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In a previous paper,1we described a method for significantly reducing the read noise of HAWAII-2RG (H2RG) and SIDECAR application specific integrated circuit (ASIC) based detector systems by making better use of reference signals. "Improved Reference Sampling & Subtraction" (IRS2; pronounced "IRS-square") is based on: (1) making better use of the H2RG's reference output, (2) sampling reference pixels more frequently in the time domain, and (3) optimal subtraction of both the reference output and reference pixels in the Fourier domain. Here we demonstrate that IRS2works as expected using an engineering grade James Webb Space Telescope (JWST) SIDECAR ASIC and H2RG detector array. We were able to reduce the read noise per frame from 25 e- rms using traditional JWST readout to 10 e- rms per frame using IRS2. The only aspect of the system that we changed to make these impressive improvements was the SIDECAR ASIC readout software -we did not change the hardware.

The interaction of the Puppis A supernova remnant (SNR) with a neighboring molecular cloud provides a unique opportunity to measure the amount of grain destruction in an SNR shock. Spitzer Space Telescope MIPS imaging of the entire SNR at 24, 70, and 160 mm shows an extremely good correlation with X-ray emission, indicating that the SNR's IR radiation is dominated by the thermal emission of swept-up interstellar dust, collisionally heated by the hot shocked gas. Spitzer IRS spectral observations targeted both the Bright Eastern Knot (BEK) of the SNR where a small cloud has been engulfed by the supernova blast wave and outlying portions of the associated molecular cloud that are yet to be hit by the shock front. Modeling the spectra from both regions reveals the composition and the grain size distribution of the interstellar dust, both in front of and behind the SNR shock front. The comparison shows that the ubiquitous polycyclic aromatic hydrocarbons of the interstellar medium are destroyed within the BEK, along with nearly 25% of the mass of graphite and silicate dust grains.

We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a five year period spanning the epochs between days [?]6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant and its radiative output is dominated by the interaction of the SN blast wave with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by emission from [?]180 K silicate dust, collisionally heated by the hot X-ray emitting gas with a temperature and density of [?]5 x 106 K and [?]3 x 104 cm-3, respectively. The mass of the radiating dust is [?]1.2 x 10-6 M on day 7554 and scales linearly with IR flux. Comparison of the IR data with the soft X-ray flux derived from Chandra observations shows that the IR-to-X-ray flux ratio, IRX, is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of IRX is most consistent with the scenario that very little grain processing or gas cooling has occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased by a factor of [?]5 with a time dependence of t'0.87+- 0.20, t' being the time since the first encounter between the blast wave and the ER. These observations are consistent with the transitioning of the blast wave from free expansion to a Sedov phase as it propagates into the main body of the ER, as also suggested by X-ray observations. The constant spectral shape of the IR emission provides strong constraints on the density and temperature of the shocked gas in which the interaction takes place. Silicate grains, with radii of [?]0.2 mm and temperature of T [?] 180 K, best fit the spectral and temporal evolution of the [?]8-30 mm data. The IR spectra also show the presence of a secondary population of very small, hot (T [?] 350 K), featureless dust. If these grains spatially coexist with the silicates, then they must have shorter lifetimes. The data show slightly different rates of increase of their respective fluxes, lending some support to this hypothesis. However, the origin of this emission component and the exact nature of its relation to the silicate emission is still a major unsolved puzzle.

Through the serendipitous discovery of infrared echoes around the Cas A supernova remnant, the Spitzer satellite has provided astronomers with a unique opportunity to study the properties of the echoing material and the history and nature of the outburst that generated these echoes. In retrospect, we find that the echoes are also clearly visible as infrared "hot spots" in IRAS images of the region. The spectra of the echoes are distinct from that of the dust in the general diffuse interstellar medium (ISM), revealing hot silicate grains that are either stochastically heated to temperatures in excess of ~150 K or radiating at an equilibrium temperature of this value. We show that the maximum luminosity that can be generated by the radioactive decay of 56Ni is not capable of producing such spectra, and could therefore not have given rise to the echoes. Instead, we find that the echoes must have been generated by an intense and short burst of EUV-UV radiation associated with the breakout of the shock through the surface of the exploding star. The inferred luminosity of the burst depends on the amount of attenuation in the intervening medium to the clouds, and we derive a burst luminosity of ~ 1.5 x 1011 L for an assumed H-column density of 1.5 x 1019 cm[?]2. The average H-column density of the IR-emitting region in the echoing clouds is about 5 x 1017 cm[?]2. Derivation of their density requires knowledge of the width of the echo that is sweeping through the ISM, which in turn is determined by the duration of the burst. A burst time of ~1 day gives a cloud density of ~400 cm[?]3, typical of dense IR cirrus.

Multiwavelength observations of supernova remnant 1987A show that its morphology and luminosity are rapidly changing at X-ray, optical, infrared (IR), and radio wavelengths as the blast wave from the explosion expands into the circumstellar equatorial ring, produced by mass loss from the progenitor star. The observed IR radiation arises from the interaction of dust grains that formed in mass outflow with the soft X-ray-emitting plasma component of the shocked gas. Spitzer Infrared Spectrograph spectra at 5-30 mm taken on day 6190 since the explosion show that the emission arises from ~ 1.1 x 10[?]6 M of silicate grains radiating at a temperature of ~ 180[?]15+20 K. Subsequent observations on day 7137 show that the IR flux had increased by a factor of 2 while maintaining an almost identical spectral shape. The observed IR-to-X-ray flux ratio (IRX) is consistent with that of a dusty plasma with standard Large Magellanic Cloud dust abundances. IRX has decreased by a factor of ~2 between days 6190 and 7137, providing the first direct observation of the ongoing destruction of dust in an expanding supernova blast wave on dynamic timescales. Detailed models consistent with the observed dust temperature, the ionization timescale of the soft X-ray emission component, and the evolution of IRX suggest that the radiating silicate grains are immersed in a 3.5 x 106 K plasma with a density of (0.3-1) x 104 cm[?]3 and have a size distribution that is confined to a narrow range of radii between 0.023 and 0.22 mm. Smaller grains may have been evaporated by the initial UV flash from the supernova.

Using the Gemini South 8 m telescope, we obtained high-resolution 11.7 and 18.3 mm mid-IR images of SN 1987A on day 6526 since the explosion. All the emission arises from the equatorial ring. Nearly contemporaneous spectra obtained at 5-38 mm with the Spitzer Space Telescope show that this is thermal emission from silicate dust that condensed out in the red giant wind of the progenitor star. The dust temperature is 166img1.gif K, and the emitting dust mass is 2.6img2.gif x 10-6 M. Comparison of the Gemini 11.7 mm image with Chandra X-ray images, HST UV-optical images, and ATCA radio synchrotron images shows generally good correlation across all wavelengths. If the dust resides in the diffuse X-ray-emitting gas then it is collisionally heated. The IR emission can then be used to derive the plasma temperature and density, which were found to be in good agreement with those inferred from the X-rays. Alternatively, the dust could reside in the dense UV-optical knots and be heated by the radiative shocks that are propagating through the knots. In either case the dust-to-gas mass ratio in the CSM around the supernova is significantly lower than that in the general interstellar medium of the LMC, suggesting either a low condensation efficiency in the wind of the progenitor star or the efficient destruction of the dust by the SN blast wave. Overall, we are witnessing the interaction of the SN blast wave with its surrounding medium, creating an environment that is rapidly evolving at all wavelengths.

The intensity of the diffuse ~1-4 mm sky emission from which solar system and Galactic foregrounds have been subtracted is in excess of that expected from energy released by galaxies and stars that formed during the z [?] 5 redshift interval.The spectral signature of this excess near-infrared background light (NIRBL) component is almost identical to that of reflected sunlight from the interplanetary dust cloud and could therefore be the result of the incomplete subtraction of this foreground emission component from the diffuse sky maps. Alternatively, this emission component could be extragalactic. Its spectral signature is consistent with that of redshifted continuum and recombination line emission from H II regions formed by the first generation of very massive stars. In this paper we analyze the implications of this spectral component for the formation rate of these Population III stars, the redshift interval during which they formed, the reionization of the universe, and evolution of collapsed halo masses. Assuming that these Population III stars are massive objects radiating at the Eddington luminosity and ending their lives by directly collapsing into black holes, we find that to reproduce the intensity and spectral shape of the NIRBL requires a peak star formation rate of ~2.5 M yr-1 Mpc-3, with a (1 + z)-2 dependence on redshift, until the epoch ends at redshifts z [?] 7-9. It requires a comoving luminosity density of about 2.7 x 1011 L Mpc-3, corresponding to a total energy input of 670-820 keV per baryon, and that about 10% of the total number of baryons in the universe be converted to Population III stars. All these numbers are higher by about a factor of 4-10 than those derived from models in which Population III stars form at a rate that is proportional to the collapse rate of halos in a cold dark matter dominated universe. Furthermore, an extragalactic origin for the NIRBL leads to physically unrealistic absorption-corrected spectra of distant TeV blazars. All these results suggest that Population III stars contribute only a fraction of the NIRBL intensity, with zodiacal light, star-forming galaxies, and/or nonnuclear sources giving rise to the remaining fraction. Further 0.1-10 mm observations of the diffuse sky and the zodiacal cloud are therefore crucial for resolving the true spectrum and origin of the NIRBL.

The 1-5 mm diffuse sky emission from which local foreground emission from the solar system and the Galaxy have been subtracted exceeds the brightness that can be attributed to normal star-forming galaxies. The nature of this excess near-infrared background light (NIRBL) is controversial. On the one hand, its sharp rise at ~1.25 mm has been interpreted as a distinct spectral feature created by the redshifted emission from primordial (Population III) stars that have formed at redshifts [?]8. On the other hand, the measured NIRBL spectrum is almost identical to that of the zodiacal cloud, raising the possibility that it is of local origin. Blazars can, in principle, offer a simple test for the nature and origin of the NIRBL. Very high energy g-ray photons emitted by these objects are attenuated en route to Earth by g-g interactions with the extragalactic background light (EBL). Assuming that the NIRBL is of extragalactic origin, its distinct spectral feature should give rise to a corresponding absorption feature in the observed g-ray spectra of these sources. This paper examines whether the extragalactic nature of the NIRBL can be determined from the analysis of the g-ray spectra of blazars. We calculate the g-ray opacity toward two blazars H1426+428 and PKS 2155-304 located at redshifts of z [?] 0.13 for several EBL scenarios with and without the alleged spectral signature of the Population III stars. We show that if the NIRBL is extragalactic, it may be very difficult to reproduce the absorption-corrected spectrum of PKS 2155-304 with a synchrotron self-Compton (SSC) model that is also consistent with the X-ray and EGRET g-ray data. This, and other arguments presented in this paper, cast serious doubts on the possibility that the NIRBL represents the spectral imprint of the first generation of stellar objects on the EBL.

We decompose the COBE/DIRBE observations of the near-IR sky brightness (minus zodiacal light) into Galactic stellar and interstellar medium (ISM) components and an extragalactic background. This empirical procedure allows us to estimate the 4.9 mm cosmic infrared background (CIB) as a function of the CIB intensity at shorter wavelengths. A weak indication of a rising CIB intensity at l > 3.5 mm hints at interesting astrophysics in the CIB spectrum, or warns that the foreground zodiacal emission may be incompletely subtracted. Subtraction of only the stellar component from the zodiacal-light-subtracted all-sky map reveals the clearest 3.5 mm ISM emission map, which is found to be tightly correlated with the ISM emission at far-IR wavelengths.

With high-sensitivity imaging arrays, accurate calibration is essential to achieve the limits of detection of space observatories. One can simultaneously extract information about the scene being observed and the calibration properties of the detector and imaging system from redundant dithered images of a scene. There are large differences in the effectiveness of dithering strategies for allowing the separation of detector properties from sky brightness measurements. In this paper, we quantify these differences by developing a figure of merit (FOM) for dithering procedures based on their usefulness for allowing calibration on all spatial scales. The figure of merit measures how well the gain characteristics of the detector are encoded in the measurements, and is independent of the techniques used to analyze the data. Patterns similar to the antenna arrangements of radio interferometers with good u-vplane coverage are found to have good figures of merit. We present patterns for both deep surveys of limited sky areas and for shallow surveys. By choosing a strategy that encodes the calibration in the observations in an easily extractable way, we enhance our ability to calibrate our detector systems and to reach the ultimate limits of sensitivity that are required to achieve the promise of many missions.

Near-infrared array detectors, like theJames Webb Space Telescope (JWST)NIRSpec's Teledyne's H2RGs, often provide reference pixels and a reference output. These are used to remove correlated noise. Improved reference sampling and subtraction (IRS2) is a statistical technique for using this reference information optimally in a least-squares sense. Compared with the traditional H2RG readout, IRS2uses a different clocking pattern to interleave many more reference pixels into the data than is otherwise possible. Compared with standard reference correction techniques, IRS2subtracts the reference pixels and reference output using a statistically optimized set of frequency-dependent weights. The benefits include somewhat lower noise variance and much less obvious correlated noise. NIRSpec's IRS2images are cosmetically clean, with less banding than in traditional data from the same system. This article describes the IRS2clocking pattern and presents the equations needed to use IRS2in systems other than NIRSpec. For NIRSpec, applying these equations is already an option in the calibration pipeline. As an aid to instrument builders, we provide our prototype IRS2calibration software and sample JWSTNIRSpec data. The same techniques are applicable to other detector systems, including those based on Teledyne's H4RG arrays. The H4RG's interleaved reference pixel readout mode is effectively one IRS2pattern.