Evidence for Centrifugal Barrier in X-ray Pulsar GRO J1744-28

We present further observational evidence of the effects of a centrifugal barrier in GRO J1744-28, based on continued monitoring of the source with RXTE. For X-ray pulsars, the centrifugal barrier manifests itself in the cessation of pulsed emission when the source becomes faint. We show that such phenomenon occurred repeatedly for GRO J1744-28, following the decay of two X-ray outbursts. This has allowed a direct measurement of the dipole field strength for this pulsar. Here we argue that some of the other peculiar properties observed of this source may also be related to an active centrifugal barrier.Comment: 4 pages. To appear in "Accretion Processes in Astrophysical Systems", Proc. of the 8th Annual Astrophysics Conference in Maryland, S. S. Holt & T. Kallman (eds.

News from a Multi-Wavelength Monitoring Campaign on Mrk 421

We conducted a daily monitoring campaign on Mrk 421 in 2003 and 2004 with the Whipple 10 m telescope and the large-area instruments aboard RXTE, simultaneously covering TeV and X-ray energies. Supporting observations at optical and radio wavelengths were also frequently carried out. Mrk 421 was observed over a wide range of fluxes (with a dynamic range of ~30 both at TeV and X-ray energies). The source was relatively quiet in 2003 but became unusually active in 2004, with flares reaching peak fluxes of ~80 mCrab in X-rays and >3 Crab at TeV energies! We will describe the multiwavelength campaign and present some preliminary results. We will also discuss the implications of the results on the proposed emission models for TeV blazars.Comment: Proc. "International Symposium on High Energy Gamma-Ray Astronomy" (Gamma-2004, Heidelberg, Germany), eds. F.A. Aharonian and H. Voelk, AIP Conf. Ser. This preprint version contains 7 pages and 8 figure

X-ray Flaring Activity of Mrk 421

We report results from a systematic search for X-ray flares from Mrk 421, using archival data from the Rossi X-ray Timing Explorer (RXTE) satellite. The flares are clearly seen over a wide range of timescales. The quasi-continuous coverage of the source with the All-Sky Monitor (ASM) reveals frequent occurrence of major flares that last for months. On a few occasions, the source was intensively monitored with the more sensitive pointing instruments aboard RXTE. The data from these observations shows the presence of X-ray flares of much shorter durations, ranging from weeks down to less than an hour. For the first time, we clearly resolved the sub-hour flares from Mrk 421. Moreover, Fourier analyses reveal variability on even shorter timescales, up to about 0.01 Hz. The source appears to behave differently in its spectral properties during different flares, large or small, which is intriguing. While significant hysteresis is observed to be associated with spectral evolution in some cases, little is seen in other cases. Sometimes, the shape of the X-ray spectrum hardly varies across a flare. Therefore, the phenomenology is complex. The observed hierarchical structure of the X-ray flares seems to imply the scale-invariant nature of the phenomenon, perhaps similar to solar flares or rapid X-ray flares observed of stellar-mass black holes in this regard. Combined with other results, the observed flaring timescales seriously constrain the physical properties of X-ray emitting regions in the jets of Mrk 421.Comment: 24 pages, 10 figures, to appear in Ap

On the Disappearance of Kilohertz Quasi-Periodic Oscillations at a High Mass Accretion Rate in Low-Mass X-ray Binaries

For all sources in which the phenomenon of kilo-Hertz quasi-periodic oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the inferred mass accretion rate exceeds a certain threshold. Although the threshold cannot at present be accurately determined (or even quantified) observationally, it is clearly higher for bright Z sources than for faint atoll sources. Here we propose that the observational manifestation of kHz QPOs {\em requires} direct interaction between the neutron star magnetosphere and the Keplerian accretion disk and that the cessation of kHz QPOs at high accretion rate is due to the lack of such an interact when the Keplerian disk terminates at the last stable orbit and yet the magnetosphere is pushed farther inward. The threshold is therefore dependent of the magnetic field strength -- the stronger the magnetic field the higher the threshold. This is certainly in agreement with the atoll/Z paradigm, but we argue that it is also generally true, even for individual sources within each (atoll or Z) category. For atoll sources, the kHz QPOs also seem to vanish at low accretion rate. Perhaps the ``disengagement'' between the magnetosphere and the Keplerian disk also takes place under such circumstances, because of, for instance, the presence of quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron star. Unfortunately, in this case, the estimation of the accretion rate threshold would require a knowledge of the physical mechanisms that cause the disengagement. If the ADAF is responsible, the threshold is likely dependent of the magnetic field of the neutron star.Comment: Minor revisions to match the published versio

Absolutely Maximally Entangled States: Existence and Applications

We investigate absolutely maximally entangled (AME) states, which are multipartite quantum states that are maximally entangled with respect to any possible bipartition. These strong entanglement properties make them a powerful resource for a variety of quantum information protocols. In this paper, we show the existence of AME states for any number of parties, given that the dimension of the involved systems is chosen appropriately. We prove the equivalence of AME states shared between an even number of parties and pure state threshold quantum secret sharing (QSS) schemes, and prove necessary and sufficient entanglement properties for a wider class of ramp QSS schemes. We further show how AME states can be used as a valuable resource for open-destination teleportation protocols and to what extend entanglement swapping generalizes to AME states