C. C. Cheung

Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar

The Fermi γ-ray Space Telescope has revolutionized our knowledge of the γ-ray pulsar population, leading to the discovery of almost 100 γ-ray pulsars and dozens of γ-ray millisecond pulsars (MSPs). Although the outer-gap model predicts different sites of emission for the radio and γ-ray pulsars, until now all of the known γ-ray MSPs have been visible in the radio. Here we report the discovery of a radio-quiet γ-ray-emitting MSP candidate by using Fermi, Chandra, Swift, and optical observations. The X-ray and γ-ray properties of the source are consistent with known γ-ray pulsars. We also found a 4.63 hr orbital period in optical and X-ray data. We suggest that the source is a black widow-like MSP with a ~0.1 M ☉ late-type companion star. Based on the profile of the optical and X-ray light curves, the companion star is believed to be heated by the pulsar while the X-ray emissions originate from pulsar magnetosphere and/or from intrabinary shock. No radio detection of the source has been reported yet, and although no γ-ray/radio pulsation has been found we estimate that the spin period of the MSP is ~3-5 ms based on the inferred γ-ray luminosity.

Discovery of millisecond pulsars in radio searches of southern Fermi Large Area Telescope sources

Using the Parkes radio telescope we have carried out deep observations of eleven unassociated gamma-ray sources. Periodicity searches of these data have discovered two millisecond pulsars, PSR J1103-5403 (1FGL J1103.9-5355) and PSR J2241-5236 (1FGL J2241.9-5236), and a long period pulsar, PSR J1604-44 (1FGL J1604.7-4443). In addition we searched for but did not detect any radio pulsations from six gammaray pulsars discovered by the Fermi satellite to a level of - 0.04 mJy (for pulsars with a 10% duty cycle). Timing of the millisecond pulsar PSR J1103-5403 has shown that its position is 9 from the centroid of the gamma-ray source. Since these observations were carried out, independent evidence has shown that 1FGL J1103.9-5355 is associated with the flat spectrum radio source PKS 1101-536. It appears certain that the pulsar is not associated with the gamma-ray source, despite the seemingly low probability of a chance detection of a radio millisecond pulsar. We consider that PSR J1604-44 is a chance discovery of a weak, long period pulsar and is unlikely to be associated with 1FGL J1604.7-4443. PSR J2241-5236 has a spin period of 2.2 ms and orbits a very low mass companion with a 3.5 hour orbital period. The relatively high flux density and low dispersion measure of PSR J2241-5236 makes it an excellent candidate for high precision timing experiments. The gamma-rays of 1FGL J2241.9-5236 have a spectrum that is well modelled by a power law with exponential cutoff, and phasebinning with the radio ephemeris results in a multi-peaked gamma-ray pulse profile. Observations with Chandra have identified a coincident X-ray source within 0.1 of the position of the pulsar obtained by radio timing

Alternatives to colonoscopy for population-wide colorectal cancer screening.

Colorectal cancer is one of the top three cancers in the world in terms of incidence. Colonoscopy, which many regard as the gold standard in diagnosis of colonic polyps and neoplasm, is costly, invasive and labour-intensive, and deemed an unsuitable population-wide index screening tool. Alternative modalities, including guaiac and immunohistochemical faecal occult blood tests, computed tomographic colonography, colon capsule endoscopy, flexible sigmoidoscopy, and double-contrast barium enema are available. The procedures, test characteristics, and their implications are reviewed. Immunohistochemical faecal occult blood testing appears to be the most suitable population-wide screening test for an average-risk population, with flexible sigmoidoscopy as an alternative. More evidence is needed to determine the role of computed tomographic colonography and colon capsule endoscopy in colorectal cancer screening.