779 research outputs found
Discovery of 21cm absorption in a DLA towards TXS 0311+430: The first low spin temperature absorber at z > 1
We report the detection of HI 21 cm absorption from the damped
Lyman- system (DLA) towards TXS 0311+430, with the Green Bank
Telescope. The 21 cm absorption has a velocity spread (between nulls) of km s and an integrated optical depth of km s. We also present new Giant Metrewave Radio
Telescope 602 MHz imaging of the radio continuum. TXS 0311+430 is unresolved at
this frequency, indicating that the covering factor of the DLA is likely to be
high. Combining the integrated optical depth with the DLA HI column density of
\nhi = \cm, yields a spin temperature of K, assuming a covering factor of unity. This is the first case of
a low spin temperature ( DLA and is among the lowest
ever measured in any DLA. Indeed, the measured for this DLA is similar to
values measured in the Milky Way and local disk galaxies. We also determine a
lower limit (Si/H) solar for the DLA metallicity, amongst the
highest abundances measured in DLAs at any redshift. Based on low redshift
correlations, the low , large 21 cm absorption width and high metallicity
all suggest that the DLA is likely to arise in a massive,
luminous disk galaxy.Comment: 5 pages, 3 figures. Accepted for publication in MNRAS (Letters
A Search for C II 158 μm Line Emission in HCM 6A, a Lyα Emitter at z = 6.56
We report a Plateau de Bure Interferometer search for CII 158μm emission from HCM 6A, a lensed Lyα emitter
(LAE) at z = 6.56. Our non-detections of CII 158μm line emission and 1.2 mm radio continuum emission yield
3σ limits of L_CII < 6.4×10^7×(ΔV/100 km s^−1)^(1/2) L_☉ for the CII 158μm line luminosity and S1.2mm < 0.68 mJy
for the 1.2 mm flux density. The local conversion factor between L_CII and the star formation rate (SFR) yields
an SFR<4.7M_☉ yr^−1, ≈2 times lower than that inferred from the ultraviolet (UV) continuum, suggesting that
the local factor may not be applicable in high-z LAEs. The non-detection of 1.2 mm continuum emission yields a
total SFR<28M_☉ yr^−1; any obscured star formation is thus within a factor of two of the visible star formation.
Our best-fit model to the rest-frame UV/optical spectral energy distribution of HCM 6A yields a stellar mass
of 1.3 × 10^9 M_☉ and an SFR of ≈10M_☉ yr^−1, with negligible dust obscuration. We fortuitously detect CO
J=3–2 emission from a z = 0.375 galaxy in the foreground cluster A370, and obtain a CO line luminosity of
L'(CO) > (8.95 ± 0.79) × 10^8 K km s^−1 pc^2 and a molecular gas mass of M(H_2) > (4.12 ± 0.36) × 10^9 M_☉, for a CO-to-H_2 conversion factor of 4.6M_☉ (K km s^−1 pc^2)^−1
Sec-Lib: Protecting Scholarly Digital Libraries From Infected Papers Using Active Machine Learning Framework
Researchers from academia and the corporate-sector rely on scholarly digital libraries to access articles. Attackers take advantage of innocent users who consider the articles' files safe and thus open PDF-files with little concern. In addition, researchers consider scholarly libraries a reliable, trusted, and untainted corpus of papers. For these reasons, scholarly digital libraries are an attractive-target and inadvertently support the proliferation of cyber-attacks launched via malicious PDF-files. In this study, we present related vulnerabilities and malware distribution approaches that exploit the vulnerabilities of scholarly digital libraries. We evaluated over two-million scholarly papers in the CiteSeerX library and found the library to be contaminated with a surprisingly large number (0.3-2%) of malicious PDF documents (over 55% were crawled from the IPs of US-universities). We developed a two layered detection framework aimed at enhancing the detection of malicious PDF documents, Sec-Lib, which offers a security solution for large digital libraries. Sec-Lib includes a deterministic layer for detecting known malware, and a machine learning based layer for detecting unknown malware. Our evaluation showed that scholarly digital libraries can detect 96.9% of malware with Sec-Lib, while minimizing the number of PDF-files requiring labeling, and thus reducing the manual inspection efforts of security-experts by 98%
Avalanche boron fusion by laser picosecond block ignition with magnetic trapping for clean and economic reactor
After the very long consideration of the ideal energy source by fusion of the
protons of light hydrogen with the boron isotope 11 (boron fusion HB11) the
very first two independent measurements of very high reaction gains by lasers
basically opens a fundamental breakthrough. The non-thermal plasma block
ignition with extremely high power laser pulses above petawatt of picosecond
duration in combination with up to ten kilotesla magnetic fields for trapping
has to be combined to use the measured high gains as proof of an avalanche
reaction for an environmentally clean, low cost and lasting energy source as
potential option against global warming. The unique HB11 avalanche reaction is
are now based on elastic collisions of helium nuclei (alpha particles) limited
only to a reactor for controlled fusion energy during a very short time within
a very small volume.Comment: 11 pages, 6 figures, Submitted to Proceedings 2nd Symposium High
Power Laser Science and Engineering, 14-18 MARCH 2016, Suzhou/Chin
The molecular gas content of z > 6.5 Lyman-alpha emitters
We present results from a sensitive search for CO J=1-0 line emission in two
z> 6.5 Lyman-alpha emitters (LAEs) with the Green Bank Telescope. CO J=1-0
emission was not detected from either object. For HCM 6A, at z ~ 6.56, the
lensing magnification factor of ~4.5 implies that the CO non-detection yields
stringent constraints on the CO J=1-0 line luminosity and molecular gas mass of
the LAE, L'(CO) < 6.1x10^9 x (dV/300)^(1/2) K km/s pc^2 and M(H_2) < 4.9x10^9 x
(dV/300)^(1/2) x (X(CO)/0.8) Msun. These are the strongest limits obtained so
far for a z >~ 6 galaxy. For IOK-1, the constraints are somewhat less
sensitive, L'(CO) < 2.3x10^10 x (dV/300)^(1/2) K km/s pc^2 and M(H_2) <
1.9x10^10 x (dV/300)^(1/2) x (X(CO)/0.8) Msun. The non-detection of CO J=1-0
emission in HCM~6A, whose high estimated star formation rate, dust extinction,
and lensing magnification make it one of the best high-z LAEs for such a
search, implies that typical z >~ 6 LAEs are likely to have significantly lower
CO line luminosities than massive sub-mm galaxies and hyperluminous infrared
quasars at similar redshifts, due to either a significantly lower molecular gas
content or a higher CO-to-H_2 conversion factor.Comment: Accepted for publication in ApJ Letter
Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A
We present observations of the possible short GRB 180418A in -rays,
X-rays, and in the optical. Early optical photometry with the TAROT and RATIR
instruments show a bright peak ( 14.2 AB mag) between and
seconds that we interpret as the signature of a reversal shock. Later
observations can be modeled by a standard forward shock model and show no
evidence of jet break, allowing us to constrain the jet collimation to
. Using deep late-time optical observations we place an
upper limit of AB mag on any underlying host galaxy. The detection of
the afterglow in the \textit{Swift} UV filters constrains the GRB redshift to
and places an upper bound on the -ray isotropic equivalent
energy erg.
The properties of this GRB (e.g. duration, hardness ratio, energetic, and
environment) lie at the intersection between short and long bursts, and we can
not conclusively identify its type. We estimate that the probability that it is
drawn from the population of short GRBs is 10\%-30\%.Comment: Accepted por publication in Ap
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