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Register a new userSN 2001em: Not so Fast
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SN 2001em is a peculiar supernova, originally classified as Type Ib/c. About two years after the SN it was detected in the radio, showing a rising radio flux with an optically thin spectral slope, and it also displayed a large X-ray luminosity (~10^{41} erg/s). Thus it was suspected to harbor a decelerating (by then, mildly) relativistic jet pointing away from us. About 3 years after its discovery the optical spectrum of SN 2001em showed a broad H-alpha line, and it was therefore reclassified as Type IIn. Here we constrain its proper motion and expansion velocity by analyzing four epochs of VLBI observations, extending out to 5.4 years after the SN. The supernova is still unresolved 5.4 years after the explosion. For the proper motion we obtain (23,000 +/- 30,000) km/s while our 2-sigma upper limit on the expansion velocity is 6000 km/s. These limits are somewhat tighter than those derived by Bietenholz & Bartel, and confirm their conclusion that late time emission from SN 2001em, a few years after the explosion, is not driven by a relativistic jet. VLA observations of the radio flux density, at 8.46 GHz, show a decay as t^{-1.23 +/- 0.40} starting ~2.7 years after the SN. Collectively, the observations suggest interaction of the SN ejecta with a very dense circumstellar medium, though the implied opacity constraints still present a challenge.
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Many graph problems are locally checkable: a solution is globally feasible if it looks valid in all constant-radius neighborhoods. This idea is formalized in the concept of locally checkable labelings (LCLs), introduced by Naor and Stockmeyer (1995). Recently, Chang et al. (2016) showed that in bounded-degree graphs, every LCL problem belongs to one of the following classes: - Easy: solvable in $O(log^* n)$ rounds with both deterministic and randomized distributed algorithms. - Hard: requires at least $Omega(log n)$ rounds with deterministic and $Omega(log log n)$ rounds with randomized distributed algorithms. Hence for any parameterized LCL problem, when we move from local problems towards global problems, there is some point at which complexity suddenly jumps from easy to hard. For example, for vertex coloring in $d$-regular graphs it is now known that this jump is at precisely $d$ colors: coloring with $d+1$ colors is easy, while coloring with $d$ colors is hard. However, it is currently poorly understood where this jump takes place when one looks at defective colorings. To study this question, we define $k$-partial $c$-coloring as follows: nodes are labeled with numbers between $1$ and $c$, and every node is incident to at least $k$ properly colored edges. It is known that $1$-partial $2$-coloring (a.k.a. weak $2$-coloring) is easy for any $d ge 1$. As our main result, we show that $k$-partial $2$-coloring becomes hard as soon as $k ge 2$, no matter how large a $d$ we have. We also show that this is fundamentally different from $k$-partial $3$-coloring: no matter which $k ge 3$ we choose, the problem is always hard for $d = k$ but it becomes easy when $d gg k$. The same was known previously for partial $c$-coloring with $c ge 4$, but the case of $c < 4$ was open.
The segmental specific heat ratio of the couple hydrogen bond defines not only the phase of Vapor, Liquid, Ice I and XI phase with a quasisolid phase that shows the negative thermal extensibility but uniquely the slope of density of water ice in different phases. Ice floats because H-O contracts less than O:H expands in the QS phase at cooling.
The HEGRA gamma-ray source TeV J2032+4130 is considered the prototypical dark accelerator, since it was the first TeV source detected with no firm counterparts at lower frequencies. The Whipple collaboration observed this source in 2003-5 and the emission hotspot appears displaced about 9 arcminutes to the northeast of the HEGRA position, though given the large positional uncertainties the HEGRA and Whipple positions are consistent. Here we report on Westerbork Synthesis Radio Telescope (WSRT), Very Large Array (VLA), Chandra and INTEGRAL data covering the locations of the Whipple and HEGRA hotspots. We confirm a dual-lobed radio source (also see Marti et al., 2007) coincident with the Whipple hotspot, as well as a weak, partially non-thermal shell-like object, with a location and morphology very similar to the HEGRA source, in our WSRT and mosaicked VLA datasets, respectively. Due to its extended nature, it is likely that the latter structure is a more plausible counterpart of the reported very high energy (VHE) gamma-ray emissions in this region. If so, TeV J2032+4130 may not be a dark accelerator after all. Further observations with the new generation of imaging Cherenkov telescopes are needed to pin down the precise location and morphology of the TeV emission region and thus clear up the confusion over its possible lower frequency counterparts.
Nonlocality plays a fundamental role in quantum information science. Recently, it has been theoretically predicted and experimentally demonstrated that the nonlocality of an entangled pair may be shared among multiple observers using weak measurements with moderate strength. Here we devise an optimal protocol of nonlocality sharing among three observers and show experimentally that nonlocality sharing may be also achieved using weak measurements with near-maximum strength. Our result sheds light on the interplay between nonlocality and quantum measurements and, may find applications in quantum steering, unbounded randomness certification and quantum communication network.
We determine Johnson $B,V$ and Cousins $R,I$ photometric CCD magnitudes for the afterglow of GRB 021211 during the first night after the GRB trigger. The afterglow was very faint and would have been probably missed if no prompt observation had been conducted. A fraction of the so-called ``dark GRBs may thus be just ``optically dim and require very deep imaging to be detected. The early-time optical light curve reported by other observers shows prompt emission with properties similar to that of GRB 990123. Following this, the afterglow emission from $sim 11$ min to $sim 33$ days after the burst is characterized by an overall power-law decay with a slope $1.1pm0.02$ in the $R$ passband. We derive the value of spectral index in the optical to near-IR region to be 0.6$pm$0.2 during 0.13 to 0.8 day after the burst. The flux decay constant and the spectral slope indicate that optical observations within a day after the burst lies between cooling frequency and synchrotron maximum frequency.
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