352 research outputs found
Multiple tidal disruption flares in the active galaxy IC 3599
Tidal disruption events occur when a star passes too close to a massive black
hole and it is totally ripped apart by tidal forces. It may also happen that
the star is not close enough to the black hole to be totally disrupted and a
less dramatic event might happen. If the stellar orbit is bound and highly
eccentric, just like some stars in the centre of our own Galaxy, repeated
flares should occur. When the star approaches the black hole tidal radius at
periastron, matter might be stripped resulting in lower intensity outbursts
recurring once every orbital period. We report on Swift observations of a
recent bright flare from the galaxy IC 3599 hosting a middle-weight black hole,
where a possible tidal disruption event was observed in the early 1990s. By
light curve modelling and spectral fitting we can consistently account for the
events as the non-disruptive tidal stripping of a star into a highly eccentric
orbit. The recurrence time is 9.5 yr. IC 3599 is also known to host a
low-luminosity active galactic nucleus. Tidal stripping from this star over
several orbital passages might be able to spoon-feed also this activity.Comment: Accepted for publication to Astronomy & Astrophysic
Polarimetric and spectroscopic optical observations of the ultra-compact X-ray binary 4U 0614+091
Aims: We present a polarimetric and spectroscopic study of the persistent
ultra compact X-ray binary 4U 0614+091 aimed at searching for the emission of a
relativistic particle jet and at unveiling the orbital period of the system.
Methods: We obtained r-band polarimetric observations with the Telescopio
Nazionale Galileo (TNG) equipped with the PAOLO polarimeter and with the Nordic
Optical Telescope (NOT) equipped with the ALFOSC instrument, covering ~ 2 hours
and ~ 0.5 hours observations, respectively. We carried out low resolution
spectroscopy of the system using the ESO Very Large Telescope equipped with
FORS1 for ~ 1.5 hours (16 spectra covering the range 430-800 nm). Results: The
polarimetric analysis performed starting from the TNG dataset revealed a
polarisation degree in the r-band of 3 % +/- 1 %. From the NOT dataset, due to
the lower S/N ratio, we could obtain only a 3 sigma upper limit of 3.4 %. From
the joining of a spectroscopic and photometric analysis, through the study of
the equivalent width variations of the CII 724 nm line and the r-band light
curve, we could find a hint of a ~ 45 min periodicity. Conclusions: A
polarisation degree P of ~ 3 % in the r-band is consistent with the emission of
a relativistic particle jet, which is supposed to emit intrinsically linearly
polarised synchrotron radiation. Since no variations of P with time have been
detected, and the accretion disc of the system does not contain ionised
hydrogen, scattering by free electrons in the accretion disc has been rejected.
The period of ~ 45 min obtained through the analysis of the system light curve
and of the equivalent width variations of the selected spectral line is
probably linked to the presence of a hot spot or a superhump in the accretion
disc, and lead to an orbital period > 1 hour for the binary system.Comment: 8 pages, 6 figures, 4 tables. Accepted for publication in Astronomy &
Astrophysics (Section 7
EXIT-chart aided hybrid multiuser detector design for frequency-domain-spread chip-interleaved MC-CDMA
With the advent of EXtrinsic Information Transfer (EXIT) charts, we are capable of analyzing, predicting and visually comparing the convergence behaviours of different turbo Multi-User Detectector (MUD)s. The different MUDs have diverse EXIT characteristics and hence their superposition allows us to create a combined EXIT curve, which closely matches that of the channel decoder. Hence a near-capacity operation is facilitated by combining the benifits of different MUDs and therefore to create a superior MUD. Thus in this contribution, we propose a novel hybrid MUD combining scheme, which combines the advantages of a high performance and low complexity in form of an advanced hybrid MUD solution. The transmitted bits are unknown at the receiver, hence it is not feasible to directly evaluate the mutual information gain of the iterative MUD in consecutive iterations, hence we propose a realistic algorithm for estimating this mutual information gain, which is then used for activating the most appropriate constituent MUD as and when it is necessary. The constituent MUDs are the Matched Filter (MF) based Soft Interference Cancellation (SoIC) and the optimum Bayesian MUDs, which are invoked in the scenario of Frequency-Domain-Spread Chip-Interleaved (FDSCI) Multiple Carrier Code Division Multiple Access (MC-CDMA). The resultant hybrid MUD is capable of outperforming both the MF-SoIC and Bayesian turbo MUDs in the terms of the attainable complexity and Bit-Error-Rate (BER) performance
The data processing pipeline for the Herschel SPIRE Fourier Transform Spectrometer
We present the data processing pipeline to generate calibrated data products from the Spectral
and Photometric Imaging Receiver (SPIRE) imaging Fourier Transform Spectrometer on the
Herschel Space Observatory. The pipeline processes telemetry from SPIRE observations and
produces calibrated spectra for all resolution modes. The spectrometer pipeline shares some
elements with the SPIRE photometer pipeline, including the conversion of telemetry packets
into data timelines and calculation of bolometer voltages.We present the following fundamental
processing steps unique to the spectrometer: temporal and spatial interpolation of the scan
mechanism and detector data to create interferograms; Fourier transformation; apodization;
and creation of a data cube. We also describe the corrections for various instrumental effects
including first- and second-level glitch identification and removal, correction of the effects
due to emission from the Herschel telescope and from within the spectrometer instrument,
interferogram baseline correction, temporal and spatial phase correction, non-linear response
of the bolometers, and variation of instrument performance across the focal plane arrays.
Astronomical calibration is based on combinations of observations of standard astronomical
sources and regions of space known to contain minimal emission
The Herschel Multi-tiered Extragalactic Survey: HerMES
The Herschel Multi-tiered Extragalactic Survey, HerMES, is a legacy program
designed to map a set of nested fields totalling ~380 deg^2. Fields range in
size from 0.01 to ~20 deg^2, using Herschel-SPIRE (at 250, 350 and 500 \mu m),
and Herschel-PACS (at 100 and 160 \mu m), with an additional wider component of
270 deg^2 with SPIRE alone. These bands cover the peak of the redshifted
thermal spectral energy distribution from interstellar dust and thus capture
the re-processed optical and ultra-violet radiation from star formation that
has been absorbed by dust, and are critical for forming a complete
multi-wavelength understanding of galaxy formation and evolution.
The survey will detect of order 100,000 galaxies at 5\sigma in some of the
best studied fields in the sky. Additionally, HerMES is closely coordinated
with the PACS Evolutionary Probe survey. Making maximum use of the full
spectrum of ancillary data, from radio to X-ray wavelengths, it is designed to:
facilitate redshift determination; rapidly identify unusual objects; and
understand the relationships between thermal emission from dust and other
processes. Scientific questions HerMES will be used to answer include: the
total infrared emission of galaxies; the evolution of the luminosity function;
the clustering properties of dusty galaxies; and the properties of populations
of galaxies which lie below the confusion limit through lensing and statistical
techniques.
This paper defines the survey observations and data products, outlines the
primary scientific goals of the HerMES team, and reviews some of the early
results.Comment: 23 pages, 17 figures, 9 Tables, MNRAS accepte
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