Observation of a sudden cessation of a very-high-energy gamma-ray flare in PKS 1510-089 with H.E.S.S. and MAGIC in May 2016

The flat spectrum radio quasar (FSRQ) PKS 1510-089 is known for its complex multiwavelength behavior, and is one of only a few FSRQs detected at very high energy (VHE, E >100 GeV) -rays. VHE -ray observations with H.E.S.S. and MAGIC during late May and early June 2016 resulted in the detection of an unprecedented flare, which reveals for the first time VHE -ray intranight variability in this source. While a common variability timescale of 1.5 hr is found, there is a significant deviation near the end of the flare with a timescale of ∼ 20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, curvature is detected in the VHE -ray spectrum of PKS 1510-089, which is fully explained through absorption by the extragalactic background light. Optical R-band observations with ATOM reveal a counterpart of the -ray flare, even though the detailed flux evolution differs from the VHE lightcurve. Interestingly, a steep flux decrease is observed at the same time as the cessation of the VHE flare. In the high energy (HE, E >100 MeV) -ray band only a moderate flux increase is observed with Fermi-LAT, while the HE -ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the -ray spectrum indicates that the emission region is located outside of the BLR. Radio VLBI observations reveal a fast moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼ 50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this correlation is indeed true, VHE rays have been produced far down the jet where turbulent plasma crosses a standing shock.Accepted manuscrip

Nascent DNA Synthesis in Ultraviolet Light-Irradiated Mouse, Human, and Chinese Hamster Cells

The technique of alkaline sucrose gradient centrifugation was used to study newly synthesized DNA in control and ultraviolet light-irradiated mouse L, human HeLa, and Chinese hamster ovary cells. Nascent DNA molecular weight distributions did not appear to differ among the three cell lines for unirradiated cells. However, at short times after ultraviolet light irradiation, human HeLa cells appeared to synthesize more low molecular weight DNA than either mouse L or Chinese hamster ovary cells. Since this difference was not related to differences in either the rate of DNA synthesis or amount of ultraviolet damage in the irradiated cells it appeared to be a phenotypic characteristic of the cell lines tested. A parallel was noted for these three cell lines between an increase in the synthesis of low molecular weight DNA, detected on alkaline sucrose gradients, and cell killing as measured by the ability of irradiated cells to form colonies