Herschel SPIRE FTS relative spectral response calibration

Herschel/SPIRE Fourier transform spectrometer (FTS) observations contain emission from both the Herschel Telescope and the SPIRE Instrument itself, both of which are typically orders of magnitude greater than the emission from the astronomical source, and must be removed in order to recover the source spectrum. The effects of the Herschel Telescope and the SPIRE Instrument are removed during data reduction using relative spectral response calibration curves and emission models. We present the evolution of the methods used to derive the relative spectral response calibration curves for the SPIRE FTS. The relationship between the calibration curves and the ultimate sensitivity of calibrated SPIRE FTS data is discussed and the results from the derivation methods are compared. These comparisons show that the latest derivation methods result in calibration curves that impart a factor of between 2 and 100 less noise to the overall error budget, which results in calibrated spectra for individual observations whose noise is reduced by a factor of 2–3, with a gain in the overall spectral sensitivity of 23 % and 21 % for the two detector bands, respectively.     

Herschel SPIRE FTS telescope model correction

Emission from the Herschel telescope is the dominant source of radiation for the majority of SPIRE Fourier transform spectrometer (FTS) observations, despite the exceptionally low emissivity of the primary and secondary mirrors. Accurate modelling and removal of the telescope contribution is, therefore, an important and challenging aspect of FTS calibration and data reduction pipeline. A dust-contaminated telescope model with time invariant mirror emissivity was adopted before the Herschel launch. However, measured FTS spectra show a clear evolution of the telescope contribution over the mission and strong need for a correction to the standard telescope model in order to reduce residual background (of up to 7 Jy) in the final data products. Systematic changes in observations of dark sky, taken over the course of the mission, provide a measure of the evolution between observed telescope emission and the telescope model. These dark sky observations have been used to derive a time dependent correction to the telescope emissivity that reduces the systematic error in the continuum of the final FTS spectra to ～0.35 Jy.     

Relative pointing offset analysis of calibration targets with repeated observations with Herschel-SPIRE Fourier-transform spectrometer

We present a method to derive the relative pointing offsets for SPIRE Fourier-Transform Spectrometer (FTS) solar system object (SSO) calibration targets, which were observed regularly throughout the Herschel mission. We construct ratios R _obs(ν) of the spectra for all observations of a given source with respect to a reference. The reference observation is selected iteratively to be the one with the highest observed continuum. Assuming that any pointing offset leads to an overall shift of the continuum level, then these R _obs(ν) represent the relative flux loss due to mispointing. The mispointing effects are more pronounced for a smaller beam, so we consider only the FTS short wavelength array (SSW, 958–1546 GHz) to derive a pointing correction. We obtain the relative pointing offset by comparing R _obs(ν) to a grid of expected losses for a model source at different distances from the centre of the beam, under the assumption that the SSW FTS beam can be well approximated by a Gaussian. In order to avoid dependency on the point source flux conversion, which uses a particular observation of Uranus, we use extended source flux calibrated spectra to construct R _obs(ν) for the SSOs. In order to account for continuum variability, due to the changing distance from the Herschel telescope, the SSO ratios are normalised by the expected model ratios for the corresponding observing epoch. We confirm the accuracy of the derived pointing offset by comparing the results with a number of control observations, where the actual pointing of Herschel is known with good precision. Using the method we derived pointing offsets for repeated observations of Uranus (including observations centred on off-axis detectors), Neptune, Ceres and NGC 7027. The results are used to validate and improve the point-source flux calibration of the FTS.     

Operations and performance of the PACS instrument 3He sorption cooler on board of the Herschel space observatory

A ³He sorption cooler produced the operational temperature of 285 mK for the bolometer arrays of the Photodetector Array Camera and Spectrometer (PACS) instrument of the Herschel Space Observatory. This cooler provided a stable hold time between 60 and 73 h, depending on the operational conditions of the instrument. The respective hold time could be determined by a simple functional relation established early on in the mission and reliably applied by the scientific mission planning for the entire mission. After exhaustion of the liquid ³He due to the heat input by the detector arrays, the cooler was recycled for the next operational period following a well established automatic procedure. We give an overview of the cooler operations and performance over the entire mission and distinguishing in-betweenthe start conditions for the cooler recycling and the two main modes of PACS photometer operations. As a spin-off, the cooler recycling temperature effects on the Herschel cryostat ⁴He bath were utilized as an alternative method to dedicated Direct Liquid Helium Content Measurements in determining the lifetime of the liquid Helium coolant.     

Herschel SPIRE FTS spectral mapping calibration

The Herschel SPIRE Fourier transform spectrometer (FTS) performs spectral imaging in the 447–1546 GHz band. It can observe in three spatial sampling modes: sparse mode, with a single pointing on sky, or intermediate or full modes with 1 and 1/2 beam spacing, respectively. In this paper, we investigate the uncertainty and repeatability for fully sampled FTS mapping observations. The repeatability is characterised using nine observations of the Orion Bar. Metrics are derived based on the ratio of the measured intensity in each observation compared to that in the combined spectral cube from all observations. The mean relative deviation is determined to be within 2 %, and the pixel-by-pixel scatter is ～ 7 %. The scatter increases towards the edges of the maps. The uncertainty in the frequency scale is also studied, and the spread in the line centre velocity across the maps is found to be ～ 15 km s ^{? 1}. Other causes of uncertainty are also discussed including the effect of pointing and the additive uncertainty in the continuum.     

On the instrumental and atmospheric stray-light for solar observations

From aureole measurements made with a 40 cm-Vacuum-Telescope at Izaña (Tenerife) in the wavelength region 417 nm < λ < 785 nm and from a comparison with other aureole measurements we conclude the following: (a) Within one solar radius from the limb, the aureole is mostly of instrumental origin, (b) Beyond that distance, the contribution of atmospheric stray-light becomes noticeable, (c) The atmospheric contribution to the aureole intensity is, under good conditions at mountain stations, a very slowly decreasing function, and amounts to some 10^?5 of the solar disk intensity. A procedure is given to separate the variable atmospheric component of the stray-light from the constant instrumental one.     

SPIRE point source photometry: within the Herschel interactive processing environment (HIPE)

The different algorithms appropriate for point source photometry on data from the SPIRE instrument on-board the Herschel Space Observatory, within the Herschel Interactive Processing Environment (HIPE) are compared. Point source photometry of a large ensemble of standard calibration stars and dark sky observations is carried out using the 4 major methods within HIPE: SUSSEXtractor, DAOphot, the SPIRE Timeline Fitter and simple Aperture Photometry. Colour corrections and effective beam areas as a function of the assumed source spectral index are also included to produce a large number of photometric measurements per individual target, in each of the 3 SPIRE bands (250, 350, 500μm), to examine both the accuracy and repeatability of each of the 4 algorithms. It is concluded that for flux densities down to the level of 30mJy that the SPIRE Timeline Fitter is the method of choice. However, at least in the 250 and 350μm bands, all 4 methods provide photometric repeatability better than a few percent down to at approximately 100mJy. The DAOphot method appears in many cases to have a systematic offset of ～8 % in all SPIRE bands which may be indicative of a sub-optimal aperture correction. In general, aperture photometry is the least reliable method, i.e. largest scatter between observations, especially in the longest wavelength band. At the faintest fluxes, <30mJy, SUSSEXtractor or DAOphot provide a better alternative to the Timeline Fitter.     

Intercomparison of the LASCO-C2, SECCHI-COR1, SECCHI-COR2, and Mk4 Coronagraphs

In order to assess the reliability and consistency of white-light coronagraph measurements, we report on quantitative comparisons between polarized brightness [pB] and total brightness [B] images taken by the following white-light coronagraphs: LASCO-C2 on SOHO, SECCHI-COR1 and -COR2 on STEREO, and the ground-based MLSO-Mk4. The data for this comparison were taken on 16?April 2007, when both STEREO spacecraft were within 3.1^° of Earth??s heliographic longitude, affording essentially the same view of the Sun for all of the instruments. Due to the difficulties of estimating stray-light backgrounds in COR1 and COR2, only Mk4 and C2 produce reliable coronal-hole values (but not at overlapping heights), and these cannot be validated without rocket flights or ground-based eclipse measurements. Generally, the agreement between all of the instruments?? pB values is within the uncertainties in bright streamer structures, implying that measurements of bright CMEs also should be trustworthy. Dominant sources of uncertainty and stray light are discussed, as is the design of future coronagraphs from the perspective of the experiences with these instruments.     

A Model for the Stray-Light Contamination of the UVCS Instrument on SOHO

We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H i Lyα emission line, but they are applicable to other spectral lines as well.     

Delta spots and great flares

Using eighteen years of observations at Big Bear, we summarize the development of δ spots and the great flares they produce. We find δ groups to develop in three ways: eruption of a single complex active region formed below the surface, eruption of large satellite spots near (particularly in front of) a large older spot, or collision of spots of opposite polarity from different dipoles. Our sample of twenty-one δ spots shows that once they lock together, they never separate, although rarely an umbra is ejected. The δ spots are already disposed to their final form when they emerge. The driving force for the shear is spot motion, either flux emergence or the forward motion of p spots in an inverted magnetic configuration. We observe the following phenomena preceding great flares:

1. δ spots, preferentially Types 1 and 2.
2. Umbrae obscured by Hα emission.
3. Bright Hα emission marking flux emergence and reconnection.
4. Greatly sheared magnetic configurations, marked by penumbral and Hα fibrils parallel to the inversion line.

We assert that with adequate spatial resolution one may predict the occurrence of great flares with these indicators.     

Relationships between lobe structure and total luminosity of radio sources

We made a statistical analysis using the Cambridge 5km, 5 GHz data on double radio sources and found highly significant correlations between the following pairs of parameters; the energies in the hot spots and the extended regions, E_h and E_e; their luminosities, L_h and L_e; energy density in the hot spots K and the total luminosity L_t and the ratio of energy densities $\text{K}\text{A}$ and L_t. These correlations exemplify the role of the hot spots in the radio sources.     

X-ray Radiation Mechanisms and the Beaming Effect of Hot Spots and Knots in AGN Jets

The observed broadband spectral energy distributions (SEDs) of 22 hot spots and 45 knots are modelled with single-zone lepton models. Considering the sources at rest, the X-rays of some hot spots can be explained by the SSC model with magnetic field being consistent with the equipartition magnetic field in magnitude of order 1, but at the same time an unreasonably low magnetic field is required to model the X-rays for all knots. When considering the relativistic bulk motion of the sources, the IC/CMB model well explains the X-ray emission for most of them under the equipartition condition. We show that the ratio of observational luminosity R _L is tentatively correlated with the co-moving equipartition magnetic field $B_{\rm eq}^{\prime}$ and the beaming factor ??. These facts suggest that the observational differences of the X-rays from the knots and hot spots may be mainly due to the differences in the Doppler boosting effect and the co-moving magnetic field of the two kinds of source.     

Water ice in the dark dune spots of Richardson crater on Mars

Interfacial liquid water has been hypothesized to form during the seasonal evolution of the dark dune spots observed in the high latitudes of Mars. In this study we assess the presence, nature and properties of ices - in particular water ice - that occur within these spots using HIRISE and CRISM observations, as well as the LMD Global Climate Model. Our studies focus on Richardson crater (72°S, 179°E) and cover southern spring and summer (L_S=175-341°). Three units have been identified of these spots: dark core, gray ring and bright halo. Each unit show characteristic changes as the season progress. In winter, the whole area is covered by CO₂ ice with H₂O ice contamination. Dark spots form during late winter and early spring. During spring, the dark spots are located in a 10 cm thick depression compared to the surrounding bright ice-rich layer. They are spectrally characterized by weak CO₂ ice signatures that probably result from spatial mixing of CO₂ ice-rich and ice-free regions within pixels, and from mixing of surface signatures due to aerosols scattering. The bright halo shaped by winds shows stronger CO₂ absorptions than the average ice-covered terrain, which is consistent with a formation process involving CO₂ re-condensation. According to spectral, morphological and modeling considerations, the gray ring is composed of a thin layer of a few tens of μm of water ice. Two sources/processes could participate to the enrichment of water ice in the gray ring unit: (i) water ice condensation at the surface in early fall (prior to the condensation of a CO₂-rich winter layer) or during wintertime (due to cold trapping of the CO₂ layer) and (ii) ejection of dust grains surrounded by water ice by the geyser activity responsible for the dark spot. In any case, water ice remains longer in the gray ring unit after the complete sublimation of the CO₂. Finally, we also looked for liquid water in the near-IR CRISM spectra using linear unmixing modeling but found no conclusive evidence for it.     

A VLBI and MERLIN Survey of faint,compact radio sources

We present preliminary results from a VLBI survey at λ=6 cm of a sample of 35 sources with flux densities of 2–100 mJy. These sources were selected from the VLA FIRST survey at λ=20 cm, in a 3 degree field around the bright calibrator 1156+295, simply by imposing S₂₀>10 mJy and θ<5 arcsec. MERLIN observations at λ6 cm detected 70/127 of these sources with a threshold of 2 mJy at 50 mas resolution and the closest 35 of these to the calibrator were observed with the VLBA+EVN in snapshot mode at λ6 cm. These sources are a mixture of flat and steep-spectrum sources and include: weak flat-spectrum nuclei of large radio galaxies, low power AGN in nearby galaxies and radio quiet quasars. With these short observations, the sensitivity is limited and most appear as either core-jets or simple point sources on the milliarcsec scale. Nonetheless, it is encouraging that with only 10 minutes observation per source, at least 35% of all sources with S₂₀>10 mJy can be detected and imaged with global 6 cm VLBI.     

Spotted Surface Structure of the Active Giant PZ Mon

Based on our photometric observations in 2015–2016 and archival photometric data for the active red giant PZ Mon, we have found the main characteristics of the stellar surface: the unspotted surface temperature T_eff = 4730 K, the spot temperature T_s = 3500 K, and the relative spot area from 30 to 40%. The best agreement with the observations has been achieved in our three-spot model including a cool polar spot with a temperature of ~3500 K as well as large and small warm spots with a temperature of ~4500 K. The stable polar spot is responsible for the long-period brightness variations. Its presence is confirmed by an analysis of the TiO 7054 Å molecular band. The small-amplitude 34-day variability is attributable to the warm spots located on the side of the secondary component, which determine the relatively stable active longitude.     

Herschel SPIRE fourier transform spectrometer: calibration of its bright-source mode

The Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board the ESA Herschel Space Observatory has two detector setting modes: (a) a nominal mode, which is optimized for observing moderately bright to faint astronomical targets, and (b) a bright-source mode recommended for sources significantly brighter than 500 Jy, within the SPIRE FTS bandwidth of 446.7–1544 GHz (or 194–671 microns in wavelength), which employs a reduced detector responsivity and out-of-phase analog signal amplifier/demodulator. We address in detail the calibration issues unique to the bright-source mode, describe the integration of the bright-mode data processing into the existing pipeline for the nominal mode, and show that the flux calibration accuracy of the bright-source mode is generally within 2 % of that of the nominal mode, and that the bright-source mode is 3 to 4 times less sensitive than the nominal mode.     

In-flight calibration of the HIFI diplexers

HIFI is a heterodyne spectrometer aboard the Herschel Space Observatory, providing high-spectral-resolution capabilities. Of its seven frequency bands, four (bands 3, 4, 6, and 7) employ Martin-Puplett diplexers to combine sky signal and local oscillator at the two linear polarizations H and V, prior to feeding them into the mixers (receivers). The optical path difference in each of these 8 diplexers must be tuned to the observed frequency. The required actuator currents were determined in flight before the start of routine science observations. We here report on regular (roughly quarterly) engineering test observations to validate the repeatability of the HIFI diplexers during the routine phase of Herschel operations. We find the optical path difference to be stable to within 0.4 % of the relevant wavelength, typically at the sub-micron level. We conclude that the repeatability and precision of the diplexer tuning mechanism are so high that science data are in no way negatively affected. With the diplexer calibration established and validated, this line of reasoning can be reversed, and the diplexers can be used as relative spectrometers to measure the local-oscillator frequency, i.e., to check the spectral purity of the local oscillator across the diplexer bands. This was done from before launch out to the last months of cryogenic operations in space.     

VLA observations of hot spots in high luminosity radio sources

VLA observations at 6 cm have been made of 16 distant luminous 3C sources that appeared to be unresolved or slightly resolved in Cambridge maps but which were known from VLBI observations to contain significant fine structure on the scale of about 1 kpc in their lobes. The general morphology of these sources is very similar to that of their nearby powerful counterparts; most of the lobes contain low brightness tails which are often directed from the hot spots towards the associated optical object. The hot spots are generally unresolved by the VLA observations; in 3C 254, 3C 268.4 and 3C 352, one of the lobes contains two hot spots.     

Jupiter's Cloud Structure as Constrained by Galileo Probe and HST Observations

The Galileo Probe sampled Jupiter's atmosphere at the edge of a 5-μm hot spot, where it found very little cloud opacity above the 700 mb level. Only τ=1-2 at λ=0.5 μm was inferred from Net Flux Radiometer observations (Sromovsky et al. 1998, J. Geophys. Res.103, 22,929-22,977), in seeming conflict with Chanover et al. (1997, Icarus128, 294-305) who inferred τ=6-8 above the 700 mb level (at λ∼0.9 μm) from 893-nm and 953-nm WFPC2 observations of a group of hot spots. Postulating a heterogeneous cloud structure is one way to resolve the conflict. We obtained a more satisfying resolution by reinterpretation of the HST observations with Probe-compatible assumptions about the vertical distribution of cloud particles. Assuming a physically thin upper (putative NH₃) cloud with adjustable optical depth and effective pressure (p_eff<440 mb) and a physically thin midlevel (putative NH₄SH) cloud with adjustable optical depth but a fixed pressure of 1.2 bars, we are able to fit WPFC2 observations with probe-consistent opacities in hot spot regions. With the same cloud pressures, but higher middle cloud opacities, we are even able to fit the visibly bright regions. Little variability is seen in the upper cloud. Best fits to October 1995 WFPC2 observations in dark regions (5-μm hot spots) yielded τ_upper=1.3-1.9 at 0.9 μm and p_eff=240 mb−270 mb, while in visibly bright regions between hot spots we obtained τ_upper=1.6-2.2 and p_eff=250 mb−290 mb. May 1996 observations yielded slightly higher values of τ_upper (1.8-2.3 and 2.0-2.8) and p_eff (250 mb−310 mb and 265 mb−320 mb). We found that the most important variable parameter is the opacity of the middle cloud, which ra nged from τ=1, 2 in dark regions, to τ=8-30 in bright regions. From limb darkening characteristics, we inferred a wavelength-dependent haze opacity ranging from 0.2±0.05 at 660 nm to 0.35±0.05 at 953 nm, and an effective haze pressure near 120 mb. We did not find it necessary to use low single scattering albedos that require effective imaginary indices, that are several orders of magnitude larger than the values of the main putative cloud components.