair 1) had blackbody temperatures of ≈0.6 keV and emitting radii of ≈100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (≈0.67 keV) and smaller radii (≈75 km). All of the PRE bursts show evidence of narrow line emission near 1 keV. By coadding the PRE phase spectra of burst pairs 1 and, separately, 2, we find, in both coadded spectra, significant, narrow, spectral features near 1.0 (emission), 1.7, and 3.0 keV (both in absorption). Remarkably, all the fitted line centroids in the coadded spectrum of burst pair 1 appear systematically blueshifted by a factor of 1.046 ± 0.006 compared to the centroids of pair 2, strongly indicative of a gravitational shift, a wind-induced blueshift, or more likely some combination of both effects. The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii, and thus have weaker gravitational redshifts, and they generate faster outflows, yielding higher blueshifts. We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models.
TL;DR: In this article, a detailed spectral and timing analysis of the coherent pulsations of IGR J17379-3747 was performed, and it was shown that they show a strong energy dependence, with soft thermal emission lagging about 640 microseconds behind the hard Comptonized emission.
Abstract: We report on the Neutron Star Interior Composition Explorer (NICER) monitoring campaign of the 468 Hz accreting millisecond X-ray pulsar IGR J17379-3747. From a detailed spectral and timing analysis of the coherent pulsations we find that they show a strong energy dependence, with soft thermal emission lagging about 640 microseconds behind the hard, Comptonized emission. Additionally, we observe uncommonly large pulse fractions, with measured amplitudes in excess of 20% sinusoidal fractional amplitude across the NICER passband and fluctuations of up to ~70%. Based on a phase-resolved spectral analysis, we suggest that these extreme properties might be explained if the source has an unusually favorable viewing geometry with a large magnetic misalignment angle. Due to these large pulse fractions, we were able to detect pulsations down to quiescent luminosities (~5 x 10^33 erg s^-1). We discuss these low-luminosity pulsations in the context of transitional millisecond pulsars.
Journal Article•
TL;DR: STROBE-X is a probe-class mission concept, selected for study by NASA, for X-ray spectral timing of compact objects across the mass scale as mentioned in this paper, which combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility, enabling a broad portfolio of high priority astrophysics.
Abstract: STROBE-X is a probe-class mission concept, selected for study by NASA, for X-ray spectral timing of compact objects across the mass scale. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility, enabling a broad portfolio of high-priority astrophysics.
TL;DR: In this paper, a detailed spectral and timing analysis of the coherent pulsations of a millisecond X-ray pulsar IGR 17379-3747 was performed and it was shown that they show a strong energy dependence, with soft thermal emission lagging about 640 μs behind the hard Comptonized emission.
Abstract: We report on the Neutron Star Interior Composition Explorer (NICER) monitoring campaign of the 468 Hz accreting millisecond X-ray pulsar IGR J17379–3747. From a detailed spectral and timing analysis of the coherent pulsations we find that they show a strong energy dependence, with soft thermal emission lagging about 640 μs behind the hard, Comptonized emission. Additionally, we observe uncommonly large pulse fractions, with measured amplitudes in excess of 20% sinusoidal fractional amplitude across the NICER passband and fluctuations of up to ∼70%. Based on a phase-resolved spectral analysis, we suggest that these extreme properties might be explained if the source has an unusually favorable viewing geometry with a large magnetic misalignment angle. Due to these large pulse fractions, we were able to detect pulsations down to quiescent luminosities ( erg ). We discuss these low-luminosity pulsations in the context of transitional millisecond pulsars.
Posted Content•
Columbia University1, University of Amsterdam2, Massachusetts Institute of Technology3, Goddard Space Flight Center4, University of Toulouse5, University of Illinois at Urbana–Champaign6, Texas Tech University7, United States Naval Research Laboratory8, University of Arizona9, Marshall Space Flight Center10
TL;DR: In this paper, the interior structure of neutron stars is extracted via sensitive observations of their exteriors using different combinations of high time resolution, superb spectral resolution, and high spatial resolution, which can produce definitive empirical constraints on the allowed dense matter equation of state.
Abstract: The unknown state of matter at ultra-high density, large proton/neutron number asymmetry, and low temperature is a major long-standing problem in modern physics. Neutron stars provide the only known setting in the Universe where matter in this regime can stably exist. Valuable information about the interior structure of neutron stars can be extracted via sensitive observations of their exteriors. There are several complementary techniques that require different combinations of high time resolution, superb spectral resolution, and high spatial resolution. In the upcoming decade and beyond, measurements of the masses and radii of an ensemble of neutron stars using these techniques, based on data from multiple proposed next-generation X-ray telescopes, can produce definitive empirical constraints on the allowed dense matter equation of state.