Extended Emission from the PSR B1259-63/SS 2883 Binary Detected with Chandra

PSR B1259-63 is a middle-aged radio pulsar (P=48 ms, tau=330 kyr, Edot=8.3*10^{35} erg/s) in an eccentric binary (P_orb =3.4 yr, e=0.87) with a high-mass Be companion, SS 2883. We observed the binary near apastron with the Chandra ACIS detector on 2009 May 14 for 28 ks. In addition to the previously studied pointlike source at the pulsar's position, we detected extended emission on the south-southwest side of this source. The pointlike source spectrum can be described by the absorbed power-law model with the hydrogen column density N_H = (2.5+/-0.6)*10^{21} cm^{-2}, photon index Gamma = 1.6+/-0.1, and luminosity L_{0.5-8 keV} = 1.3*10^{33} d_3^2 erg/s, where d_3 is the distance scaled to 3 kpc. This emission likely includes an unresolved part of the pulsar wind nebula (PWN) created by the colliding winds from the pulsar and the Be companion, and a contribution from the pulsar magnetosphere. The extended emission apparently consists of two components. The highly significant compact component looks like a southward extension of the pointlike source image, seen up to about 4 arcsec from the pulsar position. Its spectrum has about the same slope as the pointlike source spectrum, while its luminosity is a factor of 10 lower. We also detected an elongated feature extended ~15 arcsec southwest of the pulsar, but significance of this detection is marginal. We tentatively interpret the resolved compact PWN component as a shocked pulsar wind blown out of the binary by the wind of the Be component, while the elongated component could be a pulsar jet.Comment: 13 pages, 7 figures. Submitted to Ap

Probing Shock Properties with Non-thermal X-ray Filaments in Cas A

Thin non-thermal X-ray filaments are often seen in young supernova remnants. We used data from the 1 Ms Chandra observation of Cassiopeia A to study spectral properties of some of the filaments in this remnant. For all the cases that we examined, the X-ray spectrum across the filaments hardens, at about 10% level, going outward, while observed filament widths depend only weakly on the photon energy. Using a model that includes radiative cooling, advection and diffusion of accelerated particles behind the shock, we estimated the magnetic field, turbulence level and shock obliquity.Comment: 21 pages, 5 figure

Extended X-ray emission in the vicinity of the microquasar LS 5039: pulsar wind nebula?

LS 5039 is a high-mass binary with a period of 4 days, containing a compact object and an O star, one of the few high-mass binaries detected in gamma-rays. Our Chandra ACIS observation of LS 5039 provided a high-significance (~10sigma) detection of extended emission clearly visible for up to 1' from the point source. The spectrum of this emission can be described by an absorbed power-law model with photon index Gamma=1.9pm0.3, somewhat softer than the point source spectrum Gamma=1.44pm0.07, with the same absorption, N_H=(6.4pm0.6)e21 /cm2. The observed 0.5-8 keV flux of the extended emission is 8.8e-14 erg/s/cm2, or 5% of the point source flux; the latter is a factor of ~2 lower than the lowest flux detected so far. Fainter extended emission with comparable flux and a softer (Gamma~3) spectrum is detected at even greater radii (up to 2'). Two possible interpretations of the extended emission are a dust scattering halo and a synchrotron nebula powered by energetic particles escaping the binary. We discuss both of these scenarios and favor the nebula interpretation, although some dust contribution is possible. We have also found transient sources located within a narrow stripe south of LS 5039. We discuss the likelihood of these sources to be related to LS 5039.Comment: 28 pages. Accepted for publication in Ap

The Young Pulsar J1357-6429 and Its Pulsar Wind Nebula

We observed the young pulsar J1357--6429 with the {\it Chandra} and {\it XMM-Newton} observatories. The pulsar spectrum fits well a combination of absorbed power-law model ($\Gamma=1.7\pm0.6$) and blackbody model ($kT=140^{+60}_{-40}$ eV, $R\sim2$ km at the distance of 2.5 kpc). Strong pulsations with pulsed fraction of $42$, apparently associated with the thermal component, were detected in 0.3--1.1 keV. Surprisingly, pulsed fraction at higher energies, 1.1--10 keV, appears to be smaller, $23$. The small emitting area of the thermal component either corresponds to a hotter fraction of the neutron star (NS) surface or indicates inapplicability of the simplistic blackbody description. The X-ray images also reveal a pulsar-wind nebula (PWN) with complex, asymmetric morphology comprised of a brighter, compact PWN surrounded by the fainter, much more extended PWN whose spectral slopes are $\Gamma=1.3\pm0.3$ and $\Gamma=1.7\pm0.2$, respectively. The extended PWN with the observed flux of $\sim7.5\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ is a factor of 10 more luminous then the compact PWN. The pulsar and its PWN are located close to the center of the extended TeV source HESS J1356--645, which strongly suggests that the VHE emission is powered by electrons injected by the pulsar long ago. The X-ray to TeV flux ratio, $\sim0.1$, is similar to those of other relic PWNe. We found no other viable candidates to power the TeV source. A region of diffuse radio emission, offset from the pulsar toward the center of the TeV source, could be synchrotron emission from the same relic PWN rather than from the supernova remnant.Comment: 18 pages, 12 figures, and 4 tables, Accepted for publication in Ap

A systematic study of variable absorption lines in the black hole X-ray binary Cygnus X-1 with the Chandra X-ray observatory

The study of black holes is one of the most important topics in high energy astrophysics. Cygnus X-1 is one of the most well-studied stellar mass black hole systems. In Cygnus X-1, a black hole is thought to accrete material from the stellar wind of the companion star, HDE 226868. During the process the infalling matter releases enormous amounts of gravitational potential energy, and the material is heated to millions of degrees due to viscous stress. The region around the black hole then emits X-rays. Many X-ray observatories have observed Cygnus X-1, but the details of the environment of Cygnus X-1, including the physical condition of the stellar wind, are still not fully understood. High resolution spectroscopy is a valuable tool for use in such a study. After the launch of the Chandra X-ray observatory, high resolution spectral data have become available. The presence of numerous absorption lines that are associated with highly ionized materials has been revealed by Chandra. The absorption lines provided insight into the details of stellar wind geometry. However, a systematic study of the variability of the absorption lines has not been carried out. This work addresses the issue in the case of Cygnus X-1. During the study, the dramatic variability of the absorption lines on a time scale of hours was discovered for the first time. The study was based on observation carried out under a program of own. The light curve shows that Cygnus X-1 was brighter during the first half of the observation, so we divided the whole observation into “high” and “low” periods with each period having about a half of exposure time. In the first period of the observation, we detected absorption lines associated with highly ionized Ne, Na, Mg, Al, Si, S, and Fe, some of which had been seen in earlier Chandra observations. Surprisingly, however, most of the lines became undetectable in the second period. The lines weakened by more than two orders of magnitude even though the continuum varied only by 20–30%. The absorption lines are generally attributed to the absorption of X-rays by the ionized stellar wind from the companion star in the binary system. Since the overall X-ray luminosity varied only mildly, the observed variability probably arose from a sudden change in the density of the wind. To gain deeper insights into the origin of highly variable absorption lines, we examined all archival high resolution spectroscopic observations of Cygnus X-1 that were carried out with Chandra. Similar variable absorption lines were detected in one of the archival observations, so the phenomenon does not seem to be rare. We also looked into plausible dependencies of the lines on the binary orbital motion as well as on the spectral states. Although we saw evidence for such dependencies, the sporadic coverage of the orbit and spectral states makes it difficult to disentangle the two effects. Improved temporal coverage would be needed to make further progress. The study of variabilies of the absorption lines may give further details on the understand of stellar wind and Cygnus X-1 binary system. The results may also be applicable to other black hole systems. In addition to the main topics of this thesis, two more projects were carried out. In the first project, we searched systematically for X-ray counterparts to some of the unidentified TeV γ–ray sources near known pulsars, taking advantage of Chandra\u27s unprecedented spatial resolution. The results show that some of the sources are likely pulsar wind nebulae (PWNe). In the second project, the 1Ms Chandra observations of Cassiopeia A were analyzed to take a deep look at the non-thermal filaments near the forward shock. The preliminary results show an interesting trend: the X-ray spectrum of the filaments steepens going outward. Further investigation is on going. Our collaborators are now carrying out detailed modeling of the diffusion, radiation, and advection processes

Comparison of Clinical and Functional Outcomes Using Pectoralis Major and Cutaneous Free Flaps for Hypopharyngeal Squamous Cell Carcinoma

Background The purpose of this study was to compare postoperative morbidities and functional outcomes of pectoralis major myocutaneous (PMMC) flap and cutaneous free flap reconstruction approaches in hypopharyngeal squamous cell carcinoma patients. Methods We retrospectively reviewed records from 99 patients who underwent hypopharyngeal reconstruction with a cutaneous free flap (n=85) or PMMC flap (n=14) between 1995 and 2013. Morbidity was classified into hospitalization, medical, or flap-related complications. Functional outcomes were classified into oral re-alimentation and decannulation time. Results The overall flap-related complication rate was higher in the PMMC flap group (n=8, 57.1%; P=0.019), but the medical morbidity rate was higher in the cutaneous free flap group (n=68, 80%; P=0.006). The rate of pneumonia was higher in the cutaneous free flap group (n=48, 56.5%; P=0.020). Pulmonary premorbidity was the variable most significantly associated with pneumonia (odds ratio=3.012, P=0.012). There was no statistically significant difference in oral re-alimentation and decannulation time between the two groups. Conclusions Although the functional superiority of free flaps has been reported in many studies, our results do not support this hypothesis. One limitation of our study is the relatively smaller flap size and fewer PMMC flap cases compared with the cutaneous free flap group. The low postoperative medical morbidity incidence rate in the PMMC flap group was clinically significant; however, the free flap group had more flap-related complications. Thus, PMMC flaps should be considered a viable option, especially for patients with pulmonary premorbidities