Dynamic stability study for sounding rockets Final report

Joint rotation and compliance, body and fin flexibility, and aerodynamic characteristics effect on roll resonance of sounding rocket

Clustering and networking among small independent hotels: developments over ten years

This paper reports on networking activities amongst a cluster of independent hotels with a view to analysing key variables in network development for SMEs. A comparative, longitudinal element is introduced by reference to an earlier study of the same phenomenon, ten years earlier. Of particular interest is the influence of various forms of social capital in the development of informal networks and the inter-play between co-operation and competition over time. Given the time gap between the two studies, the effect of sectoral changes on network development is also examined. The research revisits and extends a previous study of many of the same hotels which were investigated in 1995 over their formal and informal links that were found to be influenced by such factors across two classifications - the business as unit (proximity, perceptions of quality) and the individual respondent (personal social networks and ethnicity). Extending that analysis, social capital concepts and relevant references to embedded networks and kinship groups and co-operative game rules will be introduced in this paper. A highly concentrated population of small, independent hotels in Central London had been identified in the previous research project and it is from this sample that the current research drew and extended its own sample of hotels. With such a high number of hotels in the area a mixture of ‘snowball’ sampling and self-selection was successfully employed. Of the original 29 hotels, 22 have been re-interviewed and complemented by another 19. Of the remainder of the original sample, several had subsequently merged or failed to survive the intervening period. Face-to-face interviews were conducted with owner-managers in all cases using a structured questionnaire that replicated as much as possible of the original questionnaire, with both open and closed questions to allow some individual expression on relevant topics. The quantitative data obtained will be analysed using UCINET software to generate visual representations of networks alongside statistical and cluster analyses. Both academic and policy implications are likely to arise from this research, such as novel insights from such an unique periodic comparison of networks development, the influence of social capital on (formal and informal) network activities and the changing influence and consolidation of hotel groups through mergers and franchising

Magnetars' Giant Flares: the case of SGR 1806-20

We first review on the peculiar characteristics of the bursting and flaring activity of the Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars. We then report on the properties of the SGR 1806-20's Giant Flare occurred on 2004 December 27th, with particular interest on the pre and post flare intensity/hardness correlated variability. We show that these findings are consistent with the picture of a twisted internal magnetic field which stresses the star solid crust that finally cracks causing the giant flare (and the observed torsional oscillations). This crustal fracturing is accompanied by a simplification of the external magnetic field with a (partial) untwisting of the magnetosphere.Comment: 6 pages, 2 figures; accepted for publication in the Chinese Journal for Astronomy and Astrophysics (Vulcano conference - 2005

The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-ray Properties of SGR 1806-20 from 1993 to 2005

On 2004 December 27, a highly-energetic giant flare was recorded from the magnetar candidate SGR 1806-20. In the months preceding this flare, the persistent X-ray emission from this object began to undergo significant changes. Here, we report on the evolution of key spectral and temporal parameters prior to and following this giant flare. Using the Rossi X-ray Timing Explorer, we track the pulse frequency of SGR 1806-20 and find that the spin-down rate of this SGR varied erratically in the months before and after the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for a discrete jump in spin frequency at the time of the December 27th flare (|dnu/nu| < 5 X 10^-6). In the months surrounding the flare, we find a strong correlation between pulsed flux and torque consistent with the model for magnetar magnetosphere electrodynamics proposed by Thompson, Lyutikov & Kulkarni (2002). As with the flare in SGR 1900+14, the pulse morphology of SGR 1806-20 changes drastically following the flare. Using the Chandra X-ray Observatory and other publicly available imaging X-ray detector observations, we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual magnetar persistent emission spectral model of a power-law plus a blackbody provides an excellent fit to the data. We confirm the earlier finding by Mereghetti et al. (2005) of increasing spectral hardness of SGR 1806-20 between 1993 and 2004. Contrary to the direct correlation between torque and spectral hardness proposed by Mereghetti et al., we find evidence for a sudden torque change that triggered a gradual hardening of the energy spectrum on a timescale of years. Interestingly, the spectral hardness, spin-down rate, pulsed, and phase-averaged of SGR 1806-20 all peak months before the flare epoch.Comment: 37 pages, 8 figures, 8 tables. Accepted for publication in ApJ. To appear in the Oct 20 2006 editio

Five years of SGR 1900+14 observations with BeppoSAX

We present a systematic analysis of all the BeppoSAX data of the soft gamma-ray repeater SGR 1900+14: these observations allowed us to study the long term properties of the source quiescent emission. In the observation carried out before the 1998 giant flare the spectrum in the 0.8-10 keV energy range was harder and there was evidence for a 20-150 keV emission, possibly associated with SGR 1900+14. This possible hard tail, if compared with the recent INTEGRAL detection of SGR 1900+14, has a harder spectrum (power-law photon index ~1.6 versus ~3) and a 20-100 keV flux ~4 times larger. In the last BeppoSAX observation (April 2002), while the source was entering the long quiescent period that lasted until 2006, the 2-10 keV flux was ~25% below the historical level. We also studied in detail the spectral evolution during the 2001 flare afterglow. This was characterized by a softening that can be interpreted in terms of a cooling blackbody-like component.Comment: Accepted for publication on Astronomy and Astrophysics on August 31, 200

Chandra and RXTE Observations of 1E 1547.0-5408: Comparing the 2008 and 2009 Outbursts

We present results from observations of the magnetar 1E 1547.0-5408 (SGR J1550-5418) taken with the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer (RXTE) following the source's outbursts in 2008 October and 2009 January. During the time span of the Chandra observations, which covers days 4 through 23 and days 2 through 16 after the 2008 and 2009 events, respectively, the source spectral shape remained stable, while the pulsar's spin-down rate in the same span in 2008 increased by a factor of 2.2 as measured by RXTE. The lack of spectral variation suggests decoupling between magnetar spin-down and radiative changes, hence between the spin-down-inferred magnetic field strength and that inferred spectrally. We also found a strong anti-correlation between the phase-averaged flux and the pulsed fraction in the 2008 and 2009 Chandra data, but not in the pre-2008 measurements. We discuss these results in the context of the magnetar model.Comment: 4 figures, accepted for publication in Ap

Normal ground state of dense relativistic matter in a magnetic field

The properties of the ground state of relativistic matter in a magnetic field are examined within the framework of a Nambu-Jona-Lasinio model. The main emphasis of this study is the normal ground state, which is realized at sufficiently high temperatures and/or sufficiently large chemical potentials. In contrast to the vacuum state, which is characterized by the magnetic catalysis of chiral symmetry breaking, the normal state is accompanied by the dynamical generation of the chiral shift parameter $\Delta$. In the chiral limit, the value of $\Delta$ determines a relative shift of the longitudinal momenta (along the direction of the magnetic field) in the dispersion relations of opposite chirality fermions. We argue that the chirality remains a good approximate quantum number even for massive fermions in the vicinity of the Fermi surface and, therefore, the chiral shift is expected to play an important role in many types of cold dense relativistic matter, relevant for applications in compact stars. The qualitative implications of the revealed structure of the normal ground state on the physics of protoneutron stars are discussed. A noticeable feature of the $\Delta$ parameter is that it is insensitive to temperature when $T \ll \mu_0$, where $\mu_0$ is the chemical potential, and {\it increases} with temperature for $T > \mu_0$. The latter implies that the chiral shift parameter is also generated in the regime relevant for heavy ion collisions.Comment: 28 pages, 6 figures; v2: title changed in journa

Exciton lifetime in InAs/GaAs quantum dot molecules

The exciton lifetimes $T_1$ in arrays of InAs/GaAs vertically coupled quantum dot pairs have been measured by time-resolved photoluminescence. A considerable reduction of $T_1$ by up to a factor of $\sim$ 2 has been observed as compared to a quantum dots reference, reflecting the inter-dot coherence. Increase of the molecular coupling strength leads to a systematic decrease of $T_1$ with decreasing barrier width, as for wide barriers a fraction of structures shows reduced coupling while for narrow barriers all molecules appear to be well coupled. The coherent excitons in the molecules gain the oscillator strength of the excitons in the two separate quantum dots halving the exciton lifetime. This superradiance effect contributes to the previously observed increase of the homogeneous exciton linewidth, but is weaker than the reduction of $T_2$. This shows that as compared to the quantum dots reference pure dephasing becomes increasingly important for the molecules

Magnetars as cooling neutron stars with internal heating

We study thermal structure and evolution of magnetars as cooling neutron stars with a phenomenological heat source in a spherical internal layer. We explore the location of this layer as well as the heating rate that could explain high observable thermal luminosities of magnetars and would be consistent with the energy budget of neutron stars. We conclude that the heat source should be located in an outer magnetar's crust, at densities rho < 5e11 g/cm^3, and should have the heat intensity of the order of 1e20 erg/s/cm^3. Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the surface.Comment: 8 pages, 5 figures, submitted to MNRA

Stacking Gravitational Wave Signals from Soft Gamma Repeater Bursts

Soft gamma repeaters (SGRs) have unique properties that make them intriguing targets for gravitational wave (GW) searches. They are nearby, their burst emission mechanism may involve neutron star crust fractures and excitation of quasi-normal modes, and they burst repeatedly and sometimes spectacularly. A recent LIGO search for transient GW from these sources placed upper limits on a set of almost 200 individual SGR bursts. These limits were within the theoretically predicted range of some models. We present a new search strategy which builds upon the method used there by "stacking" potential GW signals from multiple SGR bursts. We assume that variation in the time difference between burst electromagnetic emission and burst GW emission is small relative to the GW signal duration, and we time-align GW excess power time-frequency tilings containing individual burst triggers to their corresponding electromagnetic emissions. Using Monte Carlo simulations, we confirm that gains in GW energy sensitivity of N^{1/2} are possible, where N is the number of stacked SGR bursts. Estimated sensitivities for a mock search for gravitational waves from the 2006 March 29 storm from SGR 1900+14 are also presented, for two GW emission models, "fluence-weighted" and "flat" (unweighted).Comment: 17 pages, 16 figures, submitted to PR