The `Periodic Nulls' of Radio Pulsar J1819+1305

We present a single-pulse study of the four-component pulsar J1819+1305, whose ``null'' pulses bunch at periodic intervals of around 57 times the rotation period. The emission bursts between the null bunches exhibit characteristic modulations at two shorter periodicities of approximately 6.2 and 3 times the rotation period, the former found largely in the two outer components, and the latter only in the first component. Many bursts commence with bright emission in second component, exhibit positive six-period drift across the full profile width, and end with 3-period modulation in the leading component. The 57-period cycle can be modelled geometrically as a sparsely filled subbeam carousel with nulls appearing whenever our line of sight intersects a circulating empty region. This interpretation is compatible with other recent evidence for periodic, carousel-related nulling and appears to support the physics of a polar-gap emission model for ``drifting'' subpulses, but the subtle structure of the emission bursts defies an easy explanation.Comment: 8 pages, 9 figure

The neutron star soft X-ray transient 1H1905+000 in quiescence

In this Paper we report on our analysis of a ~25 ksec. Chandra X-ray observation of the neutron star soft X-ray transient (SXT) 1H1905+000 in quiescence. Furthermore, we discuss our findings of the analysis of optical photometric observations which we obtained using the Magellan telescope and photometric and spectroscopic observations which we obtained using the Very Large Telescope at Paranal. The X-ray counterpart of 1H1905+000 was not detected in our Chandra data, with a 95 per cent confidence limit to the source count rate of 1.2x10^-4 counts s^-1. For different spectral models this yields an upper limit on the luminosity of 1.8x10^31 erg s^-1 (for an upper limit on the distance of 10 kpc.) This luminosity limit makes 1H1905+000 the faintest neutron star SXT in quiescence observed to date. The neutron star luminosity is so low that it is similar to the lowest luminosities derived for black hole SXTs in quiescence. This low luminosity for a neutron star SXT challanges the hypothesis presented in the literature that black hole SXTs in quiescence have lower luminosities than neutron star SXTs as a result of the presence of a black hole event horizon. Furthermore, the limit on the neutron star luminosity obtained less than 20 years after the outburst has ceased, constrains the thermal conductivity of the neutron star crust. Finally, the neutron star core must be so cold that unless the time averaged mass accretion rate is lower than 2x10^-12 M_sun yr^-1, core cooling has to proceed via enhanced neutrino emission processes. We derive a limit on the absolute I-band magnitude of the quiescent counterpart of M_I>7.8 assuming the source is at 10 kpc. This is in line with 1H1905+000 being an ultra-compact X-ray binary, as has been proposed based on the low outburst V-band absolute magnitude.Comment: 10 pages, 5 figures, accepted for publication in MNRA

Temperature Equilibration Behind the Shock Front: an Optical and X-ray Study of RCW 86

We study the electron-proton temperature equilibration behind several shocks of the RCW 86 supernova remnant. To measure the proton temperature, we use published and new optical spectra, all from different locations on the remnant. For each location, we determine the electron temperature from X-ray spectra, and correct for temperature equilibration between the shock front and the location of the X-ray spectrum. We confirm the result of previous studies that the electron and proton temperatures behind shock fronts are consistent with equilibration for slow shocks and deviate for faster shocks. However, we can not confirm the previously reported trend of the electron temperature to proton temperature ratio of 1/v^2.Comment: Accepted for publication by ApJ, 8 pages, 7 figures, 2 table

Oral medicine case book 69: Burkitt lymphoma of the oral cavity

A 25-year-old female was referred to the Haematology Unit at Tygerberg Hospital for further management of a rapidly expanding and large submandibular mass which on fine needle aspiration was suggestive of lymphoma . Five months earlier she had been diagnosed with pulmonary tuberculosis and was confirmed to be HIV positive with a CD4 count of 17. She was placed on anti-retroviral (ARV) and antituberculous therapy (the ARV therapy included efavirenz, emtricitabine and tenofivir). Her CD4 count, at the time of the current consultation, was 204 and the viral load was suppressed. Lumbar puncture was normal. Significant clinical findings were a large right submandibular mass and right cervical and axillary lymphadenopathy. The submandibular mass was removed and submitted for histological examination.DHE

Proper motions of ROSAT discovered isolated neutron stars measured with Chandra: First X-ray measurement of the large proper motion of RX J1308.6+2127/RBS 1223

The unprecedented spatial resolution of the Chandra observatory opens the possibility to detect with relatively high accuracy proper motions at X-ray wavelengths. We have conducted an astrometric study of three of the "Magnificent Seven", the thermally emitting radio quiet isolated neutron stars (INSs) discovered by ROSAT. These three INSs (RX J0420.0-5022, RX J0806.4-4123 and RX J1308.6+2127/RBS 1223) either lack an optical counterpart or have one too faint to be used for astrometric purposes. We obtained ACIS observations 3 to 5 years apart to constrain or measure the displacement of the sources on the X-ray sky using as reference the background of extragalactic or remote galactic X-ray sources. Upper limits of 138 mas/yr and 76 mas/yr on the proper motion of RX J0420.0-5022 and RX J0806.4-4123, respectively, have already been presented in Motch et al. (2007). Here we report the very significant measurement (~ 10 sigma) of the proper motion of the third INS of our program, RX J1308.6+2127/RBS1223. Comparing observations obtained in 2002 and 2007 reveals a displacement of 1.1 arcsec implying a yearly proper motion of 223 mas, the second fastest measured for the ROSAT discovered INSs. The source is rapidly moving away from the galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ~ 740 (d/700pc) km/s might be the largest of the "Magnificent Seven" and among the fastest recorded for neutron stars. RX J1308.6+2127/RBS1223 is thus a young high velocity cooling neutron star. The source may have its origin in the closest part of the Scutum OB2 association about 0.8 Myr ago, an age consistent with that expected from cooling curves, but significantly younger than inferred from pulse timing measurements (1.5 Myr).Comment: 3 pages, 2 figures, proceedings of the conference "40 Years of Pulsars", 12-17 August 2007, Montreal, Canad

SAX J1808.4-3657 in Quiescence: A Keystone for Neutron Star Science

The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object between accreting X-ray binaries and millisecond radio pulsars. We have constrained the thermal radiation from its surface through XMM-Newton X-ray observations, providing strong evidence for neutrino cooling processes from the neutron star core. We have also undertaken simultaneous X-ray and optical (Gemini) observations, shedding light on whether the strong heating of the companion star in quiescence may be due to X-ray irradiation, or to a radio pulsar turning on when accretion stops.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 4 page

The decline of Alestes baremose Boulenger, 1901 and Hydrocynus forskahlii (Cuvier, 1819) stocks in Lake Albert: implications for sustainable management of their fisheries

The fish stocks of Lake Albert face immense exploitation pressure which has led to “fishingdown” of their fisheries, with some larger species having been driven to near-extinction, while others such as Citharinus citharus have almost disappeared. Both A. baremose (Angara) and H. forskahlii (Ngassia) historically formed the most important commercial species in Lake Albert until the early 2000s but recent Catch Assessment Surveys (2007-2013) revealed a sweeping decline in their contribution to the commercial catch from 72.7% in 1971 to less than 6% in 2013. The catch per unit effort also registered a two-fold decline from 45.6 and 36.1 kg/boat/day to 22.6 and 18.1 kg/boat/day for A. baremose and H. forskahlii respective between 1971 and 2007. Over 50% of illegal gillnets, below the legal minimum limit of four inches (101.6 mm) used on Lake Albert target the two species. Gillnet experiments found the three inch (76.2 mm) gill net mesh size suitable for sustained harvest of the two species. The study concludes that optimal utilization of the two species and probably other non target fish species is achievable through species specific management strategies, coupling species specific licensing, and controlling harvest of juvenile individuals, overall fishing effort and fish catch on Lake Albert and protecting the vulnerable fish habitats

Measuring the cosmic ray acceleration efficiency of a supernova remnant

Cosmic rays are the most energetic particles arriving at earth. Although most of them are thought to be accelerated by supernova remnants, the details of the acceleration process and its efficiency are not well determined. Here we show that the pressure induced by cosmic rays exceeds the thermal pressure behind the northeast shock of the supernova remnant RCW 86, where the X-ray emission is dominated by synchrotron radiation from ultra-relativistic electrons. We determined the cosmic-ray content from the thermal Doppler broadening measured with optical spectroscopy, combined with a proper-motion study in X- rays. The measured post-shock proton temperature in combination with the shock velocity does not agree with standard shock heating, implying that >50% of the post-shock pressure is produced by cosmic rays.Comment: Published in Science express, 10 pages, 5 figures and 2 table

An X-ray View of Radio Millisecond Pulsars

In recent years, X-ray observations with Chandra and XMM-Newton have significantly increased our understanding of rotation-powered (radio) millisecond pulsars (MSPs). Deep Chandra studies of several globular clusters have detected X-ray counterparts to a host of MSPs, including 19 in 47 Tuc alone. These surveys have revealed that most MSPs exhibit thermal emission from their heated magnetic polar caps. Realistic models of this thermal X-ray emission have provided important insight into the basic physics of pulsars and neutron stars. In addition, intrabinary shock X-ray radiation observed in ``black-widow'' and peculiar globular cluster ``exchanged'' binary MSPs give interesting insight into MSP winds and relativistic shock. Thus, the X-ray band contains valuable information regarding the basic properties of MSPs that are not accesible by radio timing observations.Comment: 5 pages, 5 figures, To appear in the proceedings of "40 Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007, McGill University, Montreal, Canad

The GBT350 Survey of the Northern Galactic Plane for Radio Pulsars and Transients

Using the Green Bank Telescope (GBT) and Pulsar Spigot at 350MHz, we have surveyed the Northern Galactic Plane for pulsars and radio transients. This survey covers roughly 1000 square degrees of sky within 75 deg < l < 165 deg and |b| < 5.5 deg, a region of the Galactic Plane inaccessible to both the Parkes and Arecibo multibeam surveys. The large gain of the GBT along with the high time and frequency resolution provided by the Spigot make this survey more sensitive by factors of about 4 to slow pulsars and more than 10 to millisecond pulsars (MSPs), compared with previous surveys of this area. In a preliminary, reduced-resolution search of all the survey data, we have discovered 33 new pulsars, almost doubling the number of known pulsars in this part of the Galaxy. While most of these sources were discovered by normal periodicity searches, 5 of these sources were first identified through single, dispersed bursts. We discuss the interesting properties of some of these new sources. Data processing using the data's full-resolution is ongoing, with the goal of uncovering MSPs missed by our first, coarse round of processing.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 3 pages, 2 figure