Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum-dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory

Double-Donor Energy Structure in Concentric Quantum Rings under Magnetic Field and Hydrostatic Pressure

A model for an artificial hydrogen molecule consisting of two positive on-axis Coulombic centers and two electrons coupled to them inside a double concentric quantum ring is considered. Such a nanostructure is assumed to be under the influence of external probes like hydrostatic pressure and magnetic field. By using the adiabatic approximation, the ground state energy is calculated as a function of the outer center line radius and the impurity Coulombic center separation, for different values of the hydrostatic pressure and magnetic field strength. In contrast to the single properties imposed by nature on the actual hydrogen molecule, our model allows us to explore a great variety of properties of the artificial hydrogen molecule by changing the ring dimensions. The artificial hydrogen molecule energy structure may be tuned by changing the external field strengths

Frequency of isolation of various subtypes and antimicrobial resistance of Shigella from urban slums of Karachi, Pakistan

Objectives: Shigellosis remains a major public health problem in developing countries. Antimicrobial resistance has complicated the empirical treatment. Knowledge of serotypes is crucial in vaccine development, as cross-protection between various serotypes is limited. Therefore we conducted a prospective study to determine the frequency of isolation of Shigella serotypes and antimicrobial resistance. Methods: Stool samples from 8155 individuals, collected through a surveillance study conducted in four slums of Karachi from January 2002 to March 2004, were cultured. Results: Shigella was isolated in 394 (4.8%) of 8155 Patients presenting with diarrhea. Two hundred and forty-two (62%) isolates were Shigella flexneri, 72 (18%) were Shigella sonnei, 43 (11%) were Shigella boydii, and 37 (9%) were Shigella dysenteriae. Thirteen S. flexneri serotypes were identified, of which the most frequent were 2a (38), 6 (37), and 1b (25), followed by 2b (23). Only 22 (5.6%) Shigella isolates were found to be pan-susceptible. Large proportions of isolates were resistant to cotrimoxazole (89% S. flexneri, 81% S. dysenteriae, 80% S. sonnei, and 56% S. boydii) and ampicillin (87% S. flexneri, 68% S dysenteriae, 35% S. boydii, and 4% S. sonnei). Conclusion: Concurrent circulation of multiple strains with high resistance is worrying and mandates surveillance at the national level to facilitate the control of shigellosis