arXiv subjects

E. V. Gotthelf

Publications and source records attributed to E. V. Gotthelf.

At least 109 records · Page 6Linked to original sources

arXiv2002

On the Nature of the X-ray Emission from the Galactic Center Region

The origin of the X-ray emission from the central region of the Galaxy has remained a mystery, despite extensive study over the past two decades. A fundamental question is the relative contribution of the point-source and diffuse components of this emission, which is critical to understanding the high-energy phenomena and processes unique to this Galactic nuclear environment. Here, we report on results from a large-scale imaging survey of the Galactic center with sufficient spatial resolution to allow a clean separation of the two components. The highly-ionized Fe emission, previously attributed to the diffuse emission, is found largely due to the discrete X-ray source population. The number and spectrum of such sources indicate the presence of numerous accreting white dwarfs, neutron stars, and/or black holes in the region. The diffuse X-ray emission dominates over the contribution from the faint discrete sources and is shown to be associated with distinct interstellar structures observed at radio and mid-infrared wavelengths, suggesting that it arises from the recent formation of massive stars. We have also mapped out the 6.4-keV fluorescence line emission from neutral to moderately ionized irons. The ubiquitous association of the emission with molecular gas indicates that either the X-ray radiation from the Galactic center was substantially more intense in the past than at present or non-relativistic cosmic-rays are important in producing Fe K shell vacancies.

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arXiv2001

X-ray Detection of PSR 1757-24 and its Nebular Tail

We report the first X-ray detection of the radio pulsar PSR B1757-24 using the Chandra X-ray Observatory. The image reveals point-source emission at the pulsar position, consistent with being magnetospheric emission from the pulsar. In addition, we detect a faint tail extending nearly 20" east of the pulsar, in the same direction and with comparable morphology to the pulsar's well-studied radio tail. The X-ray tail is unlikely to be emission left behind following the passage of the pulsar, but rather is probably from synchrotron-emitting pulsar wind particles having flow velocity ~7000 km/s. Assuming the point-source X-ray emission is magnetospheric, the observed X-ray tail represents only ~0.01% of the pulsar's spin-down luminosity, significantly lower than the analogous efficiencies of most known X-ray nebulae surrounding rotation-powered pulsars.

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arXiv2001

Chandra Spectroscopy of Pulsars and their Wind Nebulae

We present preliminary results from a systematic spectral study of pulsars and their wind nebulae using the Chandra X-Ray Observatory. The superb spatial resolution of Chandra allows us to differentiate the compact object's spectrum from that of its surrounding nebulae. Specifically, for six Crab-like pulsars, we compare spectral fits of the averaged pulsar wind nebulae (PWN) emission to that of the central core using an absorbed power-law model. These results suggest an empirical relationship between the bulk averaged photon indicies (Gamma) for the PWNe and the pulsar cores; Gamma_PWN = 0.8 X Gamma_core + 0.8. The photon indices of PWNe are found to fall in the range of 1.3 < Gamma_PWN < 2.3. We propose that the morphological and spectral characteristics of the pulsars observed herein seem to indicate consistent emission mechanisms common to all young pulsars. We point out that the previous spectral results obtained for most X-ray pulsars are likely contaminated by PWN emission.

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arXiv2001

Chandra X-ray Spectroscopy of Kes75, its Young Pulsar, and its Synchrotron Nebula

We have observed the young Galactic supernova remnant Kes 75 with the Chandra X-ray Observatory. This object is one of an increasing number of examples of a shell-type remnant with a central extended radio core harboring a pulsar. Here we present a preliminary spatially resolved spectroscopic analysis of the Kes~75 system. We find that the spectrum of the pulsar is significantly harder than that of the wind nebula, and both of these components can be isolated from the diffuse thermal emission that seems to follow the same distribution as the extended radio shell. When we characterize the thermal emission with a model of an under-ionized plasma and non-solar elemental abundances, we require a significant diffuse high energy component, which we model as a power-law with a photon index similar to that of the synchrotron nebula.

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arXiv2001

The Pulsar Wind Nebula of G11.2-0.3

We present high-resolution radio and X-ray studies of the composite supernova remnant G11.2-0.3. Using archival VLA data, we perform radio spectral tomography to measure for the first time the spectrum of the shell and plerion separately. We compare the radio morphology of each component to that observed in the hard and soft Chandra X-ray images. We measure the X-ray spectra of the shell and the emission in the interior and discuss the hypothesis that soft X-ray emission interior to the shell is the result of the expanding pulsar wind shocking with the supernova ejecta. We also see evidence for spatial variability in the hard X-ray emission near the pulsar, which we discuss in terms of ion mediated relativistic shocks.

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arXiv2001

15 Years Going Steady - Timing the Magnetar 1E 1841-045

We report on a long-term monitoring campaign of 1E 1841-045, the 12-s anomalous X-ray pulsar and magnetar candidate at the center of the supernova remnant Kes 73. We have obtained approximately monthly observations of the pulsar with the Rossi X-ray Timing Explorer spanning over two years, during which time 1E 1841-045 is found to be rotating with sufficient stability to derive a phase-connected timing solution. A linear ephemeris is consistent with observations of the pulse period made over the last 15 yrs with the Ginga, ASCA, XTE, & SAX observatories. Phase residuals suggest the presence of "timing noise", as is typically observed from young radio pulsars. These results confirm a rapid, constant spin-down for the pulsar, which continues to maintain a steady flux; this is inconsistent with most accretion scenarios.

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arXiv2001

Hiccups in the night: X-ray monitoring of the two Crab-like LMC pulsars

We are undertaking an extensive X-ray monitoring campaign of the two Crab-like pulsars in the Large Magellanic Clouds, PSR B0540-69 and PSR J0537-6910. We present our current phase-connected timing analysis derived from a set of 50 pointed X-ray observations spanning several years. From our initial 1.2 yr monitoring program of the young 50 ms pulsar PSR B0540-69, we find the first compelling evidence for a glitch in its rotation. This glitch is characterized by dv / v = 1.90 +/- 0.05 X 10E-9 and $dvdot/ vdot = (8.5 +/- 0.5 X 10E-5. Taking into account the glitch event, we derive a braking index of n = 1.81 +/- 0.07, significantly lower than previous reported. For the 16 ms pulsar, PSR J0537-6910, we recorded 6 large glitch events during a period of nearly 3 years, the highest rate of all known Crab-like systems. Despite the extreme timing activity, the long term spin-down of this pulsar continues to average -1.9743 X 10E-10 Hz/s.

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arXiv2001

The New Gamma-ray Pulsar PSR J2229+6114, its Pulsar Wind Nebula, and Comparison with the Vela Pulsar

With a period of 51.6 ms and spin-down power 2.2x10^{37} erg s^{-1}, PSR J2229+6114 is a compelling identification for the EGRET source 3EG J2227+6122 in which error circle it resides. Striking features of the Chandra X-ray image are an incomplete elliptical arc and a possible jet, similar to the structures that dominate the appearance of the Vela PWN. Approximately 70% of the 2-10 keV X-ray emission comes from a centrally peaked, diffuse nebula of radius 100" with a power-law spectrum of photon index 1.45 +/- 0.19. The pulsar itself has a marginally harder spectrum with photon index 0.99 +/- 0.27. For an assumed distance of 3 kpc the ratio of X-ray luminosity to spin-down power of PSR J2229+6114 is only 8 x 10^{-5}. We discuss a model in which such inefficient X-ray emission is the signature of a highly magnetized pulsar wind that prevents an internal MHD shock, at the location of the X-ray arc, from strongly compressing the flow. The incomplete X-ray arc is consistent with beaming from a relativistic equatorial outflow, while a surrounding radio shell is probably the forward shock in the surrounding ISM. An MeV source at this location was previously detected by the COMPTEL experiment on CGRO. This, plus the flat X-ray spectrum in the 2-10 keV band, suggests that PSR J2229+6114 is one of the brightest pulsars at 1 MeV, even while it is inconspicuous at radio through X-ray wavelengths, and as steep as the Crab above 100 MeV. The apparent variety of broad-band spectra displayed by high-energy pulsars bolsters the theory that rotation-powered pulsars dominate the unidentified Galactic EGRET source population.

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arXiv2001

X-ray Detection of Pulsar PSR B1757-24 and its Nebular Tail

We report the first X-ray detection of the radio pulsar PSR B1757-24 using the Chandra X-ray Observatory. We detect point-source emission at the pulsar position plus a faint tail extending nearly 20" east of the pulsar, in the same direction and with comparable morphology to the radio tail. Assuming the point-source X-ray emission is magnetospheric, the observed X-ray tail represents only ~0.01% of the pulsar's spin-down luminosity. This is significantly lower than the analogous efficiencies of most known X-ray nebulae surrounding rotation-powered pulsars. Assuming a non-thermal spectrum for the tail photons, we show that the tail is unlikely to be emission left behind following the passage of the pulsar, but rather is probably from synchrotron-emitting pulsar wind particles having flow velocity ~7000 km/s. We also show that there must be a significant break in the tail synchrotron spectrum between the radio and X-ray bands that is intrinsic to the particle spectrum. No emission is detected from the shell supernova remnant G5.4-1.2. The upper limits on remnant emission are unconstraining.

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arXiv2001

Precise Chandra Localization of the Soft Gamma-ray Repeater SGR 1806-20

We present observations of the Soft Gamma-ray Repeater SGR 1806-20 taken with the Chandra X-ray Observatory. We identify the X-ray counterpart of SGR 1806-20 based on detection of 7.5-s pulsations. Using three unrelated X-ray sources (and USNO stars) as position references, we are able to determine that the SGR is at alpha_{2000}=18h08m39.32s and delta_{2000}=-20o24'39.5", with rms uncertainty of 0.3" in each coordinate. We find that SGR 1806-20 is located within the 1-sigma error region determined by Interplanetary Network data and is 14+/-0.5 arcsec distant from the non-thermal core of SNR G10.0-0.3, excluding SGR 1806-20 as the origin of the core. We see evidence for a significant deviation of the spin-down of SGR 1806-20 from its long-term trend, demonstrating erratic spin-down behavior in this source similar to that seen in other SGRs. Finally, we show that there is a broad X-ray halo surrounding SGR 1806-20 out to radii ~1' due to scattering in the interstellar medium.

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arXiv2001

X-Ray Synchrotron Emission from 10-100 TeV Cosmic-Ray Electrons in the Supernova Remnant SN 1006

We present the results of a joint spectral analysis of RXTE PCA, ASCA SIS, and ROSAT PSPC data of the supernova remnant SN 1006. This work represents the first attempt to model both the thermal and nonthermal X-ray emission over the entire X-ray energy band from 0.12 to 17 keV. The thermal flux is described by a nonequilibrium ionization model with an electron temperature kT = 0.6 keV, an ionization timescale n0t = 9 x 10^9 s / cm^3, and a relative elemental abundance of silicon that is 10-18 times larger than the solar abundance. The nonthermal X-ray spectrum is described by a broken power law model with low- and high-energy photon indices Gamma_1 = 2.1 and Gamma_2 = 3.0, respectively. Since the nonthermal X-ray spectrum steepens with increasing energy, the results of the present analysis corroborate previous claims that the nonthermal X-ray emission is produced by synchrotron radiation. We argue that the magnetic field strength is significantly larger than previous estimates of about 1 x 10^-5 G and arbitrarily use a value of 4 x 10^-5 G to estimate the parameters of the cosmic-ray electron, proton, and helium spectra of the remnant. The results for the ratio of the number densities of protons and electrons (R = 160 at 1 GeV), the total energy in cosmic rays (E_cr = 1 x 10^50 ergs), and the spectral index of the electrons at 1 GeV (Gamma_e = 2.14 +/- 0.12) are consistent with the hypothesis that Galactic cosmic rays are accelerated predominantly in the shocks of supernova remnants. Yet, the remnant may or may not accelerate nuclei to energies as high as the energy of the "knee," depending on the reason why the maximum energy of the electrons is only 10 TeV.

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arXiv2001

Chandra Observations of G11.2-0.3: Implications for Pulsar Ages

We present Chandra X-ray Observatory imaging observations of the young Galactic supernova remnant G11.2-0.3. The image shows that the previously known young 65-ms X-ray pulsar is at position (J2000) RA 18h 11m 29.22s, DEC -19o 25' 27.''6, with 1 sigma error radius 0.''6. This is within 8'' of the geometric center of the shell. This provides strong confirming evidence that the system is younger, by a factor of ~12, than the characteristic age of the pulsar. The age discrepancy suggests that pulsar characteristic ages can be poor age estimators for young pulsars. Assuming conventional spin down with constant magnetic field and braking index, the most likely explanation for the age discrepancy in G11.2-0.3 is that the pulsar was born with a spin period of ~62 ms. The Chandra image also reveals, for the first time, the morphology of the pulsar wind nebula. The elongated hard-X-ray structure can be interpreted as either a jet or a Crab-like torus seen edge on. This adds to the growing list of highly aspherical pulsar wind nebulae and argues that such structures are common around young pulsars.

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arXiv2001

PSR J1016-5857: a young radio pulsar with possible supernova remnant, X-ray, and gamma-ray associations

We report the discovery of a young and energetic pulsar in the Parkes multibeam survey of the Galactic plane. PSR J1016-5857 has a rotation period of 107 ms and period derivative of 8e-14, implying a characteristic age of 21 kyr and spin-down luminosity of 2.6e36 erg/s. The pulsar is located just outside, and possibly interacting with, the shell supernova remnant G284.3-1.8. Archival X-ray data show a source near the pulsar position which is consistent with emission from a pulsar wind nebula. The pulsar is also located inside the error box of the unidentified EGRET source 3EG J1013-5915, for which it represents a plausible counterpart.

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arXiv2001

Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters in supernova remnants

Important constraints on the properties of the Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs) can be provided by their associations with supernova remnants (SNRs). We have made a radio search for SNRs towards the AXPs RX J170849-400910 and 4U 0142+61 - we find that the former lies near a possible new SNR with which it is unlikely to be physically associated, but see no SNR in the vicinity of the latter. We review all claimed pairings between AXPs and SNRs; the three convincing associations imply that AXPs are young (< 10 000 yr) neutron stars with low projected space velocities (< 500 km/s). Contrary to previous claims, we find no evidence that the density of the ambient medium around AXPs is higher than that in the vicinity of radio pulsars. Furthermore, the non-detection of radio emission from AXPs does not imply that these sources are radio-silent. We also review claimed associations between SGRs and SNRs. We find none of these associations to be convincing, consistent with a scenario in which SGRs and AXPs are both populations of high-field neutron stars ("magnetars") but in which the SGRs are an older or longer-lived group of objects than are the AXPs. If the SGR/SNR associations are shown to be valid, then SGRs must be high-velocity objects and most likely represent a different class of source to the AXPs.

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arXiv2001

Detection of an X-ray Pulsar Wind Nebula and Tail in SNR N157B

We report Chandra X-ray observations of the supernova remnant N157B in the Large Magellanic Cloud, which are presented together with an archival HST optical image and a radio continuum map for comparison. This remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly rotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar to within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detected in any other wavelength band. The X-ray observations resolve three distinct features: the pulsar itself, a surrounding compact wind nebula which is strongly elongated and a feature of large-scale diffuse emission trailing from the pulsar. This latter comet tail-shaped feature coexists with enhanced radio emission and is oriented nearly perpendicular to the major axis of the pulsar wind nebula. We propose the following scenario to explain these features. The bright, compact nebula is likely powered by a toroidal pulsar wind of relativistic particles which is partially confined by the ram-pressure from the supersonic motion of the pulsar. The particles, after being forced out from the compact nebula (the head of the ``comet''), are eventually dumped into a bubble (the tail), which is primarily responsible for the extended diffuse X-ray and radio emission. The ram-pressure confinement also allows a natural explanation for the observed X-ray luminosity of the compact nebula and for the unusually small X-ray to spin-down luminosity ratio, compared to similarly energetic pulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times 10^6 (with an uncertainty of a factor about 2, consistent with that inferred from the modeling of the Crab Nebula.

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arXiv2001

Ubiquity: Relativistic Winds from Young Rotation-Powered Pulsars

Recent X-ray observations of young rotation-powered pulsars are providing an unprecedented detailed view of pulsar wind nebulae. For the first time, coherent emission features involving wisps, co-aligned toroidal structures, and axial jets are fully resolved in X-rays on arc-second scales. These structures, which are remarkably coherent and symmetric, are similar to features seen in the optical Crab nebula. In this report, we present the latest high resolution Chandra X-ray images of six young rotation-powered pulsars in supernova remnants. These data suggest an X-ray morphology perhaps common to all young rotation-powered pulsars and serve as a guide for developing the next generation of theoretical models for pulsar wind nebulae.

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arXiv2001

Chandra Detection of the Forward and Reverse Shocks in Cassiopeia-A

We report the localization of the forward and reversed shock fronts in the young supernova remnant Cas-A using X-ray data obtained with the Chandra Observatory. High resolution X-ray maps resolve a previously unseen X-ray feature encompassing the extremity of the remnant. This feature consists of thin, tangential wisps of emission bordering the outer edge of the thermal X-ray and radio remnant, forming a circular rim, approx. 2.7 in radius. Radio images show a sharp rise in brightness at this X-ray rim, along with a large jump in the synchrotron polarization angle. These characteristics suggest that these wisps are the previously unresolved signature of the forward, or outer, shock. Similarly, we identify the sharp rise in emissivity of the bright shell for both the radio and X-ray line emission associated with the reverse shock. The derived ratio of the averaged forward and reverse shock radii of approx. 3:2 constrains the remnant to have swept up roughly the same amount of mass as was ejected; this suggests that Cas-A is just entering the Sedov phase. Comparison of the X-ray spectra from the two shock regions shows that the equivalent widths of prominent emission lines are significantly lower exterior to the bright shell, as expected if they are respectively identified with the shocked circumstellar material and shocked ejecta. Furthermore, the spectrum of the outer rim itself is dominated by power-law emission, likely the counterpart of the non-thermal component previously seen at energies above 10 keV.

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arXiv2001

PSR J2229+6114: Discovery of an Energetic Young Pulsar in the Error Box of the EGRET Source 3EG J2227+6122

We report the detection of radio and X-ray pulsations at a period of 51.6 ms from the X-ray source RX/AX J2229.0+6114 in the error box of the EGRET source 3EG J2227+6122. An ephemeris derived from a single ASCA observation and multiple epochs at 1412 MHz from Jodrell Bank indicates steady spin-down with P-dot = 7.83 x 10^(-14) s/s. From the measured P and P-dot we derive spin-down power E-dot = 2.2 x 10^(37) erg/s, magnetic field B = 2.0 x 10^(12) G, and characteristic age P/2P-dot = 10,460 yr. An image from the Chandra X-ray Observatory reveals a point source surrounded by centrally peaked diffuse emission that is contained within an incomplete radio shell. We assign the name G106.6+2.9 to this new supernova remnant, which is evidently a pulsar wind nebula. For a distance of 3 kpc estimated from X-ray absorption, the ratio of X-ray luminosity to spin-down power is ~8 x 10^(-5), smaller than that of most pulsars, but similar to the Vela pulsar. If PSR J2229+6114 is the counterpart of 3EG J2227+6122 then its efficiency of gamma-ray production, if isotropic, is 0.016 (d/3 kpc)^2. It obeys an established trend of gamma-ray efficiency among known gamma-ray pulsars which, in combination with the demonstrated absence of any other plausible counterpart for 3EG J2227+6122, makes the identification compelling. If confirmed, this identification bolsters the pulsar model for unidentified Galactic EGRET sources.

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