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Register a new userSouthern African Large Telescope Spectroscopy of BL Lacs for the CTA project
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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.
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We present the first release of a large-scale study of relatively bright (V < 13.5) metal-poor stars observed with the Southern African Large Telescope (SALT), based on high-resolution spectra of 50 stars with a resolving power of R ~ 40,000 and S/N ~ 20 per pixel at 4300 Angstrom. The elemental abundances of C, Sr, Ba, and Eu are reported, as well as several alpha-elements (Mg, Ca, Sc, Ti, V) and iron-peak elements (Mn, Co, Ni, Zn). We find a diverse array of abundance patterns, including several consistent with the signatures of carbon-enhanced metal-poor CEMP-i and CEMP-r stars. We find that 15 of 50 (30%) are carbon enhanced (with [C/Fe] >+ 0.70), and that a large fraction (26 of 50, 52%) are enhanced in r-process elements, among the r-process-enhanced stars, five are strongly enhanced r-II ([Eu/Fe] >+ 1.0) stars (two of which are newly discovered) and 21 are newly discovered moderately enhanced r-I (+0.3 <= [Eu/Fe] <=+ 1..0) stars. There are eight stars in our sample that, on the basis of their abundances and kinematics, are possible members of the metal-weak thick-disk population. We also compare our measured abundances to progenitor-enrichment models, and find that the abundance patterns for the majority of our stars can be attributed to a single (rather than multiple) enrichment event.
Seventeen southern sky BL Lacs were observed in UBVRI using the CCD Camera on the 1.0m telescope at the South African Astronomical Observatory (SAAO) in Aug and Nov 1999. The analyses of all the seventeen sources are now complete, and are available via anonymous ftp (ftp pukrs1.puk.ac.za/pub/Blazars). A few examples of our results are however given in this paper. Whereas PKS 2005-489 and PKS 2155-304 apear to have been in a high state, PKS 0048-097 and PKS 0521-365 showed evidence of variability on a time-scale of a few days, with the amplitude of variability increasing towards short wavelengths. This is consistent with observations of gamma-ray BL Lacs, which show similar behaviour in optical and X-rays.
Observational and theoretical results indicate that low-redshift BL Lacertae objects are the most likely extragalactic sources to be detectable at TeV energies. In this paper we present the results of observations of 4 BL Lacertae objects (PKS0521-365, EXO0423.4-0840, PKS2005-489 and PKS2316-423) made between 1993 and 1996 with the CANGAROO 3.8m imaging Cherenkov telescope. During the period of these observations the gamma-ray energy threshold of the 3.8m telescope was ~2TeV. Searches for steady long-term emission have been made, and, inspired by the TeV flares detected from Mkn421 and Mkn501, a search on a night-by-night timescale has also been performed for each source. Comprehensive Monte Carlo simulations are used to estimate upper limits for both steady and short timescale emission.
We present the results of an optical spectroscopic survey of 46 heavily obscured quasar candidates. Objects are selected using their mid-infrared (mid-IR) colours and magnitudes from the Wide-Field Infrared Survey Explorer (WISE) and their optical magnitudes from the Sloan Digital Sky Survey (SDSS). Candidate Active Galactic Nuclei (AGNs) are selected to have mid-IR colours indicative of quasar activity and lie in a region of mid-IR colour space outside previously published X-ray based selection regions. We obtain optical spectra for our sample using the Robert Stobie Spectrograph on the Southern African Large Telescope. Thirty objects (65%) have identifiable emission lines, allowing for the determination of spectroscopic redshifts. Other than one object at $zsim2.6$, candidates have moderate redshifts ranging from $z=0.1$ to $0.8$ with a median of 0.3. Twenty-one (70%) of our objects with identified redshift (46% of the whole sample) are identified as AGNs through common optical diagnostics. We model the spectral energy distributions of our sample and found that all require a strong AGN component, with an average intrinsic AGN fraction at 8$,mu$m of 0.91. Additionally, the fits require large extinction coefficients with an average $E(B-V)_textrm{AGN} = 17.8$ (average $A(V)_textrm{AGN} = 53.4$). By focusing on the area outside traditional mid-IR photometric cuts, we are able to capture and characterise a population of deeply buried quasars that were previously unattainable through X-ray surveys alone.
We measure a value for the cosmic expansion of $H(z) = 89 pm 23$(stat) $pm$ 44(syst) km s$^{-1}$ Mpc$^{-1}$ at a redshift of $z simeq 0.47$ based on the differential age technique. This technique, also known as cosmic chronometers, uses the age difference between two redshifts for a passively evolving population of galaxies to calculate the expansion rate of the Universe. Our measurement is based on analysis of high quality spectra of Luminous Red Galaxies (LRGs) obtained with the Southern African Large Telescope (SALT) in two narrow redshift ranges of $z simeq 0.40$ and $z simeq 0.55$ as part of an initial pilot study. Ages were estimated by fitting single stellar population models to the observed spectra. This measurement presents one of the best estimates of $H(z)$ via this method at $zsim0.5$ to date.
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