Hubble Space Telescope-Cosmic Origins Spectrograph Research Articles

Transient obscuration event captured in NGC 3227

From Swift monitoring of a sample of active galactic nuclei (AGN) we found a transient X-ray obscuration event in Seyfert-1 galaxy NGC 3227, and thus triggered our joint XMM-Newton, NuSTAR, and Hubble Space Telescope (HST) observations to study this event. Here in the first paper of our series we present the broadband continuum modelling of the spectral energy distribution for NGC 3227, extending from near infrared (NIR) to hard X-rays. We use our new spectra taken with XMM-Newton, NuSTAR, and the HST Cosmic Origins Spectrograph in 2019, together with archival unobscured XMM-Newton, NuSTAR, and HST Space Telescope Imaging Spectrograph data, in order to disentangle various spectral components of NGC 3227 and recover the underlying continuum. We find the observed NIR-optical-UV continuum is explained well by an accretion disk blackbody component (Tmax = 10 eV), which is internally reddened by E(B − V) = 0.45 with a Small Magellanic Cloud extinction law. We derive the inner radius (12 Rg) and the accretion rate (0.1 M⊙ yr−1) of the disk by modelling the thermal disk emission. The internal reddening in NGC 3227 is most likely associated with outflows from the dusty AGN torus. In addition, an unreddened continuum component is also evident, which likely arises from scattered radiation, associated with the extended narrow-line region of NGC 3227. The extreme ultraviolet continuum, and the ‘soft X-ray excess’, can be explained with a ‘warm Comptonisation’ component. The hard X-rays are consistent with a power-law and a neutral reflection component. The intrinsic bolometric luminosity of the AGN in NGC 3227 is about 2.2 × 1043 erg s−1 in 2019, corresponding to 3% Eddington luminosity. Our continuum modelling of the new triggered data of NGC 3227 requires the presence of a new obscuring gas with column density NH = 5 × 1022 cm−2, partially covering the X-ray source (Cf = 0.6).

Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

AbstractWe present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H2O2 are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance.

The XMM-Newton/HST View of the Obscuring Outflow in the Seyfert Galaxy Mrk 335 Observed at Extremely Low X-Ray Flux

Abstract The Seyfert galaxy Mrk 335 is known for its frequent changes of flux and spectral shape in the X-ray band that occurred during recent years. These variations may be explained by the onset of a wind that previous, noncontemporaneous high-resolution spectroscopy in X-ray and UV bands located at accretion disk scale. A simultaneous new campaign by XMM-Newton and the Hubble Space Telescope (HST) caught the source at a historically low flux in the X-ray band. The soft X-ray spectrum is dominated by prominent emission features and by the effect of a strong ionized absorber with an outflow velocity of (5–6) × 103 km s−1. The broadband spectrum obtained by the EPIC-pn camera reveals the presence of an additional layer of absorption by gas at moderate ionization covering ∼80% of the central source, as well as tantalizing evidence for absorption in the Fe K band outflowing at the same velocity of the soft X-ray absorber. The HST Cosmic Origins Spectrograph spectra confirm the simultaneous presence of broad absorption troughs in C iv, Lyα, Lyβ, and O vi, with velocities of the order of 5000 km s−1 and covering factors in the range of 20%–30%. Comparison of the ionic column densities and of other outflow parameters in the two bands shows that the X-ray and UV absorbers are likely originated by the same gas. The resulting picture from this latest multiwavelength campaign confirms that Mrk 335 undergoes the effect of a patchy, medium-velocity outflowing gas in a wide range of ionization states that seem to be persistently obscuring the nuclear continuum.

FUV line emission, gas kinematics, and discovery of [Fe XXI] λ1354.1 in the sightline toward a filament in M87

We present new Hubble Space Telescope – Cosmic Origins Spectrograph (HST-COS) G130M spectroscopy which we have obtained for a sightline toward a filament projected 1.9 kpc from the nucleus of M87, near the edge of the inner radio lobe to the east of the nucleus. The combination of the sensitivity of COS and the proximity of M87 allows us to study the structure of this filament in unparalleled detail. We propose that the filament is composed of many cold clumps, each surrounded by an FUV-emitting boundary layer, with the filament having a radius rc ~ 10 pc and the clumps filling the cylinder with a low volume filling factor. The observed velocity dispersion in emission lines from the filament results from the random motions of these clumps within the filament. We have measured fluxes and kinematics for emission lines of Lyα, C II λ1335, and N V λ1238, finding vr = 147 ± 2 km s−1, 138 ± 18 km s−1, and 148−16+14 km s−1 relative to M87, and line broadenings σr = 171 ± 2 km s−1, 189−11+12 km s−1, and 128−17+23 km s−1 respectively. We associate these three lines, as well as archival measurements of Hα, C IV λ1549, and He II λ1640, with a multitemperature boundary layer around clumps which are moving with supersonic random motions in the filament. This boundary layer is a significant coolant of the hot gas. We show that the [C II] λ158 μm flux observed by Herschel-PACS from this region implies the existence of a massive cold (T ~ 103 K) component in the filament which contains significantly more mass (M ~ 8000 M⊙ within our r ≈ 100 pc sightline) than the FUV-emitting boundary layer. It has about the same bulk velocity and velocity dispersion as the boundary layer. We also detect [Fe XXI] λ1354 in emission at 4−5σ. This line is emitted from 1 keV (T ≈ 107 K) plasma, and we use it to measure the bulk radial velocity vr = −92−22+34 km s−1 and velocity dispersion σr = 69−27+79 km s−1 of the plasma at this temperature. In contrast to the intermediate-temperature FUV lines, [Fe XXI] is blueshifted relative to M87 and matches the bulk velocity of a nearby filament to the south. We hypothesize that this line arises from the approaching face of the radio bubble expanding through this sightline, while the filament lies on the receding side of the bubble. A byproduct of our observations is the detection of absorption from interstellar gas in our Galaxy, observed in C II λ1335 and Lyα.

The Ultra-fast Outflow of the Quasar PG 1211+143 as Viewed by Time-averaged Chandra Grating Spectroscopy

Abstract We present a detailed X-ray spectral study of the quasar PG 1211+143 based on Chandra High Energy Transmission Grating Spectrometer (HETGS) observations collected in a multi-wavelength campaign with UV data using the Hubble Space Telescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the Jansky Very Large Array (VLA). We constructed a multi-wavelength ionizing spectral energy distribution using these observations and archival infrared data to create xstar photoionization models specific to the PG 1211+143 flux behavior during the epoch of our observations. Our analysis of the Chandra-HETGS spectra yields complex absorption lines from H-like and He-like ions of Ne, Mg, and Si, which confirm the presence of an ultra-fast outflow (UFO) with a velocity of approximately −17,300 km s−1 (outflow redshift z out ∼ −0.0561) in the rest frame of PG 1211+143. This absorber is well described by an ionization parameter and column density . This corresponds to a stable region of the absorber’s thermal stability curve, and furthermore its implied neutral hydrogen column is broadly consistent with a broad Lyα absorption line at a mean outflow velocity of approximately −16,980 km s−1 detected by our HST-COS observations. Our findings represent the first simultaneous detection of a UFO in both X-ray and UV observations. Our VLA observations provide evidence for an active jet in PG 1211+143, which may be connected to the X-ray and UV outflows; this possibility can be evaluated using very-long-baseline interferometric observations.

Discovery of an H i-rich Gas Reservoir in the Outskirts of SZ-effect-selected Clusters

Abstract We report on the detection of three strong H i absorbers originating in the outskirts (i.e., impact parameter, (1.6–4.7)r 500) of three massive ( M ⊙) clusters of galaxies at redshift , in the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) spectra of three background UV-bright quasars. These clusters were discovered by the 2500 deg2 South Pole Telescope Sunyaev–Zel’dovich (SZ) effect survey. All three COS spectra show a partial Lyman limit absorber with N(H i) > 1016.5 cm−2 near the photometric redshifts ( ) of the clusters. The compound probability of the random occurrence of all three absorbers is <0.02%, indicating that the absorbers are most likely related to the targeted clusters. We find that the outskirts of these SZ-selected clusters are remarkably rich in cool gas compared to existing observations of other clusters in the literature. The effective Doppler parameters of the Lyman series lines, obtained using a single-cloud curve-of-growth (COG) analysis, suggest a nonthermal/turbulent velocity of a few×10 km s−1 in the absorbing gas. We emphasize the need for uniform galaxy surveys around these fields and for more UV observations of quasar–cluster pairs in general in order to improve the statistics and gain further insights into the unexplored territory of the largest collapsed cosmic structures.

SPECTROSCOPY FROM THE HUBBLE SPACE TELESCOPE COSMIC ORIGINS SPECTROGRAPH OF THE SOUTHERN NOVA-LIKE BB DORADUS IN AN INTERMEDIATE STATE

ABSTRACT We present a spectral analysis of the spectrum from the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) of the southern VY Scl nova-like variable BB Doradus, obtained as part of a Cycle 20 HST/COS survey of accreting white dwarfs (WDs) in cataclysmic variables. BB Dor was observed with COS during an intermediate state with a low mass accretion rate, thereby allowing an estimate of the WD temperature. The results of our spectral analysis show that the WD is a significant far-ultraviolet (FUV) component of the spectrum with a temperature of about 35,000–50,000 K, assuming a WD mass of ( ). The disk, with a mass accretion rate of , contributes about 1/5 to 1/2 of the FUV flux.

AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

ABSTRACT We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright active galactic nuclei (AGNs) at redshifts z AGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H i and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths Δz H i = 21.7 and Δz O vi = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H i systems can be parametrized d  ( > N ) / dz ?> = C 14 ( N / 10 14 cm − 2 ) − ( β − 1 ) ?> , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ ( 1 + z ) 2.3 ± 0.1 ?> , and slope β(z) = 1.75–0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N H I ?> . The distribution of O vi absorbers appears to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H i absorbers on scales of Δv = 50–300 km s−1 with no significant clustering at Δv ≳ 1000 km s−1. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N H i ≳ 1013.5 cm−2, metal-bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at http://archive.stsci.edu/prepds/igm/.

CO/H2ABUNDANCE RATIO ≈ 10–4IN A PROTOPLANETARY DISK

The relative abundances of atomic and molecular species in planet-forming disks around young stars provide important constraints on photochemical disk models and provide a baseline for calculating disk masses from measurements of trace species. A knowledge of absolute abundances, those relative to molecular hydrogen (H2), are challenging because of the weak rovibrational transition ladder of H$_{2}$ and the inability to spatially resolve different emission components within the circumstellar environment. To address both of these issues, we present new contemporaneous measurements of CO and H2 absorption through the "warm molecular layer" of the protoplanetary disk around the Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode of the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2 absorption in this region for the first time. An analysis of the emission and absorption spectrum of RW Aur shows components from the accretion region near the stellar photosphere, the molecular disk, and several outflow components. The warm H2 and CO absorption lines are consistent with a disk origin. We model the 1092-1117A spectrum of RW Aur to derive log10 N(H2)~=~19.90$^{+0.33}_{-0.22}$ at T$_{rot}$(H2) ~=~440~+/-~39 K. The CO $A$~--~$X$ bands observed from 1410-1520A are best fit by log10 N(CO)~=~16.1~$^{+0.3}_{-0.5}$ at T$_{rot}$(CO) ~=~200$^{+650}_{-125}$ K. Combining direct measurements of the HI, H2, and CO column densities, we find a molecular fraction in the warm disk surface of $f_{H2}$~>=~0.47 and derive a molecular abundance ratio of CO/H2~=~1.6$^{+4.7}_{-1.3}$~x~10$^{-4}$, both consistent with canonical interstellar dense cloud values.

Synergistic observations of Io’s atmosphere in 2010 from HST–COS in the mid-ultraviolet and IRTF–TEXES in the mid-infrared

We report on mid-UV spectroscopy of Io’s SO2 atmosphere from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), and compare to contemporaneous ground-based mid-infrared spectroscopy of the atmosphere. Our motivation is to evaluate the consistency of atmospheric parameters derived by different observational techniques, and in particular to verify the atmospheric densities and longitudinal distribution derived from the disk-integrated mid-IR observations, which depend on assumptions about the atmospheric temperature and distribution. Six disk-integrated COS observations, evenly spaced over all Io longitudes, were taken in October of 2010. Spectra were obtained using the G225M grating, with wavelengths ranging from 2100Å to 2340Å. At these wavelengths, the SO2 absorption signature of Io’s atmosphere is seen in reflected sunlight. The spectra were fitted using a latitude-dependent atmospheric spatial distribution constrained by earlier disk-resolved observations, with SO2 column density, temperature, and SO abundance as variables. Equatorial SO2 column densities of between 0.26 and 1.28×1017cm−2 were derived, depending on Io’s central longitude. We compare these results with disk-averaged 19μm mid-IR spectroscopy in thermal emission from the NASA Infrared Telescope Faculty, taken at multiple central longitudes only four months prior to the mid-UV observations. The derived SO2 column abundances from the ultraviolet and mid-IR observations agree to within the uncertainties at comparable longitudes, confirming our earlier mid-IR derived abundances. In addition, both show the large longitudinal variations of atmospheric density seen previously in the mid-IR and Lyman-α observations, but never before mapped systematically in the mid-UV. Best-fit UV-derived atmospheric temperatures, constrained by UV band shape, were up to 200K, higher than the <150K temperatures derived from the IR data, though even the best-fit temperatures did not fit the UV band shapes well. Models in which the atmosphere was assumed to be in vapor pressure equilibrium with low thermal inertia SO2 frost, with column abundance thus depending only on distance from the sub-solar point, could not fit the UV or IR data without invoking very high sub-solar densities that are inconsistent with other observational constraints. Instead, the apparent latitude-dependent atmospheric distribution suggests support by some combination of low-latitude volcanoes and high thermal inertia frost. An upper limit SO abundance of 2.7% was derived from the COS data.

ON THE REDSHIFT OF THE VERY HIGH ENERGY BLAZAR 3C 66A

As a bright gamma-ray source, 3C 66A is of great interest to the high-energy astrophysics community, having a potential for placing cosmological constraints on models for the extragalactic background light (EBL) and the processes which contribute to this photon field. No firm spectroscopic redshift measurement has been possible for this blazar due to a lack of intrinsic emission and absorption features in optical spectra. We present new far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 3C 66A covering the wavelength range 1132 to 1800 Angstroms . The data show a smooth continuum with intergalactic medium absorption features which can be used to place a firm lower limit on the blazar redshift of z>0.3347. An upper limit is set by statistically treating the non-detection of additional absorbers beyond z=0.3347, indicating a redshift of less than 0.41 at 99% confidence and ruling out z>0.444 at 99.9% confidence. We conclude by showing how the redshift limits derived from the COS spectra remove the potential for this gamma-ray emitting blazar to place an upper limit on the flux of the EBL using high energy data from a flare in October of 2009.

HST-COS OBSERVATIONS OF HYDROGEN, HELIUM, CARBON, AND NITROGEN EMISSION FROM THE SN 1987A REVERSE SHOCK

We present the most sensitive ultraviolet observations of Supernova 1987A to date. Imaging spectroscopy from the Hubble Space Telescope-Cosmic Origins Spectrograph shows many narrow (dv \sim 300 km/s) emission lines from the circumstellar ring, broad (dv \sim 10 -- 20 x 10^3 km/s) emission lines from the reverse shock, and ultraviolet continuum emission. The high signal-to-noise (> 40 per resolution element) broad LyA emission is excited by soft X-ray and EUV heating of mostly neutral gas in the circumstellar ring and outer supernova debris. The ultraviolet continuum at \lambda > 1350A can be explained by HI 2-photon emission from the same region. We confirm our earlier, tentative detection of NV \lambda 1240 emission from the reverse shock and we present the first detections of broad HeII \lambda1640, CIV \lambda1550, and NIV] \lambda1486 emission lines from the reverse shock. The helium abundance in the high-velocity material is He/H = 0.14 +/- 0.06. The NV/H-alpha line ratio requires partial ion-electron equilibration (T_{e}/T_{p} \approx 0.14 - 0.35). We find that the N/C abundance ratio in the gas crossing the reverse shock is significantly higher than that in the circumstellar ring, a result that may be attributed to chemical stratification in the outer envelope of the supernova progenitor. The N/C abundance ratio may have been stratified prior to the ring expulsion, or this result may indicate continued CNO processing in the progenitor subsequent to the expulsion of the circumstellar ring.

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H2AND ACCRETION SHOCKS

We present deep spectroscopic observations of the classical T Tauri stars DF Tau and V4046 Sgr in order to better characterize two important sources of far-ultraviolet continuum emission in protoplanetary disks. These new Hubble Space Telescope-Cosmic Origins Spectrograph observations reveal a combination of line and continuum emission from collisionally excited H2 and emission from accretion shocks. H2 is the dominant emission in the 1400-1650 A band spectrum of V4046 Sgr, while an accretion continuum contributes strongly across the far-ultraviolet spectrum of DF Tau. We compare the spectrum of V4046 Sgr to models of electron-impact induced H2 emission to constrain the physical properties of the emitting region, after making corrections for attenuation within the disk. We find reasonable agreement with the broad spectral characteristics of the H2 model, implying N(H2) ~ 10^{18} cm^{-2}, T(H2) = 3000^{+1000}_{-500} K, and a characteristic electron energy in the range of ~ 50 - 100 eV. We propose that self-absorption and hydrocarbons provide the dominant attenuation for H2 line photons originating within the disk. For both DF Tau and V4046 Sgr, we find that a linear fit to the far-UV data can reproduce near-UV/optical accretion spectra. We discuss outstanding issues concerning how these processes operate in protostellar/protoplanetary disks, including the effective temperature and absolute strength of the radiation field in low-mass protoplanetary environments. We find that the 912-2000A continuum in low-mass systems has an effective temperature of ~10^{4} K with fluxes 10^{5-7} times the interstellar level at 1 AU.

HST-COS observations of the transiting extrasolar planetary system HD 209458b

We summarize results from deep spectroscopic observations of the HD 209458 planetary system, carried out with the Hubble Space Telescope—Cosmic Origins Spectrograph. Orbitally resolved observations are used to show that hot gas emission lines, arising only in the stellar atmosphere, are not variable, while lower ionizations species found in the upper atmosphere of the hot Jupiter HD 209458b absorb stellar photons during transit. For both C II and Si III, we find mean transit attenuation of ∼8%. The firm detection of silicon is in direct conflict with previous low-resolution studies, which we attribute to long-term variability in the system. We also use these observations to search for auroral emission from the planet, detecting a statistically significant emission feature at 1582 A that is consistent with H2 photoexcited by stellar O I photons.

METAL DEPLETION AND WARM H2IN THE BROWN DWARF 2M1207 ACCRETION DISK

We present new far-ultraviolet observations of the young M8 brown dwarf 2MASS J12073346-3932539, which is surrounded by an accretion disk. The data were obtained using the Hubble Space Telescope-Cosmic Origins Spectrograph. Moderate resolution spectra (R~17,000-18,000) obtained in the 1150-1750 A and 2770-2830 A bandpasses reveal H2 emission excited by HI Ly$\alpha$ photons, several ionization states of carbon (CI - CIV), and hot gas emission lines of HeII and NV (T ~ 10^4-5 K). Emission from some species that would be found in a typical thermal plasma at this temperature (SiII, SiIII, SiIV, and MgII) are not detected. The non-detections indicate that these refractory elements are depleted into grains, and that accretion shocks dominate the production of the hot gas observed on 2MASS J12073346-3932539. We use the observed CIV luminosity to constrain the mass accretion rate in this system. We use the kinematically broadened H2 profile to confirm that the majority of the molecular emission arises in the disk, measure the radius of the inner hole of the disk (R_{hole}~3R_{*}), and constrain the physical conditions of the warm molecular phase of the disk (T(H2)~2500-4000 K). A second, most likely unresolved H2 component is identified. This feature is either near the stellar surface in the region of the accretion shock or in a molecular outflow, although the possibility that this Jovian-like emission arises on the day-side disk of a 6 M_{J} companion (2M1207b) cannot be conclusively ruled out. In general, we find that this young brown dwarf disk system is a low-mass analog to classical T Tauri stars that are observed to produce H2 emission from a warm layer in their disks, such as the well studied TW Hya and DF Tau systems.

SEARCHING FOR FAR-ULTRAVIOLET AURORAL/DAYGLOW EMISSION FROM HD 209458b

We present recent observations from the Hubble Space Telescope-Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD 209458b. We obtained medium-resolution (R ∼ 20,000) far-ultraviolet (1150–1700 Å) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8σ) feature at +15(± 20) km s−1 in the rest frame of the planet, at λ1582 Å. This feature is consistent with emission from H2 B–X (2–9) P(4) (λrest = 1581.11 Å); however, the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short-period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres.