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تسجيل مستخدم جديدTransient effects in Herschel/PACS spectroscopy
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Dario Fadda
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The Ge:Ga detectors used in the PACS spectrograph onboard the Herschel space telescope react to changes of the incident flux with a certain delay. This generates transient effects on the resulting signal which can be important and last for up to an hour. The paper presents a study of the effects of transients on the detected signal and proposes methods to mitigate them especially in the case of the unchopped mode. Since transients can arise from a variety of causes, we classified them in three main categories: transients caused by sudden variations of the continuum due to the observational mode used; transients caused by cosmic ray impacts on the detectors; transients caused by a continuous smooth variation of the continuum during a wavelength scan. We propose a method to disentangle these effects and treat them separately. In particular, we show that a linear combination of three exponential functions is needed to fit the response variation of the detectors during a transient. An algorithm to detect, fit, and correct transient effects is presented. The solution proposed to correct the signal for the effects of transients substantially improves the quality of the final reduction with respect to the standard methods used for archival reduction in the case where transient effects are most pronounced. The programs developed to implement the corrections are offered through two new interactive data reduction pipelines in the latest releases of the Herschel Interactive Processing Environment.
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The Herschel Space Observatory was the fourth cornerstone mission in the European Space Agency (ESA) science programme. It had excellent broad band imaging capabilities in the far-infrared (FIR) and sub-millimetre part of the electromagnetic spectrum
. Although the spacecraft finished observing in 2013, it left a large legacy dataset that is far from having been fully explored and still has a great potential for new scientific discoveries. The PACS and SPIRE photometric cameras observed about 8% of the sky in six different wavebands. This document describes the Herschel/PACS Point Source Catalogue (HPPSC), a FIR catalogue based on the broad-band photometric observations of the PACS instrument with filters centred at 70, 100 and 160 microns. We analysed 14842 combined, Level 2.5/Level 3 Herschel/PACS photometric observations. The PACS photometer maps were generated by the JScanam task of the Herschel Interactive Processing Environment (HIPE) v13.0.0. Sources were identified with the HIPE implementation of SUSSEXtractor, and the flux densities obtained by aperture photometry. We found a total of 108 319 point sources that are considered to be reliable in the 70 micron maps, 131 322 at 100 micron and 251 392 point sources in the 160 micron maps. In addition, our quality control algorithm identified 546 587 candidate sources that were found to be extended and 7 185 160 features which did not pass the signal-to-noise and other criteria to be considered reliable sources. These sources were included in the Extended Source List and Rejected Source List of the HPPSC, respectively. The calculated completeness and photometric accuracy values are based on simulations, where artificial sources were injected into the observational timeline with well controlled flux density values. The actual completeness is a complex function of the source flux, photometric band and the background complexity.
We present Herschel-PACS observations of rest-frame mid-infrared and far-infrared spectral line emissions from two lensed, ultra-luminous infrared galaxies at high redshift: MIPS J142824.0+352619 (MIPS J1428), a starburst-dominated system at z = 1.3,
and IRAS F10214+4724 (F10214), a source at z = 2.3 hosting both star-formation and a luminous AGN. We have detected [OI]63 micron and [OIII]52 micron in MIPS J1428, and tentatively [OIII]52 micron in F10214. Together with the recent ZEUS-CSO [CII]158 micron detection in MIPS J1428 we can for the first time combine [OI], [CII] and far-IR (FIR) continuum measurements for photo-dissociation (PDR) modeling of an ultra-luminous (L_IR > 10^12 L_sun) star forming galaxy at the peak epoch of cosmic star formation. We find that MIPS J1428, contrary to average local ULIRGs, does not show a deficit in [OI] relative to FIR. The combination of far-UV flux G_0 and gas density n (derived from the PDR models), as well as the star formation efficiency (derived from CO and FIR) is similar to normal or starburst galaxies, despite the high infrared luminosity of this system. In contrast, F10214 has stringent upper limits on [OIV] and [SIII], and an [OIII]/FIR ratio at least an order of magnitude lower than local starbursts or AGN, similar to local ULIRGs.
This is the first of a series of papers presenting the THROES (A caTalogue of HeRschel Observations of Evolved Stars) project, intended to provide a comprehensive overview of the spectroscopic results obtained in the far-infrared (55-670 microns) wit
h the Her- schel space observatory on low-to-intermediate mass evolved stars in our Galaxy. Here we introduce the catalogue of interactively reprocessed PACS (Photoconductor Array Camera and Spectrometer) spectra covering the 55-200 microns range for 114 stars in this category for which PACS range spectroscopic data is available in the Herschel Science Archive (HSA). Our sample includes objects spanning a range of evolutionary stages, from the asymptotic giant branch to the planetary nebula phase, displaying a wide variety of chemical and physical properties. The THROES/PACS catalogue is accessible via a dedicated web-based inter- face (https://throes.cab.inta-csic.es/) and includes not only the science-ready Herschel spectroscopic data for each source, but also complementary photometric and spectroscopic data from other infrared observatories, namely IRAS (Infrared Astronomical Satellite), ISO (Infrared Space Observatory) or AKARI, at overlapping wavelengths. Our goal is to create a legacy-value Herschel dataset that can be used by the scientific community in the future to deepen our knowledge and understanding of these latest stages of the evolution of low-to-intermediate mass stars.
Our aims are to determine flux densities and their photometric accuracy for a set of seventeen stars that range in flux from intermediately bright (<2.5 Jy) to faint (>5 mJy) in the far-infrared (FIR). We also aim to derive signal-to-noise dependence
with flux and time, and compare the results with predictions from the Herschel exposure-time calculation tool. The PACS faint star sample has allowed a comprehensive sensitivity assessment of the PACS photometer. Accurate photometry allows us to establish a set of five FIR primary standard candidates, namely alpha Ari, epsilon Lep, omega,Cap, HD41047 and 42Dra, which are 2 -- 20 times fainter than the faintest PACS fiducial standard (gamma Dra) with absolute accuracy of <6%. For three of these primary standard candidates, essential stellar parameters are known, meaning that a dedicated flux model code may be run.
A new concept of bolometer arrays is used for the imager of PACS, one of the three instruments aboard the future Herschel space observatory. Within the framework of PACS photometer characterization, irradiation tests were performed on a dedicated bol
ometer array in order to study long-term and short-term radiation effects. The main objective was to study particles impacts on the detectors applicable to future observations in orbit and possible hard and/or soft curing to restore its performances. Cobalt-60 gamma ray irradiations did not show significant degradation, so we mainly focused on single events effects (SEE). Protons and alphas irradiations were then performed at the Van de Graaf tandem accelerator at the Institut de Physique Nucleaire (IPN, Orsay, France), respectively at 20MeV and 30MeV. Observation showed that the shape of signal perturbations clearly depends on the location of the impacts either on the detector itself or the read-out circuit. Software curing has then to be anticipated in order to deglitch the signal. This test gives also a unique opportunity to measure some parameters of the detector: electrical crosstalk and thermo- electrical time constant. However a detailed bolometer model is necessary to understand the contribution of the thermal response in relation with the electrical response. It will be the second step of our study. Finally the complete radiation evaluation proved that this detector can be used in spatial experiments.
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