Narrow Absorption Features Research Articles

Discovery of X-ray absorption lines from the low-mass X-ray binaries 4U 1916-053 and X 1254-690 with XMM-Newton

We report the discovery of narrow X-ray absorption features from the two dipping low-mass X-ray binary 4U 1916-053 and X 1254-690 during XMM-Newton observations. The features detected are identified with resonant scattering absorption lines of highly ionized iron (Fe XXV and Fe XXVI). Resonant absorption features are now observed in a growing number of low-mass X-ray binaries (LMXBs): the two superluminal jet sources GRS 1915+105 and GRO J1655-40, the bright LMXB GX 13+1 and the four dipping sources MXB 1658-298, X 1624-490, 4U 1916-053 and X 1254-690. The early hypothesis that their origin could be related to the presence of superluminal jets is thus ruled out. Ionized absorption features may be common characteristics of accreting systems. Furthermore, their presence may depend on viewing angle, as suggested by their detection in dippers which are viewed close to the disk plane, and by the fact that GRS 1915+105, GRO J1655-40 and GX 13+1, although not dippers, are suspected to be also viewed at high inclination.

Probing the Early Stages of Low‐Mass Star Formation in LDN 1689N: Dust and Water in IRAS 16293−2422A, B, and E

We present deep images of dust continuum emission at 450, 800, and 850 mum of the dark cloud LDN 1689N, which harbors the low-mass young stellar objects (YSOs) IRAS 16293-2422 A and B (I16293A and I16293B) and the cold prestellar object I16293E. Toward the positions of I16293A and I16293E we also obtained spectra of CO-isotopomers and deep submillimeter observations of chemically related molecules with high critical densities (HCO+, (HCO+)-C-13, DCO+, H2O, HDO, and H2D+). Toward I16293A we report the detection of the HDO 1(01)-0(00) and H2O 1(10-)1(01) ground-state transitions as broad self-reversed emission profiles with narrow absorption and a tentative detection of H2D+ 1(10)-1(11). Toward I16293E we detect weak emission of subthermally excited HDO 1(01)-0(00). Based on this set of submillimeter continuum and line data, we model the envelopes around I16293A and I16293E. The density and velocity structure of I16293A is fitted by an inside-out collapse model, yielding a sound speed of a 0: 7 kms(-1), an age of t = (0.6-2.5) x 10(4) yr, and amass of 6.1 M.. The density in the envelope of I16293E is fitted by a radial power law with index -1.0 +/- 0.2, a mass of 4.4 M., and a constant temperature of 16 K. These respective models are used to study the chemistry of the envelopes of these pre- and protostellar objects. We made a large, fully sampled CO J = 2-1 map of LDN 1689N, which clearly shows the two outflows from I16293A and I16293B and the interaction of one of the flows with I16293E. An outflow from I16293E reported elsewhere is not confirmed. Instead, we find that the motions around I16293E identified from small maps are part of a larger scale fossil flow from I16293B. Modeling of the I16293A outflow shows that the broad HDO, water ground state, and CO J = 6-5 and 7-6 emission lines originate in this flow, while the HDO and H2O line cores originate in the envelope. The narrow absorption feature in the ground-state water lines is due to cold gas in the outer envelope. The derived H2O abundance is 3 x 10(-9) in the cold regions of the envelope of I16293A (T-kin 14 K), and 10(-8) in the outflow. The HDO abundance is constant at a few times 10(-10) throughout the envelopes of I16293A and I16293E. Because the derived H2O and HDO abundances in the two objects can be understood through shock chemistry in the outflow and ion-molecule chemistry in the envelopes, we argue that both objects are related in chemical evolution. The [HDO]/[H2O] abundance ratio in the warm inner envelope of I16293A of a few times 10(-4) is comparable to that measured in comets. This supports the idea that the [HDO]/[ H2O] ratio is determined in the cold prestellar core phase and conserved throughout the formation process of low-mass stars and planets.

ChandraX‐Ray Observations of NGC 4258: Iron Absorption Lines from the Nucleus

We report sub-arcsecond resolution X-ray imaging spectroscopy of the low luminosity active galactic nucleus of NGC 4258 and its immediate surroundings with the Chandra X-ray Observatory. NGC 4258 was observed four times, with the first two observations separated by one month, followed over a year later by two consecutive observations. The spectrum of the nucleus is well described by a heavily absorbed, hard X-ray power law of variable luminosity, plus a constant, thermal soft X-ray component. We do not detect an iron K alpha emission line with the upper limit to the equivalent width of a narrow, neutral iron line ranging between 94 and 887 eV (90% confidence) for the different observations. During the second observation on 2000-04-17, two narrow absorption features are seen with >99.5% confidence at ~6.4 keV and ~6.9 keV, which we identify as resonant absorption lines of Fe XVIII - Fe XIX K alpha and Fe XXVI K alpha, respectively. In addition, the 6.9 keV absorption line is probably variable on a timescale of ~6000 sec. The absorption lines are analyzed through a curve of growth analysis, which allows the relationship between ionic column and kinematic temperature or velocity dispersion to be obtained for the observed equivalent widths. We discuss the properties of the absorbing gas for both photo and collisionally ionized models. Given that the maser disk is viewed at an inclination i = 82 degrees, the gas responsible for the 6.9 keV absorption line may be in an inner disk, a disk-wind boundary layer or be thermal gas entrained at the base of the jet. The gas which gives rise to the photoelectric absorption may be the same as that which causes the 6.4 keV Fe K alpha absorption provided that the gas has a bulk velocity dispersion of a few thousand km/s. (Abstract abridged).

Production and structural characterization of carbon soot with narrow UV absorption feature

In the present study, by a simple method, carbon soot with a narrow absorption features at around 220 nm was produced by pyrolysis of methane gas at a hot (1400–2800 °C) filament. A detailed correlation between the structure and the optical spectra of carbon soot was carried out using a high-resolution transmission electron microscope. It appears that the origin of the narrow absorption peak is an onion-like graphitic structure. Interestingly, we found that the absorption peak position and the size of the soot can be controlled by the production temperature. The absorption peak is located at 217.5 nm, i.e., at the position of an intense interstellar absorption, when the onion structure has about 5 nm diameter. Infrared spectroscopy was applied to further elucidate the chemical composition of the obtained soot.

Discovery of narrow X-ray absorption features from the low-mass X-ray binary X 1254-690 with XMM-Newton

We report on two XMM-Newton observations of the low-mass X-ray binary X 1254-690. During an XMM-Newton observation of the low-mass X-ray binary in 2001 January a deep X-ray dip was seen while in a second observation one year later no dips were evident. The 0.5–10 keV EPIC spectra from both non-dipping intervals are very similar being modeled by a disk-blackbody and a power-law continuum with additional structure around 1 keV and narrow absorption features at 7.0 keV and 8.2 keV which are identified with the Kα and Kβ absorption lines of Fe xxvi. The low-energy structure may be modeled as a 175 eV (σ) wide emission line at ~0.95 keV. This feature is probably the same structure that was modeled as an absorption edge in an earlier BeppoSAX observation. The absorption line properties show no obvious dependence on orbital phase and are similar in both observations suggesting that the occurrence of such features is not directly related to the presence of dipping activity. Narrow Fe absorption features have been observed from the two superluminal jet sources GRO J1655-40 and GRS 1915+105, and the four low-mass X-ray binaries GX 13+1, MXB 1658-298, X 1624-490 and X 1254-690. Since the latter 3 sources are dipping sources, which are systems viewed close to the accretion disk plane, and the two microquasars are thought to be viewed at an inclination of ~70°, this suggests that these features are more prominent when viewed at high-inclination angles. This, together with the lack of any orbital dependence, implies a cylindrical geometry for the absorbing material.

Hubble Space TelescopeSpectroscopy of the Unexpected 2001 July Outburst of the Dwarf Nova WZ Sagittae

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph E140M spectra of the dwarf nova WZ Sge, following the early superoutburst of 2001 July. Our four far-ultraviolet (FUV) spectra, obtained over a time span of 4 months, monitor changes in the hot component of the system during the decline phase. The spectra cover the wavelength interval 1150-1708 Å. They reveal Stark-broadened Lyα and He II (1640) absorption and absorption lines due to metals (Si, C, N, Al) from a range of ionization states. Single-temperature white dwarf models provide reasonable qualitative agreement with the HST spectra. We find that the white dwarf appears to dominate the spectra from October through December. However, it is not clear that the absorption lines of metals form in the white dwarf photosphere. Therefore, the derived abundances and rotational velocity must be viewed with caution. Only a modest improvement in the fits to the data results when an accretion belt component is included. If the FUV spectra arise from the white dwarf alone, then we measure a cooling in response to the outburst of at least 11,000 K (29,000-18,000 K). The absence of broad underlying absorption features due to metals at this stage suggests slow rotation (~200 km s-1). It is possible that the white dwarf envelope has expanded due to the heating by the outburst or that the relatively narrow absorption features we observe are forming in an inflated disk atmosphere or curtain associated with the outburst.

Detection of OH Absorption against PSR B1849+00

We have searched for OH absorption against seven pulsars using the Arecibo telescope. In both OH main lines (at 1665 and 1667 MHz), deep and narrow absorption features were detected toward PSR B1849+00. In addition, we have detected several absorption and emission features against B33.6+0.1, a nearby supernova remnant (SNR). The most interesting result of this study is that a pencil-sharp absorption sample against the PSR differs greatly from the large-angle absorption sample observed against the SNR. If both the PSR and the SNR probe the same molecular cloud, then this finding has important implications for absorption studies of the molecular medium, since it shows that the statistics of absorbing OH depends on the size of the background source. We also show that the OH absorption against the PSR most likely originates from a small ( 105 cm-3) molecular clump.

HiNarrow Self‐Absorption in Dark Clouds

We have used the Arecibo telescope to carry out a survey of 31 dark clouds in the Taurus/Perseus region for narrow absorption features in H I (21 cm) and OH (1667 and 1665 MHz) emission. We detected H I narrow self-absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH emission, observed simultaneously, are remarkably well correlated. Spectrally, they have the same nonthermal line width and the same line centroid velocity. Spatially, they both peak at the optically selected central position of each cloud, and both fall off toward the cloud edges. Sources with clear HINSA features have also been observed in transitions of CO, 13CO, C18O, and C I. HINSA exhibits better correlation with molecular tracers than with C I. The line width of the absorption feature, together with analyses of the relevant radiative transfer, provides upper limits to the kinetic temperature of the gas producing the HINSA. Some sources must have a temperature close to or lower than 10 K. The correlation of column densities and line widths of HINSA with those characteristics of molecular tracers suggests that a significant fraction of the atomic hydrogen is located in the cold, well-shielded portions of molecular clouds and is mixed with the molecular gas. The average number density ratio [H I]/[H2] is 1.5 × 10-3. The inferred H I density appears consistent with, but slightly higher than, the value expected in steady state equilibrium between formation of H I via cosmic-ray destruction of H2 and destruction via formation of H2 on grain surfaces. The distribution and abundance of atomic hydrogen in molecular clouds are critical tests of dark cloud chemistry and structure, including the issues of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.

The Effect of Vacuum Polarization and Proton Cyclotron Resonances on Photon Propagation in Strongly Magnetized Plasmas

We consider the effects of vacuum polarization and proton cyclotron resonances on the propagation of radiation through a strongly magnetized plasma. We analyze the conditions under which the photons evolve adiabatically through the resonant density and find that the adibaticity condition is satisfied for most photon energies of interest, allowing for a normal-mode treatment of the photon propagation. We then construct radiative equilibrium atmosphere models of strongly magnetized neutron stars that includes these effects, employing a new numerical method that resolves accurately the sharp changes of the absorption and mode-coupling cross sections at the resonant densities. We show that the resulting spectra are modified by both resonances and are harder at all field strengths than a blackbody at the effective temperature. We also show that the narrow absorption features introduced by the proton cyclotron resonance have small equivalent widths. We discuss the implications of our results for properties of thermal emission from the surfaces of young neutron stars.

Spectroscopic studies of AGN

High resolution X-ray spectra of AGN have been obtained for the first time due to the launch of the Chandra and XMM-Newton X-ray observatories. This paper is focussed upon the X-ray absorption properties of AGN as observed with the grating spectrometers of both satellites. It is shown that absorption line studies provide a powerful tool to study the properties of the outflowing warm winds around the active nucleus. Up to now no narrow absorption features have been identified in BL Lac objects, while Seyfert 1 galaxies show rich absorption spectra, with tens of lines from 15–20 different ions. Contrary, the first Seyfert 2 spectrum shows only emission lines, whose X-ray properties are consistent with the expected fluxes from Seyfert 1 galaxies viewed from a different angle.

Discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 with XMM-Newton

We report the discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 during an XMM-Newton observation in 2001 February. The features are identified with the K alpha absorption lines of Fe xxv and Fe xxvi and have energies of 6.72 +/- 0.03 keV and 7.00 +/- 0.02 keV and equivalent widths (EWs) of -7.5 +1.7 -6.3 eV and -16.6 +1.9 -5.9 eV, respectively. The EWs show no obvious dependence on orbital phase, except during a dip, and correspond to a column of greater than 10^17.3 Fe atom /cm2. In addition, faint absorption features tentatively identified with Ni xxvii K alpha and Fe xxvi K beta may be present. A broad emission feature at 6.58 +0.07 -0.04 keV with an EW of 78 +19 -6 eV is also evident. This is probably the 6.4 keV feature reported by earlier missions since fitting a single Gaussian to the entire Fe-K region gives an energy of 6.39 +0.03 -0.04 keV. A deep absorption feature is present during the dip with an energy consistent with Fe xxv K alpha. This is the second dipping LMXRB source from which narrow Fe absorption features have been observed. Until recently the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized absorption features may be common characteristics of accreting systems with accretion disks.

Laser induced fluorescence spectra of the D 1∑u+→B′ 1∑g+ and C 1Πg→A 1∏u systems of C2 in solid Ne

In our study of carbon vapor molecules trapped in Ne matrices at 6 K, we observed laser induced fluorescence spectra of the D 1∑u+→B′ 1∑g+ system of C2 upon excitation of the Mulliken transition D 1∑u+←X 1∑g+ of C2 at 232 nm. A vibrational progression was clearly observed going from the upper vibrational v′=0 level of the electronic D state into several lower v″=0–6 levels of the B′ state. We found that the progression spans from 359 to 517 nm with a large Franck–Condon shift showing the intensity maximum for the 0–2 or 0–3 transition. The vibrational constants for the B′ state were derived as ωe=1427 cm−1 and ωexe=2.1 cm−1. We also observed the C→A and the Swan band emissions, showing that from the excited D state also additional states were populated by internal conversion and intersystem crossing. The constants for the A state were derived as ωe=1613.5 cm−1 and ωexe=18.5 cm−1. Fluorescence excitation spectra of the D→B′ vibronic transitions were found to reproduce well the relatively narrow absorption feature of the Mulliken D←X system. We discuss the entirely different excitation spectra occurring in argon matrices in terms of matrix-site effects. Each vibronic band of the D→B′ progression shows fine structures probably due to translations and librations of C2 molecules coupled with the surrounding lattice of Ne atoms.

ITAL]Hubble Space Telescope[/ITAL] STIS Spectroscopy of VW Hydri during Early Quiescence following a Superoutburst

Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) observations of VW Hydri 2 and 7 days after the end of a superoutburst reveal a heated white dwarf with deep broad Lyα, narrow metallic absorption features and evidence of a hotter Keplerian-broadened component. We confirm the existence of enhanced abundances of odd-numbered nuclear species P, Mn, and Al as well as an N/C ratio indicative of CNO H-burning thermonuclear processing. Our best single-temperature white dwarf reduced χ2 fit to the first spectrum reveals (1) a DAZQ white dwarf with Teff = 22,500 ± 500 K, log g = 8.0, and photospheric abundances C = 0.3 solar, N = 3.0 solar, O = 3.0 solar, Si = 0.3 solar, Al = 2 solar, Fe = 0.5 solar, Mg = 3.0 solar, Mn = 50 solar, Ni = 0.3 solar, P = 15 solar, and Ti = 0.1 solar. The best-fit white dwarf + accretion belt composite model yields a large improvement in the reduced χ2 value. The accretion belt temperature is 32,000 K and covers a fractional area of 3%, contributing 11% of the flux. The second spectrum 5 days later reveals slightly increased metal abundances except that P is elevated to 20 times solar while Fe has declined to 0.05 times solar. The white dwarf has cooled by ≈1000 K, the belt temperature is 32,000 K, and the fractional area and flux contribution of the belt are 5% and 20%, respectively. These STIS observations confirm that a past (prehistoric?) thermonuclear runaway has occurred on the white dwarf in VW Hyi. It is expected that the thermonuclear runaway would be strong enough to produce a nova outburst. Therefore, these two classes of close binaries, namely, dwarf novae and classical novae, are linked and can overlap.

An XMM-Newton study of the X-ray binary MXB 1659-298 and the discovery of narrow X-ray absorption lines

We report the discovery of narrow X-ray absorption lines from the low-mass X-ray binary MXB 1659- 298 during an XMM-Newton observation in 2001 February. The 7.1 hr orbital cycle is clearly evident with narrow X-ray eclipses preceded by intense dipping activity. A sinusoid-like OM B-band modulation with a peak-to-peak modulation of0.5 mag and a minimum coincident with the X-ray eclipse is visible. EPIC and RGS spectra reveal the presence of narrow resonant absorption features identied with O viii 1s-2p, 1s-3p and 1s-4p, Ne x 1s-2p, Fe xxv 1s-2p, and Fe xxvi 1s-2p transitions, together with a broad Fe emission feature at 6:47 +0:18 0:14 keV. The equivalent widths of the Fe absorption features show no obvious dependence on orbital phase, even during dipping intervals. The equivalent widths of the other features are consistent with having the same values during persistent and dipping intervals. Previously, the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. This now appears unlikely, and instead their presence may be related to the viewing angle of the system. The MXB 1659-298 0.6{12 keV continuum is modeled using absorbed cuto power-law and blackbody components. During dips the blackbody is more strongly absorbed than the power-law. The spectral shape of the 3.6% of 0:5 10 keV emission that remains during eclipses is consistent with that during non-dipping intervals.

A method to lower the detection limit for optical beat signals froma frequency-dithered stabilized laser

A narrow absorption feature in an atomic or molecular gas (such as iodineor methane) is used as the frequency reference in many stabilized lasers.As part of the stabilization scheme an optical frequency dither is appliedto the laser. In optical heterodyne experiments, this dither is transferredto the RF beat signal, reducing the spectral power density and hence thesignal to noise ratio over that in the absence of dither.We removed the dither by mixing the raw beat signal with a dithered localoscillator signal. When the dither waveform is matched to that of thereference laser the output signal from the mixer is rendered dither free.Application of this method to a Winters iodine-stabilized helium-neonlaser reduced the bandwidth of the beat signal from 6 MHz to 390 kHz,thereby lowering the detection threshold from 5 pW of laser power to 3 pW.In addition, a simple signal detection model is developed which predictssimilar threshold reductions.

Bands of solid CO$_\mathsf{2}$ in the 2-3μm spectrum of S 140:IRS1

We investigate the 2-3 ISO-SWS spectrum of the luminous protostellar object S 140:IRS1. Two narrow absorption features are detected at 2.70 and 2.77 μm, which are well fitted with laboratory spectra of the and the combination modes of solid . The ice in this line of sight must have been subjected to significant heating, in agreement with previously studied CO2 bands. A combined laboratory fit to all CO2 bands detected toward S 140:IRS1 shows, among others, the need for particle shape calculations for the CO2 stretch mode. Finally, we discuss the absence of features of isolated H2O and dangling OH groups in the spectrum of S 140:IRS1.

NICS-TNG infrared spectroscopy of trans-neptunian objects 2000 EB173 and 2000 WR106

We report complete near-infrared (0.9-2.4 $\mu$m) spectral observations of trans-neptunian objects (TNOs) 2000 EB173 and 2000 WR106 collected using the new Near Infrared Camera Spectrometer (NICS) attached to the 3.56m Telescopio Nazionale Galileo (TNG). Both spectra are very red and with a quite strong and broad drop extending throughout the K band. However, while 2000 EB173 does not show any evidence of narrow absorption features, the spectrum of 2000 WR106 has quite deep water ice absorption at 1.5 and 2.0 $\mu$m. Moreover, the latter object is significantly less red than the former indicating, therefore, that the surface of 2000 WR106 is ``cleaner'' (i.e. less processed by particle irradiation) than that of 2000 EB173.

Evolution of Carbon Dust in Aromatic Infrared Emission Sources: Formation of Nanodiamonds

Particles of crystalline diamond showing extremely narrow (4-6 cm-1) IR absorption features attributable to tertiary CH groups have been prepared in the laboratory by deposition of hydrogenated amorphous carbon (HAC) in the presence of oxygen at 300 K. These tertiary CH features are also observed in emission in type C aromatic infrared (AIR) objects, typified by the post-asymptotic giant branch star HR 4049. We have investigated the connection between the chemistry of HAC and the formation of nanodiamond material and conclude that this may occur via dehydrogenation of nonaromatic precursor molecules in this material. To trace the role of these precursors, we have obtained high-temperature (1200 K) infrared emission spectra of HAC in the region between 3.0 and 3.7 μm (3200 and 2700 cm-1) and find a broad continuum similar to the plateau emission observed in many type A AIR sources, typified by NGC 7027. This continuum is produced primarily from the overlap of two distinct features: a band centered at 3.24 μm (3090 cm-1) assigned to C=CH2 groups in nonaromatic molecules and a broad band at 3.46 μm (2890 cm-1) attributed to the stretching vibration in tertiary CH. Sharp (15-30 cm-1) features associated with CH2 are also observed at 3.425 and 3.51 μm (2920 and 2850 cm-1). We find that the tertiary CH diamond bands are likely formed by dissociation of these groups. It is concluded that hydrogenation of polycyclic aromatic hydrocarbon followed by exposure to ultraviolet radiation in the presence of oxygen is a chemical route to the formation of nanodiamond material in interstellar clouds. We suggest that carbonaceous material in type B AIR sources such as IRAS 05341+0852 can evolve into either nanodiamond dust or aromatic-rich components depending on the time spent under hydrogenating conditions. We also find that the chemical components responsible for plateau emission in AIR sources are present in 3 μm spectra of young stellar objects with ice absorption bands.

Caltech Faint Galaxy Redshift Survey. XIII. Spectral Energy Distributions for Galaxies in the Region of the Hubble Deep Field North*

We introduce a new empirical function for modeling the spectral energy distributions of galaxies. We apply it to a sample of 590 galaxies in the region of the HDF with $z < 1.5$ using multi-color photometry with wide wavelength coverage combined with spectroscopic redshifts from our 93% complete R-selected redshift survey there. We find the following: As expected, galaxies with strong signs of recent star formation (i.e. those which show emission lines) have bluer continua in both the rest frame UV and the optical/near-infrared. The redder galaxies tend to be more luminous. The SEDs of actively star forming galaxies become bluer in the mean in the rest-frame UV at higher redshift, which trend might arise from SED modeling errors. Aside from this, we discern no change with redshift in the relationship between SED characteristics and galaxy spectral type based on the strength of narrow emission and absorption features. There is no evidence in our R-selected sample that supports the existence of a substantial population of very dusty star forming galaxies at z < 1.5. The overall pattern of the L(K) - z relationship suggests that passive evolution at constant stellar mass is a good approximation to the actual behavior of at least the most luminous galaxies in this large sample of galaxies in the region of the HDF out to z \sim 1.5. (abridged)

High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data. V. Hobby-Eberly Telescope Observations

We report the discovery of 27 quasars with redshifts between 3.58 and 4.49. The objects were identified as high-redshift candidates based on their colors in Sloan Digital Sky Survey commissioning data. The redshifts were confirmed with low-resolution spectra obtained at the Hobby-Eberly Telescope. The quasars' i* magnitudes range from 18.55 to 20.97. Nearly 60% of the quasar candidates observed are confirmed spectroscopically as quasars. Two of the objects are broad absorption line quasars, and several other quasars appear to have narrow associated absorption features.