Using ACIS on the Chandra X-ray Observatory as a particle radiation monitor II

The Advanced CCD Imaging Spectrometer is an instrument on the Chandra X-ray Observatory. CCDs are vulnerable to radiation damage, particularly by soft protons in the radiation belts and solar storms. The Chandra team has implemented procedures to protect ACIS during high-radiation events including autonomous protection triggered by an on-board radiation monitor. Elevated temperatures have reduced the effectiveness of the on-board monitor. The ACIS team has developed an algorithm which uses data from the CCDs themselves to detect periods of high radiation and a flight software patch to apply this algorithm is currently active on-board the instrument. In this paper, we explore the ACIS response to particle radiation through comparisons to a number of external measures of the radiation environment. We hope to better understand the efficiency of the algorithm as a function of the flux and spectrum of the particles and the time-profile of the radiation event.Comment: 10 pages, 5 figures, to be published in Proc. SPIE 8443, "Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray

A very deep Chandra observation of Abell 1795: The Cold Front and Cooling Wake

We present a new analysis of very deep Chandra observations of the galaxy cluster Abell 1795. Utilizing nearly 750 ks of net ACIS imaging, we are able to resolve the thermodynamic structure of the Intracluster Medium (ICM) on length scales of ~ 1 kpc near the cool core. We find several previously unresolved structures, including a high pressure feature to the north of the BCG that appears to arise from the bulk motion of Abell 1795's cool core. To the south of the cool core, we find low temperature (~ 3 keV), diffuse ICM gas extending for distances of ~ 50 kpc spatially coincident with previously identified filaments of H-alpha emission. Gas at similar temperatures is also detected in adjacent regions without any H-alpha emission. The X-ray gas coincident with the H-alpha filament has been measured to be cooling spectroscopically at a rate of ~ 1 Solar Masses/ yr, consistent with measurements of the star formation rate in this region as inferred from UV observations, suggesting that the star formation in this filament as inferred by its H$\alpha$ and UV emission can trace its origin to the rapid cooling of dense, X-ray emitting gas. The H-alpha filament is not a unique site of cooler ICM, however, as ICM at similar temperatures and even higher metallicities not cospatial with H$\alpha$ emission is observed just to the west of the H-alpha filament, suggesting that it may have been uplifted by Abell 1795's central active galaxy. Further simulations of cool core sloshing and AGN feedback operating in concert with one another will be necessary to understand how such a dynamic cool core region may have originated and why the H-alpha emission is so localized with respect to the cool X-ray gas despite the evidence for a catastrophic cooling flow.Comment: 14 Pages, 10 Figures, Resubmitted to ApJ after first referee report, Higher Resolution Figures available upon reques

Chandra X-ray Observations of the Hydra A Cluster: An Interaction Between the Radio Source and the X-Ray-Emitting Gas

We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the cluster's X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, $\sim 25-35$ kpc diameter, that are coincident with Hydra A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is $\sim 600$ Myr in the inner 10 kpc. These properties are consistent with the presence of a \sim 34 \msunyr cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently-accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material cooling at larger radii remains elusive.Comment: 4 pages, 3 figures; submitted to ApJ Letter

Detecting high redshift evolved galaxies as the hosts of optically faint hard X-ray sources

We combine deep Subaru near-infrared images of the massive lensing clusters A2390 and A370 with Keck optical data to map the spectral energy distributions (SEDs) of Chandra X-ray sources lying behind the clusters. The three sources behind A2390 are found to have extremely red colors with SEDs consistent with evolved galaxies at redshifts z>1.4. One source has extremely anomalous colors, which we interpret as evidence for a type Sa SED at a redshift around 2.5. The photometric redshift of another source has been confirmed at z=1.467 from near-infrared spectroscopy using the CISCO spectrograph on Subaru. Mapping of optically faint hard X-ray sources may prove to be an extremely efficient way to locate luminous evolved galaxies at high redshifts.Comment: 5 pages, ApJ Letters, in pres

Chandra Detects a Rapid Flare in the Gravitationally Lensed Mini-BALQSO RX J0911.4+0551

The mini Broad Absorption Line (BAL) quasar RX J0911.4+0551 was observed with the Advanced CCD Imaging Spectrometer (ACIS) of the Chandra X-ray Observatory for ~ 29 ks as part of a gravitational lens (GL) survey aimed at measuring time-delays. Timing analysis of the light-curve of the lensed image A2 shows a rapid flux variation with a duration of about 2000s. A Kolmogorov-Smirnov test shows that the probability that a constant-intensity source would produce the observed variability is less than ~ 0.2 percent. We discuss possible origins for the observed short-term X-ray variability. Our gravitational lens models for the RX J0911.4+0551 GL system predict a time-delay of less than a day between images A1 and A2. The rapid variability combined with the predicted short-time delay make RX J0911.4+0551 an ideal system to apply the GL method for estimating the Hubble constant. We describe the prospects of measuring H_0 within single X-ray observations of GL systems with relatively short time delays. Modeling of the spectrum of the mini-BAL quasar RX J0911.4+0551 suggests the presence of an intrinsic absorber. Partial covering models are slightly preferred over models that contain absorption due to intrinsic ionized or neutral gas.Comment: 17 pages, includes 5 figures, Accepted for publication in Ap

The Redshift Evolution of the 2-8 keV X-ray Luminosity Function

The high angular resolution and sensitivity of the Chandra X-ray Observatory has yielded large numbers of faint X-ray sources with measured redshifts in the soft (0.5-2 keV) and hard (2-8 keV) energy bands. Many of these sources show few obvious optical signatures of active galactic nuclei (AGN). We use Chandra observations of the Hubble Deep Field North region, A370, and the Hawaii Survey Fields SSA13 and SSA22, together with the ROSAT Ultra Deep Survey soft sample and the ASCA Large Sky Survey hard sample, to construct rest-frame 2-8 keV luminosity functions versus redshift for all the X-ray sources, regardless of their optical AGN characteristics. At z=0.1-1 most of the 2-8 keV light density arises in sources with luminosities in the 10^42 erg/s to 10^44 erg/s range. We show that the number density of sources in this luminosity range is rising, or is at least constant, with decreasing redshift. Broad-line AGN are the dominant population at higher luminosities, and these sources show the well-known rapid positive evolution with increasing redshift to z~3. We argue that the dominant supermassive black hole formation has occurred at recent times in objects with low accretion mass flow rates rather than at earlier times in more X-ray luminous objects with high accretion mass flow rates.Comment: 5 pages, accepted by The Astrophysical Journal Letter

Chandra X-ray Observations of the Quadruply Lensed Quasar RX J0911.4+0551

We present results from X-ray observations of the quadruply lensed quasar RX J0911.4+0551 using data obtained with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray Observatory. The 29 ks observation detects a total of ~404 X-ray photons (0.3 to 7.0 keV) from the four images of the lensed quasar. Deconvolution of the aspect corrected data resolves all four lensed images, with relative positions in good agreement with optical measurements. When compared to contemporaneous optical data, one of the lensed images (component A3) is dimmer by a factor of ~6 in X-rays with respect to the 2 brighter images (components A1 and A2). Spectral fitting for the combined images shows significant intrinsic absorption in the soft (0.2 to 2.4 keV) energy band, consistent with the mini-BAL nature of this quasar, while a comparison with ROSAT PSPC observations from 1990 shows a drop of ~6.5 in the total soft bandpass flux. The observations also detect ~157 X-ray photons arising from extended emission of the nearby cluster (peaked ~42" SW of RXJ0911.4+0551) responsible for the large external shear present in the system. The Chandra observation reveals the cluster emission to be complex and non-spherical, and yields a cluster temperature of kT = 2.3^{+1.8}_{-0.8} keV and a 2.0 to 10 keV cluster luminosity within a 1 Mpc radius of L_X = 7.6_{-0.2}^{+0.6} x 10^{43} ergs/s (error bars denote 90% confidence limits). Our mass estimate of the cluster within its virial radius is 2.3^{+1.8}_{-0.7} x 10^{14} solar, and is a factor of 2 smaller than, although consistent with, previous mass estimates based on the observed cluster velocity dispersion.Comment: 16 pages, 3 figures (figure 1 is color ps). Accepted by Ap

X-ray Sources in the Hubble Deep Field Detected by Chandra

We present first results from an X-ray study of the Hubble Deep Field North (HDF-N) and its environs obtained using 166 ks of data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray Observatory. This is the deepest X-ray observation ever reported, and in the HDF-N itself we detect six X-ray sources down to a 0.5--8 keV flux limit of 4E-16 erg cm^-2 s^-1. Comparing these sources with objects seen in multiwavelength HDF-N studies shows positional coincidences with the extremely red object NICMOS J123651.74 +621221.4, an active galactic nucleus (AGN), three elliptical galaxies, and one nearby spiral galaxy. The X-ray emission from the ellipticals is consistent with that expected from a hot interstellar medium, and the spiral galaxy emission may arise from a `super-Eddington' X-ray binary or ultraluminous supernova remnant. Four of the X-ray sources have been detected at radio wavelengths. We also place X-ray upper limits on AGN candidates found in the HDF-N, and we present the tightest constraints yet on X-ray emission from the SCUBA submillimeter source population. None of the 10 high-significance submillimeter sources reported in the HDF-N and its vicinity is detected with Chandra ACIS. These sources appear to be dominated by star formation or have AGN with Compton-thick tori and little circumnuclear X-ray scattering.Comment: 11 pages, ApJ, in press, also available from http://www.astro.psu.edu/users/niel/hdf/hdf-chandra.htm