Study of Four Young TeV Pulsar Wind Nebulae with a Spectral Evolution Model

We study four young Pulsar Wind Nebulae (PWNe) detected in TeV gamma-rays, G21.5-0.9, G54.1+0.3, Kes 75, and G0.9+0.1, using the spectral evolution model developed and applied to the Crab Nebula in our previous work. We model the evolution of magnetic field and particle distribution function inside a uniformly expanding PWN considering a time-dependent injection from the pulsar and radiative and adiabatic losses. Considering uncertainties in the interstellar radiation field (ISRF) and their distance, we study two cases for each PWN. Because TeV PWNe have a large TeV gamma-rays to X-rays flux ratio, the magnetic energy of the PWNe accounts for only a small fraction of the total energy injected (typically a few x 10^{-3}). The gamma-ray emission is dominated by inverse Compton scattering off the infrared photons of the ISRF. A broken power-law distribution function for the injected particles reproduces the observed spectrum well, except for G0.9+0.1. For G0.9+0.1, we do not need a low energy counterpart because adiabatic losses alone are enough to reproduce the radio observations. High energy power-law indices at injection are similar (2.5 -- 2.6), while low energy power-law indices range from 1.0 to 1.6. The lower limit of the particle injection rate indicates that the pair multiplicity is larger than 10^4. The corresponding upper limit of the bulk Lorentz factor of the pulsar winds is close to the break energy of the broken power-law injection, except for Kes 75.Comment: Accepted for publication in ApJ; 56pages, 15figure

X-ray investigation of the diffuse emission around plausible gamma-ray emitting pulsar wind nebulae in Kookaburra region

We report on the results from {\it Suzaku} X-ray observations of the radio complex region called Kookaburra, which includes two adjacent TeV $\gamma$-ray sources HESS J1418-609 and HESS J1420-607. The {\it Suzaku} observation revealed X-ray diffuse emission around a middle-aged pulsar PSR J1420-6048 and a plausible PWN Rabbit with elongated sizes of $\sigma_{\rm X}=1^{\prime}.66$ and $\sigma_{\rm X}=1^{\prime}.49$, respectively. The peaks of the diffuse X-ray emission are located within the $\gamma$-ray excess maps obtained by H.E.S.S. and the offsets from the $\gamma$-ray peaks are $2^{\prime}.8$ for PSR J1420-6048 and $4^{\prime}.5$ for Rabbit. The X-ray spectra of the two sources were well reproduced by absorbed power-law models with $\Gamma=1.7-2.3$. The spectral shapes tend to become softer according to the distance from the X-ray peaks. Assuming the one zone electron emission model as the first order approximation, the ambient magnetic field strengths of HESS J1420-607 and HESS J1418-609 can be estimated as 3 $\mu$G, and $2.5 \mu$G, respectively. The X-ray spectral and spatial properties strongly support that both TeV sources are pulsar wind nebulae, in which electrons and positrons accelerated at termination shocks of the pulsar winds are losing their energies via the synchrotron radiation and inverse Compton scattering as they are transported outward.Comment: To appear in Ap

Multi-Zone Modeling of The Pulsar Wind Nebula HESS J1825-137

The pulsar wind nebula associated with PSR J1826-1334, HESS J1825-137, is a bright very high energy source with an angular extent of ~1 degree and spatially-resolved spectroscopic TeV measurements. The gamma-ray spectral index is observed to soften with increasing distance from the pulsar, likely the result of cooling losses as electrons traverse the nebula. We describe analysis of X-ray data of the extended nebula, as well as 3-D time-dependent spectral energy distribution modeling, with emphasis on the spatial variations within HESS J1825-137. The multi-wavelength data places significant constraints on electron injection, transport, and cooling within the nebula. The large size and high nebular energy budget imply a relatively rapid initial pulsar spin period of 13 \pm 7 ms and an age of 40 \pm 9 kyr. The relative fluxes of each VHE zone can be explained by advective particle transport with a radially decreasing velocity profile with $v(r) \propto r^{-0.5}$. The evolution of the cooling break requires an evolving magnetic field which also decreases radially from the pulsar, $B(r,t) \propto r^{-0.7} \dot{E}(t)^{1/2}$. Detection of 10 TeV flux ~80 pc from the pulsar requires rapid diffusion of high energy particles with $\tau_{esc} \approx 90 (R / 10 pc)^2 (E_e/100 TeV)^{-1}$ year, contrary to the common assumption of toroidal magnetic fields with strong magnetic confinement. The model predicts a rather uniform Fermi LAT surface brightness out to ~1 degree from the pulsar, in good agreement with the recently discovered LAT source centered 0.5 degree southwest of PSR J1826-1334 with extension 0.6 \pm 0.1 degree.Comment: Updated to published versio

A search for VHE counterparts of Galactic Fermi bright sources and MeV to TeV spectral characterization

Very high-energy (VHE; E>100 GeV) gamma-rays have been detected from a wide range of astronomical objects, such as pulsar wind nebulae (PWNe), supernova remnants (SNRs), giant molecular clouds, gamma-ray binaries, the Galactic Center, active galactic nuclei (AGN), radio galaxies, starburst galaxies, and possibly star-forming regions as well. At lower energies, observations using the Large Area Telescope (LAT) onboard Fermi provide a rich set of data which can be used to study the behavior of cosmic accelerators in the MeV to TeV energy bands. In particular, the improved angular resolution of current telescopes in both bands compared to previous instruments significantly reduces source confusion and facilitates the identification of associated counterparts at lower energies. In this paper, a comprehensive search for VHE gamma-ray sources which are spatially coincident with Galactic Fermi/LAT bright sources is performed, and the available MeV to TeV spectra of coincident sources are compared. It is found that bright LAT GeV sources are correlated with TeV sources, in contrast to previous studies using EGRET data. Moreover, a single spectral component seems unable to describe the MeV to TeV spectra of many coincident GeV/TeV sources. It has been suggested that gamma-ray pulsars may be accompanied by VHE gamma-ray emitting nebulae, a hypothesis that can be tested with VHE observations of these pulsars.Comment: Astronomy and Astrophysics, in press, 17 pages, 12 figures, 5 table

Synchrotron Radiation from the Crab Nebula Discriminates between Models of Space-Time Foam

It has been argued by Jacobson, Liberati and Mattingly that synchrotron radiation from the Crab Nebula imposes a stringent constraint on any modification of the dispersion relations of the electron that might be induced by quantum gravity. We supplement their analysis by deriving the spectrum of synchrotron radiation from the coupling of an electrically-charged particle to an external magnetic fields in the presence of quantum-gravity effects of the general form $(E/M_{QG})^\alpha$. We find that the synchrotron constraint from the Crab Nebula practically excludes \alpha \lsim 1.74 for $M_{QG} \sim m_P = 1.2 \times 10^{19}$ GeV. On the other hand, this analysis does not constrain any modification of the dispersion relation of the photon that might be induced by quantum gravity. We point out that such quantum-gravity effects need not obey the equivalence principle, a point exemplified by the Liouville-string D-particle model of space-time foam. This model suggests a linear modification of the dispersion relation for the photon, but not for the electron, and hence is compatible with known constraints from the Crab Nebula and elsewhere.Comment: 26 pages LATEX, two eps figures incorporated; the version published in Astroparticle Physic

X-ray and Radio Timing of the Pulsar in 3C 58

We present timing data spanning 6.4 yr for the young and energetic PSR J0205+6449, in the supernova remnant 3C 58. Data were obtained with the Rossi X-ray Timing Explorer, the Jodrell Bank Observatory and the Green Bank Telescope. We present phase-coherent timing analyses showing timing noise and two spin-up glitches with fractional frequency increases of ~3.4E-7 near MJD 52555, and ~3.8E-6 between MJDs 52777 and 53062. These glitches are unusually large if the pulsar was created in the historical supernova in 1181 as has been suggested. For the X-ray timing we developed a new unbinned maximum-likelihood method for determining pulse arrival times which performs significantly better than the traditional binned techniques. In addition, we present an X-ray pulse profile analysis of four years of RXTE data showing that the pulsar is detected up to ~40 keV. We also present the first measurement of the phase offset between the radio and X-ray pulse for this source, showing that the radio pulse leads the X-ray pulse by phi=0.10+/-0.01 in phase. We compile all known measurements of the phase offsets between radio and X-ray and radio and gamma-ray pulses for X-ray and gamma-ray pulsars. We show that there is no relationship between pulse period and phase offset, supported by our measurement of the phase offset for PSR J0205+6449.Comment: 19 pages, 12 figures. Published in the Astrophysical Journal. Includes additional data analysis and two new figure

Deconvolving the contributions of cell-type heterogeneity on cortical gene expression

Complexity of cell-type composition has created much skepticism surrounding the interpretation of bulk tissue transcriptomic studies. Recent studies have shown that deconvolution algorithms can be applied to computationally estimate cell-type proportions from gene expression data of bulk blood samples, but their performance when applied to brain tissue is unclear. Here, we have generated an immunohistochemistry (IHC) dataset for five major cell-types from brain tissue of 70 individuals, who also have bulk cortical gene expression data. With the IHC data as the benchmark, this resource enables quantitative assessment of deconvolution algorithms for brain tissue. We apply existing deconvolution algorithms to brain tissue by using marker sets derived from human brain single cell and cell-sorted RNA-seq data. We show that these algorithms can indeed produce informative estimates of constituent cell-type proportions. In fact, neuronal subpopulations can also be estimated from bulk brain tissue samples. Further, we show that including the cell-type proportion estimates as confounding factors is important for reducing false associations between Alzheimer\u27s disease phenotypes and gene expression. Lastly, we demonstrate that using more accurate marker sets can substantially improve statistical power in detecting cell-type specific expression quantitative trait loci (eQTLs)

PSR J1856+0245: Arecibo Discovery of a Young, Energetic Pulsar Coincident with the TeV Gamma-ray Source HESS J1857+026

We present the discovery of the Vela-like radio pulsar J1856+0245 in the Arecibo PALFA survey. PSR J1856+0245 has a spin period of 81ms, a characteristic age of 21kyr, and a spin-down luminosity Edot = 4.6 x 10^36 ergs/s. It is positionally coincident with the TeV gamma-ray source HESS J1857+026, which has no other known counterparts. Young, energetic pulsars create wind nebulae, and more than a dozen pulsar wind nebulae have been associated with very-high-energy (100GeV-100TeV) gamma-ray sources discovered with the HESS telescope. The gamma-ray emission seen from HESS J1857+026 is potentially produced by a pulsar wind nebula powered by PSR J1856+0245; faint X-ray emission detected by ASCA at the pulsar's position supports this hypothesis. The inferred gamma-ray efficiency is epsilon_gamma = L_gamma/Edot = 3.1% (1-10TeV, for a distance of 9kpc), comparable to that observed in similar associations.Comment: 13 pages, 1 figure, accepted for publication in The Astrophysical Journal Letter

Observations of PSR J2021+3651 and its X-ray Pulsar Wind Nebula G75.2+0.1

We present results from X-ray and radio observations of the recently discovered young Vela-like pulsar PSR J2021+3651, which is coincident with the EGRET gamma-ray source GeV 2020+3658. A 19.0-ks Chandra ACIS-S observation has revealed a ~20'' x 10'' pulsar wind nebula that is reminiscent of the equatorial tori seen around some young pulsars, along with thermal emission from an embedded point source (kT = 0.15 +/- 0.02 keV). We name the nebula G75.2+0.1. Its spectrum is well fit by an absorbed power-law model with photon index 1.7 +/- 0.3, hydrogen column density nH = 7.8 +/- 1.7 x 10^21 cm^-2, and an unabsorbed 0.3-10.0 keV flux of 1.9 +/- 0.3 x 10^-12 erg cm^-2 s^-1. We have spatially fit G75.2+0.1 with a model that assumes a toroidal morphology, and from this we infer that the torus is highly inclined 83 deg +/- 1 deg to the line of sight. A 20.8-ks Chandra observation in continuous-clocking mode reveals a possible pulse detection, with a pulsed fraction of ~37% and an H-test probability of occuring by chance of 1.2 x 10^-4. Timing observations with the Arecibo radio telescope spanning two years show that PSR J2021+3651 glitched sometime between MJDs 52616 and 52645 with parameters delta(v)/v = (2.587 +/- 0.002) x 10^-6 and delta(dot(v))/v = (6.2 +/- 0.3) x 10^-3, similar to those of the largest glitches observed in the Vela pulsar. PSR J2021+3651 is heavily scattered (T_sc = 17.7 ms +/- 0.9 ms at 1 GHz) and exhibits a significant amount of timing noise.Comment: 23 pages, 6 figures. Submitted to the Astrophysical Journa

Implications of H.E.S.S. observations of pulsar wind nebulae

In this review paper on pulsar wind nebulae (PWN) we discuss the properties of such nebulae within the context of containment against cross-field diffusion (versus normal advection), the effect of reverse shocks on the evolution of offset ``Vela-like'' PWN, constraints on maximum particle energetics, magnetic field strength estimates based on spectral and spatial properties, and the implication of such field estimates on the composition of the wind. A significant part of the discussion is based on the High Energy Stereoscopic System ({\it H.E.S.S.} or {\it HESS}) detection of the two evolved pulsar wind nebulae Vela X (cocoon) and HESS J1825-137. In the case of Vela X (cocoon) we also review evidence of a hadronic versus a leptonic interpretation, showing that a leptonic interpretation is favored for the {\it HESS} signal. The constraints discussed in this review paper sets a general framework for the interpretation of a number of offset, filled-center nebulae seen by {\it HESS}. These sources are found along the galactic plane with galactic latitudes $|b|\sim 0$, where significant amounts of molecular gas is found. In these regions, we find that the interstellar medium is inhomogeneous, which has an effect on the morphology of supernova shock expansion. One consequence of this effect is the formation of offset pulsar wind nebulae as observed.Comment: to appear in Springer Lecture Notes on Neutron Stars and Pulsars: 40 years after their discovery, eds. W. Becke