R. L. Kelley

Signal processing for microcalorimeters

Most of the power in the signals from microcalorimeters occurs at relatively low frequencies. At these frequencies, typical amplifiers will have significant amounts of 1/f noise. Our laboratory systems can also suffer from pickup at several harmonics of the AC power line, and from microphonic pickup at frequencies that vary with the configuration of the apparatus. We have developed some optimal signal processing techniques in order to construct the best possible estimates of our pulse heights in the presence of these non-ideal effects. In addition to a discussion of our laboratory systems, we present our plans for providing this kind of signal processing in flight experiments.

Millisecond Oscillations in the Persistent and Bursting Flux of Aquila X-1 during an Outburst

The Rossi X-Ray Timing Explorer observed the soft X-ray transient Aquila X-1 during its outburst in 1997 February and March. We report the discovery of quasi-periodic oscillations (QPOs) in its persistent flux with frequencies in the range of 740-830 Hz, a Q-value of over 100, a fractional rms amplitude of 6.8%±0.6%, and nearly coherent oscillations (NCOs) during a type I burst with a frequency of 549 Hz. The frequency of the QPOs in the persistent flux is correlated with the mass accretion rate on a timescale of hours, but not on a timescale of days. This is most likely the manifestation in a single source of the kilohertz QPO puzzle observed among many sources, i.e., on the one hand, individual sources show a correlation between the QPO frequency and the inferred mass accretion rate, and on the other hand, the dozen or so sources with luminosities spanning two decades have essentially the same QPO frequencies. We propose that this multivalued QPO frequency and mass accretion rate correlation indicates the existence of many similar regimes of the accretion disk. These regimes, with a very similar energy spectrum and QPO frequency, are distinguished from each other by the mass accretion rate or the total X-ray flux. The NCOs during the burst can be made almost perfectly coherent by taking into account a large . This strongly suggests that this frequency is related to the neutron star spin frequency. The large is attributable to the expansion or contraction of the neutron star photosphere during the burst.

A new X-ray spectral observation of NGC 1068

A new X-ray observation of NGC 1068, in which improved spectral resolution (R is approximately equal to 40) and broad energy range provide important new constraints on models for this galaxy, is reported. The observed X-ray continuum of NGC 1068 from 0.3 to 10 keV is well fitted as the sum of two power-law spectra with no evidence for absorption intrinsic to the source. Strong Fe K emission lines with a total equivalent width of 2700 eV were detected due to iron less ionized than Fe XX and to iron more ionized than Fe XXIII. No evidence was seen for lines due to the recombination of highly ionized oxygen with an upper limit for the O Ly-alpha emission line of 40 eV. The discovery of multiple Fe K and Fe L emission lines indicates a broad range of ionization states for this gas. The X-ray emission from the two components is modeled for various geometries using a photoionization code that calculates the temperature and ionization state of the gas. Typical model parameters are a total Compton depth of a few percent, an inner boundary of the hot component of about 1 pc, and an inner boundary of the warm component of about 20 pc.

Close-packed arrays of transition-edge x-ray microcalorimeters with high spectral resolution at 5.9 keV

We present measurements of high fill-factor arrays of superconducting transition-edge x-ray microcalorimeters designed to provide rapid thermalization of the x-ray energy. We designed an x-ray absorber that is cantilevered over the sensitive part of the thermometer itself, making contact only at normal-metal features. With absorbers made of electroplated gold, we have demonstrated an energy resolution between 2.4 and 3.1 eV at 5.9 keV on 13 separate pixels. We have determined the thermal and electrical parameters of the devices throughout the superconducting transition and, using these parameters, have modeled all aspects of the detector performance.

Discovery of 13.5 S X-ray pulsations from LMC X-4 and an orbital determination

X-ray pulsations with a 13.5-sec period have been detected from the 1.4-d X-ray binary LMC X-4. By measuring the apparent pulse period at several binary orbital phases, and assuming the orbit to be nearly circular, the semimajor axis of the orbit is determined to be 30 + or - 5 ly-sec. This result, together with a revised orbital velocity amplitude of 37.9 + or - 2.4 km/sec, and other available information, suffice for the determination of the component masses of the binary system and the radius of the companion star. The mass of the neutron star is found to be 1.6 +1.0 -0.5 solar masses, while the mass, radius, and effective temperature of the companion star indicate that it may be undermassive for its luminosity.

Orbital elements of the binary X-ray pulsar GX 301-2

X-ray outbursts from GX 301-2 were observed with Hakucho on two occasions during April and May 1982 and with Tenma in April 1984. Pulse arrival times measured with Hakucho were combined with those previously measured with Ariel 5 and SAS 3. The joint timing analysis of these data sets yields an orbital period of 41.5 days as the only acceptable solution; this is consistent with the period derived from the X-ray flaring events of the source. The orbital elements of the binary system, established from the joint timing analysis, are presented. The orbital solution suggests that all the peaks of X-ray flares observed during the past 10 yr occur at the orbital period, but consistently appear about 1.4 days before the time of periastron passage of the X-ray star. 25 references.

Cyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7 × 1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.

Laboratory astrophysics using a spare XRS microcalorimeter

The XRS instrument on Astro-E is a fully self-contained microcalorimeter x-ray instrument capable of acquiring, optimally filtering, and characterizing events for 32 independent pixels. We have recently integrated a full engineering model XRS detector system into a laboratory cryostat for use on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory. The detector system contains a microcalorimeter array with 32 instrumented pixels heat sunk to 60 mK using an adiabatic demagnetization refrigerator. The instrument has a composite resolution of 8 eV at 1 keV and 11 eV at 6 keV with a minimum of 98% quantum efficiency and a total collecting area of 13 mm2. This will allow high spectral resolution, broadband observations of plasmas with known ionization states that are produced in the EBIT experiment. Unique to our instrument are exceptionally well characterized 1000 Angstrom thick aluminum on polyimide infrared blocking filters. The detailed transmission function including the edge fine structure of these filters has been measured in our laboratory using a variable spaced grating spectrometer. This will allow the instrument to perform the first broadband absolute flux measurements with the EBIT instrument. The instrument performance as well as the results of preliminary measurements of Fe K and L shell at fixed electron energy, Fe emission with Maxwellian electron distributions, and phase resolved spectroscopy of ionizing plasmas will be discussed.

Laboratory Measurements of the Relative Intensity of the 3s → 2p and 3d → 2p Transitions in Fe XVII

The intensity ratios of the 3s → 2p and 3d → 2p lines in Fe XVII were measured on the Livermore electron beam ion trap employing a complementary set of spectrometers, including a high-resolution crystal spectrometer and the Goddard 32 pixel calorimeter. The resulting laboratory data are in agreement with satellite measurements of the Sun and astrophysical sources in collisional equilibrium such as Capella, Procyon, and NGC 4636. The results disagree with earlier laboratory measurements and assertions that processes not accounted for in laboratory measurements must play a role in the formation of the Fe XVII spectra in solar and astrophysical plasmas.

Microcalorimeter arrays for high resolution soft X-ray spectroscopy

Abstract We have produced monolithic silicon detector arrays comprising 36 one-square-millimeter microcalorimeter pixels capable of achieving a resolution of ∼8 eV FWHM for soft X-rays. Each silicon pixel is suspended by silicon beams that provide a weak link to a ∼60 mK heat sink. The thermometer element of each pixel is an ion-implanted thermistor. X-ray thermalization is achieved in a 1 μm absorbing layer of MOCVD-grown HgTe. The HgTe is grown on thin Si substrates that are individually attached to the array elements using epoxy. We have determined that the value of the conductance of the link between absorber and thermometer critically affects device performance. We have performed preliminary investigations using arrays with small suspended absorber isolators etched in the center of each pixel. When an absorber is attached to the center of an absorber isolator, the beams supporting that isolator provide the thermal link to the rest of the pixel.