F. J. Lu

G359.95-0.04: an energetic pulsar candidate near Sgr A*

We report the discovery of a prominent non-thermal X-ray feature located near the Galactic centre that we identify as an energetic pulsar wind nebula. This feature, G359.95-0.04, lies 1-lyr north of Sgr A* (in projection), is comet like in shape, and has a power-law spectrum that steepens with increasing distance from the putative pulsar. The distinct spectral and spatial X-ray characteristics of the feature are similar to those belonging to the rare class of ram-pressure confined pulsar wind nebulae. The luminosity of the nebula at the distance of Sgr A*, consistent with the inferred X-ray absorptions, is L(x)similar to 1 x 10(34) erg s(-1) in the 2-10 keV energy band. The cometary tail extends back to a region centred at the massive stellar complex IRS 13 and surrounded by an enhanced diffuse X-ray emission, which may represent an associated supernova remnant. Furthermore, the inverse Compton scattering of the strong ambient radiation by the nebula consistently explains the observed TeV emission from the Galactic centre. We also briefly discuss plausible connections of G359.95-0.04 to other high-energy sources in the region, such as the young stellar complexes IRS 13 and SNR Sgr A East.

EMERGENT SPECTRA FROM SLIM ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI

Recently the vertical structure of accretion disks has received much attention because of the available comparison with observations of the big blue bump, but all the calculations are based on the standard accretion disk model. In this paper we calculate the vertical structure and the emergent spectrum based on the radial structure of slim disks from the height-averaged equations. With the a-prescription of energy generation, we obtain the density profile as a Gaussian distribution, then solve the energy transfer problem in the z-direction. We find that the sub-Keplerian rotation affects the density profile in the z-direction. The advection process plays significant roles in the emergent spectrum, especially for high accretion rates. We show that there are two prominent characteristics of the spectrum: the presence of a maximum frequency in the emergent spectrum, which is very weakly dependent on the accretion rate, and a flattened component. These results are beyond the scope of explanation of the standard model of accretion disks even for the model with the modified blackbody spectrum. The present model is suitable for a wide range of disk parameters.

THE ENERGY DEPENDENCE OF THE CENTROID FREQUENCY AND PHASE LAG OF THE QUASI-PERIODIC OSCILLATIONS IN GRS 1915+105

We present a study of the centroid frequencies and phase lags of quasi-periodic oscillations (QPOs) as functions of photon energy for GRS 1915+105. It is found that the centroid frequencies of the 0.5-10 Hz QPOs and their phase lags are both energy dependent, and there exists an anticorrelation between the QPO frequency and phase lag. These new results challenge the popular QPO models, because none of them can fully explain the observed properties. We suggest that the observed QPO phase lags are partially due to the variation of the QPO frequency with energy, especially for those with frequency higher than 3.5 Hz.

X-Ray Thread G0.13–0.11: A Pulsar Wind Nebula?

We have examined Chandra observations of the recently discovered X-ray thread G0.13-0.11 in the Galactic center Radio Arc region. Part of the Chandra data was studied by Yusef-Zadeh, Law, & Wardle, who reported the detection of 6.4 keV line emission in this region. We find, however, that this line emission is not associated with G0.13-0.11. The X-ray spectrum of G0.13-0.11 is well-characterized by a simple power law with an energy slope of 1.8 (90% confidence uncertainties). Similarly, the X-ray spectrum of the pointlike source embedded in G0.13-0.11 has a power-law energy slope of 0.9. The 2-10 keV band luminosities of these two components are ~3.2 × 1033 ergs s-1 (G0.13-0.11) and ~7.5 × 1032 ergs s-1 (point source) at the Galactic center distance of 8 kpc. The morphological, spectral, and luminosity properties strongly indicate that G0.13-0.11 represents the leading edge of a pulsar wind nebula, produced by a pulsar (point source) moving in a strong magnetic field environment. The main body of this pulsar wind nebula is likely traced by a bow-shaped radio feature, which is apparently bordered by G0.13-0.11 and is possibly associated with the prominent nonthermal radio filaments of the Radio Arc. We speculate that young pulsars may be responsible for various unique nonthermal filamentary radio and X-ray features observed in the Galactic center region.

The Radio Properties and Magnetic Field Configuration in the Crab-Like Pulsar Wind Nebula G54.1+0.3

We present a multifrequency radio investigation of the Crab-like pulsar wind nebula (PWN) G54.1+0.3 using the Very Large Array. The high resolution of the observations reveals that G54.1+0.3 has a complex radio structure which includes filamentary and loop-like structures that are magnetized, a diffuse extent similar to the associated diffuse X-ray emission. But the radio and X-ray structures in the central region differ strikingly, indicating that they trace very different forms of particle injection from the pulsar and/or particle acceleration in the nebula. No spectral index gradient is detected in the radio emission across the PWN, whereas the X-ray emission softens outward in the nebula. The extensive radio polarization allows us to image in detail the intrinsic magnetic field, which is well-ordered and reveals that a number of loop-like filaments are strongly magnetized. In addition, we determine that there are both radial and toroidal components to the magnetic field structure of the PWN. Strong mid-infrared (IR) emission detected in Spitzer Space Telescope data is closely correlated with the radio emission arising from the southern edge of G54.1+0.3. In particular, the distributions of radio and X-ray emission compared with the mid-IR emission suggest that the PWN may be interacting with this interstellar cloud. This may be the first PWN where we are directly detecting its interplay with an interstellar cloud that has survived the impact of the supernova explosion associated with the pulsars progenitor.

Cr-K EMISSION LINE AS A CONSTRAINT ON THE PROGENITOR PROPERTIES OF SUPERNOVA REMNANTS

We perform a survey of the Cr, Mn, and Fe-K emission lines in young supernova remnants (SNRs) with the Japanese X-ray astronomy satellite Suzaku. The Cr and/or Mn emission lines are detected in 3C 397 and 0519–69.0 for the first time. We also confirm the detection of these lines in Kepler, W49B, N103B, and Cas A. We derive the line parameters (i.e., the line centroid energy, flux, and equivalent width (EW)) for these six sources and perform a correlation analysis for the line center energies of Cr, Mn, and Fe. Also included in the correlation analysis are Tycho and G344.7–0.1 for which the Cr, Mn, and Fe-K line parameters were available in the literature through Suzaku observations. We find that the line center energies of Cr correlate very well with that of Fe and that of Mn. This confirms our previous findings that Cr, Mn, and Fe are spatially co-located, share a similar ionization state, and have a common origin in the supernova nucleosynthesis. We find that the ratio of the EW of the Cr emission line to that of Fe () provides useful constraints on the SNR progenitors and on the SN explosion mechanisms: for SNRs with γCr/Fe > 2%, a Type Ia origin is favored (e.g., N103B, G344.7–0.1, 3C 397, and 0519–69.0); for SNRs with γCr/Fe < 2%, they could be of either core-collapse origin or carbon-deflagration Ia origin.

A multiwavelength study of 1WGA J1346.5-6255 : A new γ cas analog unrelated to the background supernova remnant G309.2-00.6

WGA J1346.5-6255 is a ROSAT X-ray source found within the radio lobes of the supernova remnant (SNR) G309.2-00.6. This source also appears to coincide with the bright and early-type star HD 119682, which is in the middle of the galactic open cluster NGC 5281. The radio morphology of the remnant, consisting of two brightened and distorted arcs of emission on opposite sides of the 1WGA J1346.5-6255 source and of a jet-like feature and break in the shell, led to the suggestion that 1WGA J1346.5-6255/G309.2-00.6 is a young analog of the microquasar SS 433 powering the W50 nebula. This motivated us to study this source at X-ray and optical wavelengths. We here present new Chandra observations of 1WGA J1346.5-6255, archival XMM- Newton observations of G309.2-00.6, and optical spectroscopic observations of HD 119682, in order to search for X-ray jets from 1WGA J1346.5-6255, study its association with the SNR, and test for whether HD 119682 represents its optical counterpart. We do not nd evidence for jets from 1WGA J1346.5-6255 down to an unabsorbed ux of 2.6 10 -13 ergs cm -2 s (0.5n7.5 keV), we rule out its association with G309.2-00.6, and we conrm that HD 119682 is its optical counterpart. We derive a distance of 1.2 0.3 kpc, which is consistent with the distance estimate to NGC 5281 (1.3 0.3 kpc), and much smaller than the distance derived to the SNR G309.2-00.6. We discuss the nature of the source, unveil that HD 119682 is a Be star and suggest it is a new member of the recently proposed group of -Cas analogs. The Chandra and XMM-Newton X-ray lightcurves show variability on timescales of hundreds of seconds, and the presence of a possible period of 1500 s that could be the rotational period of an accreting neutron star or white dwarf in this -Cas analog.

X-Ray Timing Observations of PSR J1930+1852 in the Crab-like SNR G54.1+0.3

We present new X-ray timing and spectral observations of PSR J1930+1852, the young energetic pulsar at the center of the nonthermal supernova remnant G54.1+0.3. Using data obtained with the Rossi X-Ray Timing Explorer (RXTE) and Chandra X-ray observatories we have derived an updated timing ephemeris of the 136 ms pulsar spanning 6 years. During this interval, however, the period evolution shows significant variability from the best-fit constant spin-down rate of = 7.5112(6) × 10-13 s s-1, suggesting strong timing noise and/or glitch activity. The X-ray emission is highly pulsed (71% ± 5% modulation) and is characterized by an asymmetric, broad profile (~70% duty cycle) that is nearly twice the radio width. The spectrum of the pulsed emission is well fitted with an absorbed power law of photon index Γ = 1.2 ± 0.2; this is marginally harder than that of the unpulsed component. The total 2-10 keV flux of the pulsar is 1.7 × 10-12 ergs cm-2 s-1. These results confirm PSR J1930+1852 as a typical Crab-like pulsar.

Phase Evolution of the Crab Pulsar between Radio and X-Ray

We study the X-ray phases of the Crab pulsar utilizing the 11-year observations from the Rossi X-ray Timing Explorer, 6-year radio observations from the Nanshan Telescope, and the ephemeris from Jodrell Bank Observatory. It is found that the X-ray phases in different energy bands and the radio phases from Nanshan Telescope show similar behaviors, including long-time evolution and short-time variations. Such strong correlations between the X-ray and radio phases imply that the radio and X-ray timing noises are both generated from the pulsar spin that cannot be well described by the the monthly ephemeris from the Jodrell Bank observatory. When using the Nanshan phases as references to study the X-ray timing noise, it has a significantly smaller variation amplitude and shows no long-time evolution, with a change rate of

Discovery of a 136 Millisecond Radio and X-Ray Pulsar in Supernova Remnant G54.1+0.3

(-1.1\pm1.1)\times10^{-7}