Calibration of the Herschel SPIRE Fourier Transform Spectrometer

The Herschel SPIRE instrument consists of an imaging photometric camera and an imaging Fourier Transform Spectrometer (FTS), both operating over a frequency range of 450-1550 GHz. In this paper, we briefly review the FTS design, operation, and data reduction, and describe in detail the approach taken to relative calibration (removal of instrument signatures) and absolute calibration against standard astronomical sources. The calibration scheme assumes a spatially extended source and uses the Herschel telescope as primary calibrator. Conversion from extended to point-source calibration is carried out using observations of the planet Uranus. The model of the telescope emission is shown to be accurate to within 6% and repeatable to better than 0.06% and, by comparison with models of Mars and Neptune, the Uranus model is shown to be accurate to within 3%. Multiple observations of a number of point-like sources show that the repeatability of the calibration is better than 1%, if the effects of the satellite absolute pointing error (APE) are corrected. The satellite APE leads to a decrement in the derived flux, which can be up to ~10% (1 sigma) at the high-frequency end of the SPIRE range in the first part of the mission, and ~4% after Herschel operational day 1011. The lower frequency range of the SPIRE band is unaffected by this pointing error due to the larger beam size. Overall, for well-pointed, point-like sources, the absolute flux calibration is better than 6%, and for extended sources where mapping is required it is better than 7%.Comment: 20 pages, 18 figures, accepted for publication in MNRA

Thermal H₂O emission from the Herbig-Haro flow HH 54

The first detection of thermal water emission from a Herbig-Haro object is presented. The observations were performed with the LWS (Long Wavelength Spectrograph) aboard ISO (Infrared Space Observatory). Besides H2O, rotational lines of CO are present in the spectrum of HH 54. These high-J CO lines are used to derive the physical model parameters of the FIR (far-infrared) molecular line emitting regions. This model fits simultaneously the observed OH and H2O spectra for an OH abundance X(OH)=10-6 and a water vapour abundance X(H2O)=10-5. At a distance of 250pc, the total CO, OH and H2O rotational line cooling rate is estimated to be 1.3x10-2 L⊙, which is comparable to the mechanical luminosity generated by the 10km s-1 shocks, suggesting that practically all of the cooling of the weak-shock regions is done by these three molecular species alone

Looking at the bright side of the ρ Ophiuchi dark cloud. Far infrared spectrophotometric observations of the ρ Oph cloud with the ISO-LWS

We present far infrared (45-195 μm) spectrophotometric observations with the ISO-LWS of the active star forming ρOph main cloud (L 1688). The [CII] 158 μm and [OI] 63 μm lines were detected at each of the 33 positions observed, whereas the [OI] 145 μm line was clearly seen toward twelve. The principal observational result is that the [CII] 158 μm line fluxes exhibit a clear correlation with projected distance from the dominant stellar source in the field (HD 147889). We interpret this in terms of PDR-type emission from the surface layers of the ρOph cloud. The observed [CII] 158 μm/[OI] 63 μm flux ratios are larger than unity everywhere. A comparison of the [CII] 158 μm line emission and the FIR dust continuum fluxes yields estimates of the efficiency at which the gas in the cloud converts stellar to [CII] 158 μm photons (χCII≳0.5%). We first develop an empirical model, which provides us with a three dimensional view of the far and bright side of the dark ρOph cloud, showing that the cloud surface towards the putative energy source is concave. This model also yields quantitative estimates of the incident flux of ultraviolet radiation (G0 ~ 101 - 102) and of the degree of clumpiness/texture of the cloud surface (filling of the 80" beam ~0.2). Subsequently, we use theoretical models of PDRS to derive the particle density, n(H), and the temperature structures, for Tgas and Tdust, in the surface layers of the ρOph cloud. Tgas is relatively low, ~60 K, but higher than Tdust ( ~30 K), and densities are generally found within the interval (1-3) 104 cm-3 . These PDR models are moderately successful in explaining the LWS observations. They correctly predict the [OI] 63 μm and [CII] 158 μm line intensities and the observed absence of any molecular line emission. The models do fail, however, to reproduce the observed small [OI] 63 μm/[OI] 145 μm ratios. We examine several possible explanations, but are unable to uniquely identify (or to disentangle) the cause(s) of this discrepancy. From pressure equilibrium arguments we infer that the total mass of the ρOph main cloud (2pc2) is ~2 500 M⊙, which implies that the star formation efficiency to date is ≲4%, significantly lower than previous estimates

Detection of [O I] 63 <i>μ</i>m in absorption toward Sgr B2

A high signal-to-noise 52-90 μm spectrum is presented for the central part of the Sagittarius B2 complex. The data were obtained with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The [O I] 63 μm line is detected in absorption even at the grating spectral resolution of 0.29 μm. A lower limit for the column density of atomic oxygen of the order of 1019 cm-2 is derived. This implies that more than 40% of the interstellar oxygen must be in atomic form along the line of sight toward the Sgr B2 molecular cloud

The Hot Galactic Corona and the Soft X-ray Background

I characterize the global distribution of the 3/4 keV band background with a simple model of the hot Galactic corona, plus an isotropic extragalactic background. The corona is assumed to be approximately polytropic (index = 5/3) and hydrostatic in the gravitational potential of the Galaxy. The model accounts for X-ray absorption, and is constrained iteratively with the ROSAT all-sky X-ray survey data. Regions where the data deviate significantly from the model represent predominantly the Galactic disk and individual nearby hot superbubbles. The global distribution of the background, outside these regions, is well characterized by the model; the 1 sigma relative dispersion of the data from the model is about 15%. The electron density and temperature of the corona near the Sun are about 1.1 x 10^{-3} cm^{-3} and about 1.7 x 10^6 K. The same model also explains well the 1.5 keV band background. The model prediction in the 1/4 keV band, though largely uncertain, qualitatively shows large intensity and spectral variations of the corona contribution across the sky.Comment: An invited talk at IAU Colloquium No. 166: The Local Bubble and Beyond. 10 pages (including b/w figures). Color versions of Figs. 1 and 4 are provided separately and may also be found at www.astro.nwu.edu/astro/wqd/paper/hal

Constraints on a Local Group X-ray Halo

A simple model for a hot Local Group halo is constructed, using the standard beta-model for the halo density and by choosing model parameters based on all available observations of X-ray emission in other groups of galaxies and on optical data on Local Group morphology. From the predicted X-ray intensities, total Local Group mass, and central cooling time of the halo, we derive very conservative upper limits on the central halo density N_0 and global temperature T of N_0 = 5e-4 cm-3 and kT = 0.5 keV, irrespective of realistic values of the density profile parameters r_c and beta. A typical poor group value of beta = 0.5 requires kT < 0.15 keV and N_0 < 1e-4 cm-3, from which it is concluded that the Local Group is very unlikely to possess a significant X-ray halo. The prospects for further constraining of halo parameters from UV absorption line observations are considered. We explicitly calculate the ability of the halo to distort the cosmic microwave background (CMB) in terms of the resulting CMB temperature variations and multipole anisotropies.Comment: 24 pages, 16 figures. Accepted for publication in the Astrophysical Journa

Extended fine structure and continuum emission from S140/L1204

Grating spectra, covering the wavelength range 45 to 187μm have been taken with the ISO Long Wavelength Spectrometer (LWS) at a series of pointing positions over the S 140 region, centred on the cluster of embedded young stellar objects at the south-west corner of the L1204 molecular cloud. Extended emission from [CII]158μm and [OI]63μm is seen, peaking near the position of the embedded stars. The measurements of the fine structure lines are interpreted in terms of PDR models for the emission, as well as the underlying thermal continuum for the heated gas and dust

First results on Martian carbon monoxide from Herschel/HIFI observations

We report on the initial analysis of Herschel/HIFI carbon monoxide (CO) observations of the Martian atmosphere performed between 11 and 16 April 2010. We selected the (7-6) rotational transitions of the isotopes ^{13}CO at 771 GHz and C^{18}O at 768 GHz in order to retrieve the mean vertical profile of temperature and the mean volume mixing ratio of carbon monoxide. The derived temperature profile agrees within less than 5 K with general circulation model (GCM) predictions up to an altitude of 45 km, however, show about 12-15 K lower values at 60 km. The CO mixing ratio was determined as 980 \pm 150 ppm, in agreement with the 900 ppm derived from Herschel/SPIRE observations in November 2009.Comment: Accepted for publication in Astronomy and Astrophysics (special issue on HIFI first results); minor changes to match published versio

LWS-spectroscopy of Herbig Haro objects and molecular outflows in the Cha II dark cloud

We present the first far infrared spectra of the Herbig Haro objects HH 52-53-54 and of IRAS 12496-7650, all located in the nearby star forming region known as Chamaleon II dark cloud, obtained with the Long Wavelength Spectrometer (LWS) onboard the Infrared Space Observatory (ISO). The richest spectrum is found in HH 54, showing molecular transitions (CO with Ju from 19 to 14, water vapour mainly in its ortho form and OH) and low excitation fine structure lines ([OI]63, 145μm, [CII]158μm). In HH 52 and HH 53, only the [OI] and [CII] lines are detected. The LWS spectrum of IRAS 12496-7650 shows both fine structure and CO lines. The [CII]158μm line is ubiquitous in the region, as proved by its presence in all ISO pointings, including the raster scan maps. The fine structure lines are used to evaluate the physical parameters of the emitting regions. In particular, the mass loss rates of each outflow present in the region, are derived from the [OI]63μm line luminosity

Calibration and performance of the ISO Long-Wavelength Spectrometer

The wavelength and flux calibration, and the in-orbit performance of the Infrared Space Observatory Long-Wavelength Spectrometer (LWS) are described. The LWS calibration is mostly complete and the instrument's performance in orbit is largely as expected before launch. The effects of ionising radiation on the detectors, and the techniques used to minimise them are outlined. The overall sensitivity figures achieved in practice are summarised. The standard processing of LWS data is described