654 research outputs found
NEOWISE observations of comet C/2013 A1 (Siding Spring) as it approaches Mars
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission
observed comet C/2013 A1 (Siding Spring) three times at 3.4 {\mu}m and 4.6
{\mu}m as the comet approached Mars in 2014. The comet is an extremely
interesting target since its close approach to Mars in late 2014 will be
observed by various spacecraft in-situ. The observations were taken in 2014
Jan., Jul. and Sep. when the comet was at heliocentric distances of 3.82 AU,
1.88 AU, and 1.48 AU. The level of activity increased significantly between the
Jan. and Jul. visits but then decreased by the time of the observations in
Sep., approximately 4 weeks prior to its close approach to Mars. In this work
we calculate Af\r{ho} values, and CO/CO2 production rates.Comment: 9 pages, 3 figures, accepted by Astrophysical Journal Letter
On the relation between the seismic activity and the Hurst exponent of the geomagnetic field at the time of the 2000 Izu swarm
Many papers document the observation of earthquake-related precursory signatures in geomagnetic field data. However, the significance of these findings is ambiguous
because the authors did not adequately take into account that these signals could have been generated by other sources, and the seismogenic origin of these signals have not been validated by comparison with independent
datasets. Thus, they are not reliable examples of magnetic disturbances induced by the seismic activity. Hayakawa et al.(2004) claim that at the time of the 2000 Izu swarm the Hurst exponent of the Ultra- Low-Frequency (ULF: 0.001â10 Hz) band of the geomagnetic field varied in accord with the energy released by the seismicity. The present paper demonstrates that the behaviour of the Hurst exponent was insufficiently investigated and also misinterpreted by the authors.
We clearly show that during the Izu swarm the changes of the Hurst exponent were strongly related to the level of global
geomagnetic activity and not to the increase of the local seismic activity
Comment on âUltra low frequency (ULF) electromagnetic anomalies associated with large earthquakes in Java Island, Indonesia by using wavelet transform and detrended fluctuation analysisâ by Febriani et al. (2014)
We examine the recent report of Febriani et al. (2014) in which the authors show changes in ULF magnetic field data prior to the M7.5 Tasikmalaya earthquake that occurred south of Java, Indonesia, on 2 September 2009. Febriani et al. (2014) state that the magnetic changes they found may be related to the impending earthquake. We do not agree that the pre-earthquake magnetic changes shown in Febriani et al. (2014) are seismogenic. These magnetic changes, indeed, are too closely related to global geomagnetic disturbances to be regarded as being of seismic origin
Breaking the Habit: The Peculiar 2016 Eruption of the Unique Recurrent Nova M31N 2008-12a
Since its discovery in 2008, the Andromeda galaxy nova M31N 2008-12a has been observed in eruption every single year. This unprecedented frequency indicates an extreme object, with a massive white dwarf and a high accretion rate, which is the most promising candidate for the single-degenerate progenitor of a Type Ia supernova known to date. The previous three eruptions of M31N 2008-12a have displayed remarkably homogeneous multiwavelength properties: (i) from a faint peak, the optical light curve declined rapidly by two magnitudes in less than two days, (ii) early spectra showed initial high velocities that slowed down significantly within days and displayed clear He/N lines throughout, and (iii) the supersoft X-ray source (SSS) phase of the nova began extremely early, six days after eruption, and only lasted for about two weeks. In contrast, the peculiar 2016 eruption was clearly different. Here we report (i) the considerable delay in the 2016 eruption date, (ii) the significantly shorter SSS phase, and (iii) the brighter optical peak magnitude (with a hitherto unobserved cusp shape). Early theoretical models suggest that these three different effects can be consistently understood as caused by a lower quiescence mass accretion rate. The corresponding higher ignition mass caused a brighter peak in the freeâfree emission model. The less massive accretion disk experienced greater disruption, consequently delaying the re-establishment of effective accretion. Without the early refueling, the SSS phase was shortened. Observing the next few eruptions will determine whether the properties of the 2016 outburst make it a genuine outlier in the evolution of M31N 2008-12a
On a report that the 2012 M6:0 earthquake in Italy was predicted after seeing an unusual cloud formation
Several recently published reports have suggested
that semi-stationary linear-cloud formations might be
causally precursory to earthquakes.We examine the report of
Guangmeng and Jie (2013), who claim to have predicted the
2012 M6:0 earthquake in the Po Valley of northern Italy after
seeing a satellite photograph (a digital image) showing a
linear-cloud formation over the eastern Apennine Mountains
of central Italy. From inspection of 4 years of satellite images
we find numerous examples of linear-cloud formations over
Italy. A simple test shows no obvious statistical relationship
between the occurrence of these cloud formations and earthquakes
that occurred in and around Italy. All of the linearcloud
formations we have identified in satellite images, including
that which Guangmeng and Jie (2013) claim to have
used to predict the 2012 earthquake, appear to be orographic
â formed by the interaction of moisture-laden wind flowing
over mountains. Guangmeng and Jie (2013) have not clearly
stated how linear-cloud formations can be used to predict the
size, location, and time of an earthquake, and they have not
published an account of all of their predictions (including
any unsuccessful predictions). We are skeptical of the validity
of the claim by Guangmeng and Jie (2013) that they have
managed to predict any earthquakes
Variability Flagging in the Wide-field Infrared Survey Explorer Preliminary Data Release
The Wide-field Infrared Survey Explorer Preliminary Data Release Source Catalog contains over 257 million objects. We describe the method used to flag variable source candidates in the Catalog. Using a method based on the chi-square of single-exposure flux measurements, we generated a variability flag for each object, and have identified almost 460,000 candidate sources that exhibit significant flux variability with greater than ~7Ï confidence. We discuss the flagging method in detail and describe its benefits and limitations. We also present results from the flagging method, including example light curves of several types of variable sources including Algol-type eclipsing binaries, RR Lyr, W UMa, and a blazar candidate
Unravelling Active Galactic Nuclei
A complete flat-spectrum radio-loud sample of AGN includes a significant
fraction of Seyfert-like AGN including a NLS1. Analysis of their optical
spectra suggests that the reddest continuum colours are either associated with
AGN in nearby resolved galaxies, or distant quasars showing relatively narrow
permitted emission lines.Comment: Poster contribution presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
- âŠ