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Extremely variable quasars can also show strong changes in broad-line emission strength and are known as changing-look quasars (CLQs). To study the CLQ transition mechanism, we present a pilot sample of CLQs with X-ray observations in both the bright and faint states. From a sample of quasars with bright-state archival SDSS spectra and (Chandra or XMM-Newton) X-ray data, we identified five new CLQs via optical spectroscopic follow-up and then obtained new target-of-opportunity X-ray observations with Chandra. No strong absorption is detected in either the bright- or the faint-state X-ray spectra. The intrinsic X-ray flux generally changes along with the optical variability, and the X-ray power-law slope becomes harder in the faint state. Large-amplitude mid-infrared variability is detected in all five CLQs, and it echoes the variability in the optical with a time lag expected from the light-crossing time of the dusty torus for CLQs with robust lag measurements. The changing-obscuration model is not consistent with the observed X-ray spectra and spectral energy distribution changes seen in these CLQs. It is highly likely that the observed changes are due to the changing accretion rate of the supermassive black hole, so the multiwavelength emission varies accordingly, with promising analogies to the accretion states of X-ray binaries.
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Dual-angle light scattering spectroscopy for calibration-free measurements of scatterer suspensions
It can be difficult to employ optical techniques for analyzing biological structures smaller than or comparable to the wavelength of light, such as extracellular vesicles or some types of bacteria. Biological light scattering spectroscopy (LSS), developed to address this problem, has been successfully used for characterizing tissue on cellular and subcellular scales. At the same time, calibration with a reference sample of known optical properties can complicate LSS measurements. In this Letter, we present dual-angle LSS (daLSS), which is designed for calibration-free measurements of scatterer suspensions. It employs measurements of a sample at two distinct angles, which then allows system effects to be removed entirely. Not only does daLSS simplify and speed up the measurement procedure, but it also makes spectra more reproducible, an important feature for diagnostic techniques. We validated the technique by accurately reconstructing the sizes of polystyrene microspheres with diameters less than 100 nm and then demonstrated that not only are the daLSS spectra of several common bacteria strains easily distinguishable but they are also highly reproducible.
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- PAR ID:
- 10560408
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Letters
- Volume:
- 50
- Issue:
- 1
- ISSN:
- 0146-9592; OPLEDP
- Format(s):
- Medium: X Size: Article No. 33
- Size(s):
- Article No. 33
- Sponsoring Org:
- National Science Foundation
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