Julia C. Lee

Measurements of Omega and Lambda from 42 high redshift supernovae

We report measurements of the mass density, Omega_M, and cosmological-constant energy density, Omega_Lambda, of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these SNe, at redshifts between 0.18 and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of interest (Omega_M 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. The conclusions are robust whether or not a width-luminosity relation is used to standardize the SN peak magnitudes.

A long hard look at MCG–6-30-15 with XMM-Newton

We present the first results from a 325-ks observation of the Seyfert 1 galaxy MCG–6-30-15 with XMM-Newton and BeppoSAX. The strong, broad, skewed iron line is clearly detected and is well characterized by a steep emissivity profile within 6rg (i.e. 6GM/c2) and a flatter profile beyond. The inner radius of the emission appears to lie at about 2rg, consistent with results reported from both an earlier XMM-Newton observation of MCG–6-30-15 by Wilms et al. and part of an ASCA observation by Iwasawa et al. when the source was in a lower flux state. The radius and steep emissivity profile do depend however on an assumed incident power-law continuum and a lack of complex absorption above 2.5 keV. The blue wing of the line profile is indented, either by absorption at about 6.7 keV or by a hydrogenic iron emission line. The broad iron line flux does not follow the continuum variations in a simple manner.

Accretion disk winds as the jet suppression mechanism in the microquasar GRS 1915+105

Stellar-mass black holes with relativistic jets, also known as microquasars, mimic the behaviour of quasars and active galactic nuclei. Because timescales around stellar-mass black holes are orders of magnitude smaller than those around more distant supermassive black holes, microquasars are ideal nearby ‘laboratories’ for studying the evolution of accretion disks and jet formation in black-hole systems. Whereas studies of black holes have revealed a complex array of accretion activity, the mechanisms that trigger and suppress jet formation remain a mystery. Here we report the presence of a broad emission line in the faint, hard states and narrow absorption lines in the bright, soft states of the microquasar GRS 1915+105. (‘Hard’ and ‘soft’ denote the character of the emitted X-rays.) Because the hard states exhibit prominent radio jets, we argue that the broad emission line arises when the jet illuminates the inner accretion disk. The jet is weak or absent during the soft states, and we show that the absorption lines originate when the powerful radiation field around the black hole drives a hot wind off the accretion disk. Our analysis shows that this wind carries enough mass away from the disk to halt the flow of matter into the radio jet.

High-Resolution Chandra HETGS and Rossi X-Ray Timing Explorer Observations of GRS 1915+105: A Hot Disk Atmosphere and Cold Gas Enriched in Iron and Silicon

The Chandra AO1 HETGS observation of the micro-quasar GRS 1915+105 in the low hard state reveals (1) neutral K absorption edges from Fe, Si, Mg, and S in cold gas, and (2) highly ionized (Fe XXV and Fe XXVI) absorption attributed to a hot disk, disk wind, or corona. The neutral edges reveal anomalous Si and Fe abundances which we attribute to surrounding cold material in/near the environment of GRS 1915+105. We also point out the exciting possibility for the first astrophysical detection of XAFS attributed to material in interstellar grains. We place constraints on the ionization parameter, temperature, and hydrogen equivalent number density of the absorber near the accretion disk based on the detection of the H- and He-like Fe absorption. Observed spectral changes in the ionized lines which track the light curve point to changes in both the ionizing flux and density of the absorber, supporting the presence of a flow. Details can be found in Lee et al., 2002, ApJ., 567, 1102The time-averaged 30 ks Chandra HETGS observation of the micro-quasar GRS 1915+105 in the low hard state reveals for the first time in this source neutral K absorption edges from Fe, Si, Mg, & S. Ionized resonance absorption from H-, and He-like Fe (XXV, XXVI), Ca XX and possibly emission from neutral Fe Kalpha and ionized Fe XXV (forbidden, or the resonance emission component of a P-Cygni profile) are also seen. We report the tentative detection of the first astrophysical signature of XAFS in the photoelectric edge of Si (and possibly Fe and Mg), attributed to material in grains. The large column densities measured from the neutral edges reveal anomalous Si and Fe abundances. Scenarios for which the anomalous abundances can be attributed to surrounding cold material associated with GRS 1915+105 and/or that the enrichment may signify either a highly unusual supernova/hypernova, or external supernova activity local to the binary are discussed. We attribute the ionized features to a hot disk, disk-wind, or corona environment. These features allow for constraints on the ionization parameter (log xi > 4.15), temperature (T > 2.4 x 10^6 K), and hydrogen equivalent number density (n > 10^{12} cm^{-3}) for this region. Variability studies with simultaneous RXTE data show that the light curve count rate tracks changes in the disk blackbody and the power-law flux. Spectral changes in the Chandra data also track the behavior of the light curve, and may point to changes in both the ionizing flux and density of the absorber. A 3.69 Hz QPO and weak first harmonic is seen in the RXTE data.

Revealing the Dusty Warm Absorber in MCG ?6-30-15 with the Chandra High-Energy Transmission Grating

We present detailed evidence for a warm absorber in the Seyfert 1 galaxy MCG -6-30-15 and dispute earlier claims for relativistic O line emission. The High-Energy Transmission Grating spectra show numerous narrow, unresolved (FWHM 200 km s-1) absorption lines from a wide range of ionization states of N, O, Mg, Ne, Si, S, Ar, and Fe. The O VII edge and the 1s2-1snp resonance line series to n = 9 are clearly detected at rest in the active galactic nucleus frame. We attribute previous reports of an apparently highly redshifted O VII edge to the 1s2-1snp (n > 5) O VII resonance lines and a neutral Fe L absorption complex. The shape of the Fe L feature is nearly identical to that seen in the spectra of several X-ray binaries and in laboratory data. The implied dust column density agrees with that obtained from reddening studies and gives the first direct X-ray evidence for dust embedded in a warm absorber. The O VIII resonance lines and the weak edge are also detected, and the spectral rollover below ~2 keV is explained by the superposition of numerous absorption lines and edges. We identify, for the first time, a KLL resonance in the O VI photoabsorption cross section, giving a measure of the O VI column density. The O VII (f) emission detected at the systemic velocity implies a covering fraction of ~5% (depending on the observed vs. time-averaged ionizing flux). Our observations show that a dusty warm absorber model is not only adequate to explain all the spectral features 0.48 keV (26 A) but that the data require it. This contradicts the interpretation of Branduardi-Raymont and coworkers that this spectral region is dominated by highly relativistic line emission from the vicinity of the black hole.

Testing the Seyfert unification theory: Chandra HETGS observations of NGC 1068

We present spatially resolved Chandra HETGS observations of the Seyfert 2 galaxy NGC 1068. X-ray imaging and high resolution spectroscopy are used to test the Seyfert unification theory. Fe K emission is concentrated in the nuclear region, as are neutral and ionized continuum reflection. This is consistent with reprocessing of emission from a luminous, hidden X-ray source by the obscuring molecular torus and X-ray narrow-line region (NLR). We detect extended hard X-ray emission surrounding the X-ray peak in the nuclear region, which may come from the outer portion of the torus. Detailed modeling of the spectrum of the X-ray NLR confirms that it is excited by photoionization and photoexcitation from the hidden X-ray source. K-shell emission lines from a large range of ionization states of H-like and He-like N, O, Ne, Mg, Al, Si, S, and Fexvii-xxiv L-shell emission lines are modeled. The emission measure distribution indicates roughly equal masses at all observed ionization levels in the range log= 1-3. We separately analyze the spectrum of an o-nuclear cloud. We find that it has a lower column density than the nuclear region, and is also photoionized. The nuclear X-ray NLR column density, optical depth, outflow velocity, and electron temperature are all consistent with values predicted by optical spectropolarimetry for the region which provides a scattered view of the hidden Seyfert 1 nucleus.

Chandra Observations of the X-Ray Narrow-Line Region in NGC 4151

We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy NGC 4151. Observations with the Chandra High-Energy Transmission Grating Spectrometer reveal a spectrum dominated by narrow emission lines from a spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line region is composite, consisting of both photoionized and collisionally ionized components. The X-ray emission lines have similar velocities, widths, and spatial extent to the optical emission lines, showing that they arise in the same region. The clouds in the narrow-line region must contain a large range of ionization states in order to explain both the optical and X-ray photoionized emission. Chandra data give the first direct evidence of X-ray line emission from a hot plasma (T ~ 107 K) that may provide pressure confinement for the cooler (T = 3 × 104 K) photoionized clouds.

The distant type Ia supernova rate

We present a measurement of the rate of distant Type Ia supernovae derived using 4 large subsets of data from the Supernova Cosmology Project. Within this fiducial sample,which surveyed about 12 square degrees, thirty-eight supernovae were detected at redshifts 0.25--0.85. In a spatially flat cosmological model consistent with the results obtained by the Supernova Cosmology Project, we derive a rest-frame Type Ia supernova rate at a mean red shift z {approx_equal} 0.55 of 1.53 {sub -0.25}{sub -0.31}{sup 0.28}{sup 0.32} x 10{sup -4} h{sup 3} Mpc{sup -3} yr{sup -1} or 0.58{sub -0.09}{sub -0.09}{sup +0.10}{sup +0.10} h{sup 2} SNu(1 SNu = 1 supernova per century per 10{sup 10} L{sub B}sun), where the first uncertainty is statistical and the second includes systematic effects. The dependence of the rate on the assumed cosmological parameters is studied and the redshift dependence of the rate per unit comoving volume is contrasted with local estimates in the context of possible cosmic star formation histories and progenitor models.

The First High-Resolution X-Ray Spectrum of Cygnus X-1: Soft X-Ray Ionization and Absorption

We observed the black hole candidate Cyg X-1 for 15 ks with the High-Energy Transmission Grating Spectrometer aboard the Chandra X-Ray Observatory. The source was observed during a period of intense flaring activity, so it was about a factor of 2.5 brighter than usual, with a 0.5-10 keV (1-24 ?) luminosity of 1.6 ? 1037 ergs s-1 (at a distance of 2.5 kpc). The spectrum of the source shows prominent absorption edges, some of which have a complicated substructure. We use the most recent results from laboratory measurements and calculations to model the observed substructure of the edges. From the model, we derive a total absorption column of (6.21 ? 0.22) ? 1021 cm-2. Furthermore, the results indicate that there are ~ 10%-25% abundance variations relative to solar values for neon, oxygen, and iron. The X-ray continuum is well described by a two-component model that is often adopted for black hole candidates: a soft multicolor disk component (with kT = 203 eV) and a hard power-law component (with a photon index of ~2). Comparing the fit results to those of the hard and soft states, we conclude that the source was in a transitional state. Finally, the spectrum also shows the presence of faint emission lines, which could be attributed to highly ionized species.

A CHANDRA HETGS SPECTRAL STUDY OF THE IRON K BANDPASS IN MCG -6-30-15: A NARROW VIEW OF THE BROAD IRON LINE

We present a high-resolution X-ray spectrum of the iron K bandpass in MCG -6-30-15 based on a 522 ks observation with Chandras High Energy Transmission Grating Spectrometer (HETGS). The Chandra spectrum is consistent with the presence of a relativistically broadened, highly redshifted iron Kα emission line with a profile similar to previous observations. A number of narrow features are detected above 2 keV, including a narrow Fe Kα emission line and narrow absorption lines from H- and He-like Fe, H-like S, and H-like Si. This absorption is well described by a photoionized plasma with a column density log NH = 23.2 and an ionization parameter log ξ = 3.6, assuming the iron abundance has the solar value and a velocity dispersion parameter b = 100 km s-1. Applying this absorption model to a high-fidelity XMM-Newton EPIC pn spectrum, we find that a broad iron line is still required with emission extending to within 1.9rg of the black hole. If the iron line comes from an accretion disk truncated at the innermost stable circular orbit, this indicates that the black hole must be spinning rapidly with a > 0.95. Ionized absorption models attempting to explain the 3-6 keV spectral curvature without strong gravity predict absorption lines in the 6.4-6.6 keV range that are inconsistent with the Chandra spectrum. The H- and He-like iron absorption lines in the Chandra spectrum are blueshifted by 2.0 × 103 km s-1 compared to the source frame and may originate in a high-velocity, high-ionization component of the warm absorber outflow. This high-ionization component may dominate the energy budget of the outflow and account for a significant fraction of the outflowing mass. Detailed modeling of the warm absorber below 2 keV will be addressed in a later paper, but our results are robust to the broader details of the warm absorber behavior. The difference spectrum between the high- and low-flux states is well described by a power law, in agreement with previous studies.