Time Dependent Modeling of the Markarian 501 X-ray and TeV Gamma-Ray Data Taken During March and April, 1997

If the high-energy emission from TeV blazars is produced by the Synchrotron Self-Compton (SSC) mechanism, then simultaneous X-ray and Gamma-ray observations of these objects are a powerful probe of the electron (and/or positron) populations responsible for this emission. Understanding the emitting particle distributions and their evolution in turn allow us to probe physical conditions in the inner blazar jet and test, for example, various acceleration scenarios. By constraining the SSC emission model parameters, such observations also allow us to predict the intrinsic (unabsorbed) Gamma-ray spectra of these sources, a major uncertainty in current attempts to use the observed Gamma-ray spectra to constrain the intensity of the extragalactic background at optical/infrared wavelengths. As a next step in testing the SSC model and as a demonstration of the potential power of coordinated X-ray and Gamma-ray observations, we attempt to model in detail the X-ray and Gamma-ray light curves of the TeV Blazar Mrk 501 during its April-May 1997 outburst using a time dependent SSC emission model. Extensive, quasi-simultaneous X-ray and gamma-ray coverage exists for this period. We discuss and explore quantitatively several of the flare scenarios presented in the literature. We show that simple two-component models (with a soft, steady X-ray component plus a variable SSC component) involving substantial pre-acceleration of electrons to Lorentz factors on the order of 1E+5 describe the data train surprisingly well. All considered models imply an emission region that is strongly out of equipartition and low radiative efficiencies (ratio between kinetic jet luminosity and comoving radiative luminosity) of 1 per-mill and less.Comment: 16 pages, Refereed Manuscript. Minor changes to previous versio

Galactic Models of Gamma-Ray Bursts

We describe observational evidence and theoretical calculations which support the high velocity neutron star model of gamma-ray bursts. We estimate the energetic requirements in this model, and discuss possible energy sources. we also consider radiative processes involved in the bursts.Comment: 16 pages Latex file in revtex format. Fourteen postscript figures come in a separate file. To appear in the Proceedings of the 1995 La Jolla Workshop "High Velocity Neutron Stars and Gamma-Ray Bursts", eds. R. Rorschild etal., AIP, New Yor

Higher Order Variability Properties of Accreting Black Holes

To better constrain the emission mechanism underlying the hard state of galactic black hole candidates, we use high-time resolution RXTE lightcurves for Cyg X-1 and GX 399-4 to compute two higher order variability statistics for these objects, the skewness and the Fourier bispectrum. Similar analyses, in particular using the skewness measure, have been attempted before, but the photon collection area of RXTE allows us to present results of much greater statistical significance. The results for the two objects are qualitatively similar, reinforcing the idea that the same basic mechanisms are at work in both. We find a significantly positive skewness for variability timescales less than $\sim 1$ second, and a {\it negative} skewness for timescale $1- 5$ sec. Such a skewness pattern cannot be reproduced by the simplest shot variability models where individual shots have a fixed profile and intensity and are uncorrelated in time. Further evidence against simple shot models comes from the significant detection of a non-zero bicoherence for Fourier periods $\sim 0.1-10$ sec, implying that significant coupling does exist between variations on these timescales. We discuss how current popular models for variability in black hole systems can be modified to match these observations. Using simulated light curves, we suggest that the most likely way to reproduce this observed behavior is to have the variability come in groups of many shots, with the number of shots per unit time fitting an envelope function which has a rapid rise and slow decay, while the individual shots have a slow rise and a rapid decay. Invoking a finite energy reservoir that is depleted by each shot is a natural way of producing the required shot correlations.Comment: Accepted to MNRAS, 10 pages, 7 figures, substantially improved discussion of errors, minor changes to interpretation sectio

Klein-Nishina Effects in the Spectra of Non-Thermal Sources Immersed in External Radiation Fields

We study Klein-Nishina (KN) effects in the spectrum produced by a steady state, non-thermal source where rapidly accelerated electrons cool by emitting synchrotron radiation and Compton upscattering ambient photons produced outside the source. We focus on the case where the radiation density inside the source exceeds that of the magnetic field. We show that the KN reduction in the electron Compton cooling rate causes the steady-state electron spectrum to harden at energies above \gamma_{KN}, where \gamma_{KN}= 1/4\epsilon_0 and \epsilon_0=h\nu_0/m_ec^2 is the characteristic ambient photon energy. The source synchrotron spectrum thus shows a high-energy ``bump'' or excess even though the electron acceleration spectrum has no such excess. In contrast, the low-energy Compton gamma-ray spectrum shows little distortion because the electron hardening compensates for the KN decline in the scattering rate. For sufficiently high electron energies, however, Compton cooling becomes so inefficient that synchrotron cooling dominates -- an effect omitted in most previous studies. The hardening of the electron distribution thus stops, leading to a rapid decline in Compton gamma-ray emission, i.e., a strong spectral break whose location does not depend on the maximum electron energy. This break can limit the importance of Compton gamma-ray pair production on ambient photons and implies that a source's synchrotron luminosity may exceed its Compton luminosity even though the source magnetic field energy density is smaller than the ambient radiation energy density. We discuss the importance of these KN effects in blazars, micro-quasars, and pulsar binaries.Comment: 36 pages, 10 figures. MNRAS LaTeX. Abtract slightly shortened. Submitted to Monthly Notice

Thoracoscopic oesophageal atresia/tracheo-oesophageal fistula (OA/TOF) repair is associated with a higher stricture rate: a single institution’s experience

Purpose: Thoracoscopic OA/TOF repair was first described in 1999. Currently, less than 10% of surgeons routinely employ minimally access surgery. Our primary aim was to review our immediate-, early- and long-term outcomes with this technique compared with the open approach. Methods: A retrospective review of all patients undergoing primary OA/TOF (Type C) repair at our institution from 2009 was conducted. Outcome measures included length of surgery, conversion rate from thoracoscopy, early complications such as anastomotic leak and post-operative complications such as anastomotic strictures needing dilatations. Fisher’s exact and Kruskal–Wallis tests were used for statistical analysis. Results: 95 patients in total underwent OA/TOF repair during the study period of which 61 (64%) were completed via an open approach. 34 were attempted thoracoscopically of which 11 (33%) were converted. There was only one clinically significant anastomotic leak in our series that took place in the thoracoscopic group. We identified a significantly higher stricture rate in our thoracoscopic cohort (72%) versus open surgery (43%, P < 0.05). However, the median number of dilations (3) performed was not significantly different between the groups. There was one recurrent fistula in the thoracoscopic converted to open group. Our median follow-up was 60 months across the groups. Conclusion: In our experience, the clinically significant leak rate for both open and thoracoscopic repair as well as recurrent fistula is much lower than has been reported in the literature. We do not routinely perform contrast studies and are, thus, reporting clinically significant leaks only. The use of post-operative neck flexion, ventilation and paralysis is likely to be protective towards a leak. Thoracoscopic OA/TOF repair is associated with a higher stricture rate compared with open surgery; however, these strictures respond to a similar number of dilatations and are no more refractory. Larger, multicentre studies may be useful to investigate these finding further