Extreme mass ratio inspiral rates: dependence on the massive black hole mass

We study the rate at which stars spiral into a massive black hole (MBH) due to the emission of gravitational waves (GWs), as a function of the mass M of the MBH. In the context of our model, it is shown analytically that the rate approximately depends on the MBH mass as M^{-1/4}. Numerical simulations confirm this result, and show that for all MBH masses, the event rate is highest for stellar black holes, followed by white dwarfs, and lowest for neutron stars. The Laser Interferometer Space Antenna (LISA) is expected to see hundreds of these extreme mass ratio inspirals per year. Since the event rate derived here formally diverges as M->0, the model presented here cannot hold for MBHs of masses that are too low, and we discuss what the limitations of the model are.Comment: Accepted to CQG, special LISA issu

Far-field scattering microscopy applied to analysis of slow light, power enhancement, and delay times in uniform Bragg waveguide gratings

A novel method is presented for determining the group index, intensity enhancement and delay times for waveguide gratings, based on (Rayleigh) scattering observations. This far-field scattering microscopy (FScM) method is compared with the phase shift method and a method that uses the transmission spectrum to quantify the slow wave properties. We find a minimum group velocity of 0.04c and a maximum intensity enhancement of ~14.5 for a 1000-period grating and a maximum group delay of ~80 ps for a 2000-period grating. Furthermore, we show that the FScM method can be used for both displaying the intensity distribution of the Bloch resonances and for investigating out of plane losses. Finally, an application is discussed for the slow-wave grating as index sensor able to detect a minimum cladding index change of $10^{-8}$, assuming a transmission detection limit of $10^{-4}$

Warping the young stellar disc in the Galactic Centre

We examine influence of the circum-nuclear disc (CND) upon the orbital evolution of young stars in the Galactic Centre. We show that gravity of the CND causes precession of the orbits which is highly sensitive upon the semi-major axis and inclination. We consider such a differential precession within the context of an ongoing discussion about the origin of the young stars and suggest a possibility that all of them have originated in a thin disc which was partially destroyed due to the influence of the CND during the period of ~6Myr.Comment: proc. conf. "The Universe Under the Microscope - Astrophysics at High Angular Resolution", 21-25 April 2008, Bad Honnef, German

Simulations of Extreme-Mass-Ratio Inspirals Using Pseudospectral Methods

Extreme-mass-ratio inspirals (EMRIs), stellar-mass compact objects (SCOs) inspiralling into a massive black hole, are one of the main sources of gravitational waves expected for the Laser Interferometer Space Antenna (LISA). To extract the EMRI signals from the expected LISA data stream, which will also contain the instrumental noise as well as other signals, we need very accurate theoretical templates of the gravitational waves that they produce. In order to construct those templates we need to account for the gravitational backreaction, that is, how the gravitational field of the SCO affects its own trajectory. In general relativity, the backreaction can be described in terms of a local self-force, and the foundations to compute it have been laid recently. Due to its complexity, some parts of the calculation of the self-force have to be performed numerically. Here, we report on an ongoing effort towards the computation of the self-force based on time-domain multi-grid pseudospectral methods.Comment: 6 pages, 4 figures, JPCS latex style. Submitted to JPCS (special issue for the proceedings of the 7th International LISA Symposium

The binding study advice in medical education: a 2-year experience.

To improve the effectiveness of higher education, Dutch universities implemented the binding study advice at medical faculties. Accordingly, medicine students of Radboud University need to gain ≥ 42 out of 60 European Credit Transfer System (ECTS) credits to obtain a positive binding study advice and to continue their study programme. In case of a negative advice, the student is obliged to terminate the study, and he/she cannot register for the same study programme in the Netherlands within the next three years. The purpose of this manuscript is to evaluate the effect of implementation of the binding study advice on study outcomes. First, the binding study advice did not impact on student performance, as the average ECTS credits were comparable before and after its introduction. Second, study progress improved 8 % with 93 % of the students obtaining access to the second year of the study programme after binding study advice implementation. Third, the binding study advice did not impact propaedeutic graduation rates. These data demonstrate that the implementation of the binding study advice in medical faculties has only a small impact on study outcomes. The high performance levels of medical students compared with peers at other faculties are likely to contribute to these findings and suggest a 'ceiling effect' in the potential improvement of study outcomes at medical faculties

Probing Stellar Dynamics in Galactic Nuclei

Electromagnetic observations over the last 15 years have yielded a growing appreciation for the importance of supermassive black holes (SMBH) to the evolution of galaxies, and for the intricacies of dynamical interactions in our own Galactic center. Here we show that future low-frequency gravitational wave observations, alone or in combination with electromagnetic data, will open up unique windows to these processes. In particular, gravitational wave detections in the 10^{-5}-10^{-1} Hz range will yield SMBH masses and spins to unprecedented precision and will provide clues to the properties of the otherwise undetectable stellar remnants expected to populate the centers of galaxies. Such observations are therefore keys to understanding the interplay between SMBHs and their environments.Comment: 8 pages, Science white paper for the Astro2010 Decadal Surve

Elevation in blood flow and shear rate prevents hyperglycemia-induced endothelial dysfunction in healthy subjects and those with type 2 diabetes

Hyperglycemia, commonly present after a meal, causes transient impairment in endothelial function. We examined whether increases in blood flow (BF) protect against the hyperglycemia- mediated decrease in endothelial function in healthy subjects and patients with type 2 diabetes mellitus (T2DM). Ten healthy subjects and 10 age- and sex-matched T2DM patients underwent simultaneous bilateral assessment of brachial artery endothelial function by means of flow- mediated dilation (FMD), using high-resolution echo-Doppler. FMD was examined before and 60, 120 and 150 minutes after a 75-gr oral glucose challenge. We unilaterally manipulated BF by heating one arm between minute 30 and 60. Oral glucose administration caused a statistically significant, transient increase in blood glucose in both groups (P<0.001). Forearm skin temperature, brachial artery BF and shear rate significantly increased in the heated arm (P<0.001), and to a greater extent compared to the non-heated arm in both groups (interaction- effect, P<0.001). The glucose load caused a transient decrease in FMD% (P<0.05), whilst heating significantly prevented the decline (interaction-effect: P<0.01). Also when correcting for changes in diameter and shear rate, we found that the hyperglycemia-induced decrease in FMD can be prevented by local heating (P<0.05). These effects on FMD were observed in both groups. Our data indicate that non-metabolically driven elevation in BF and shear rate can similarly prevent the hyperglycemia-induced decline in conduit artery endothelial function in healthy volunteers and in patients with type 2 diabetes. Additional research is warranted to confirm that other interventions increasing BF and shear rate equally protect the endothelium when challenged by hyperglycemia