Chandra Observations of the Gravitationally Lensed System 2016+112

An observation of the gravitationally lensed system 2016+112 with the Chandra X-ray Observatory has resolved a mystery regarding the proposed presence of a dark matter object in the lens plane of this system. The Chandra ACIS observation has clearly detected the lensed images of 2016+112 with positions in good agreement with those reported in the optical and also detects 13 additional X-ray sources within a radius of 3.5 arcmin. Previous X-ray observations in the direction of 2016+112 with the ROSAT HRI and ASCA SIS have interpreted the X-ray data as arising from extended emission from a dark cluster. However, the present Chandra observation can account for all the X-ray emission as originating from the lensed images and additional point X-ray sources in the field. Thus cluster parameters based on previous X-ray observations are unreliable. We estimate an upper limit on the mass-to-light ratio within a radius of 800 h_(50)^(-1) kpc of M/L_(V) < 190 h_(50) (M/L_(V))_Sun. The lensed object is quite unusual, with reported narrow emission lines in the optical that suggest it may be a type-2 quasar (Yamada et. al. 1999). Our modeling of the X-ray spectrum of the lensed object implies that the column density of an intrinsic absorber must lie between 3 and 85 x 10^22 cm^-2 (3 sigma confidence level). The 2-10 keV luminosity of the lensed object, corrected for the lens magnification effect and using the above range of intrinsic absorption, is 3 x 10^43 - 1.4 x 10^44 erg/s.Comment: 9 pages, includes 2 figures, Accepted for publication in ApJ

Elastic energy of proteins and the stages of protein folding

We propose a universal elastic energy for proteins, which depends only on the radius of gyration $R_{g}$ and the residue number $N$. It is constructed using physical arguments based on the hydrophobic effect and hydrogen bonding. Adjustable parameters are fitted to data from the computer simulation of the folding of a set of proteins using the CSAW (conditioned self-avoiding walk) model. The elastic energy gives rise to scaling relations of the form $R_{g}\sim N^{\nu}$ in different regions. It shows three folding stages characterized by the progression with exponents $\nu = 3/5, 3/7, 2/5$, which we identify as the unfolded stage, pre-globule, and molten globule, respectively. The pre-globule goes over to the molten globule via a break in behavior akin to a first-order phase transition, which is initiated by a sudden acceleration of hydrogen bonding