X-ray spectroscopy of neutron star low-mass X-ray binaries

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (p. 133-150).In this thesis, I present work spanning a variety of topics relating to neutron star lowmass X-ray binaries (LMXBs) and utilize spectral information from X-ray observations to further our understanding of these sources. First, I give an overview of important X- ray astrophysics relevant to the work I present in subsequent chapters, as well as information about the X-ray observatories from which I obtained my data. In the next three chapters, I consider spectra-both high- and low-resolution--of accretion-powered millisecond X-ray pulsars, a unique and relatively new class of objects. In addition to analysis of the pulsar XTE J1814-338, I compare a broader sample of pulsars with a sample of atoll sources in order to better understand why the latter class do not contain persistently pulsating neutron stars. In particular, I test the hypothesis that pulsations in the atoll sources are suppressed by a high-optical- depth scattering region. Using X-ray color-color diagrams to define a selection criterion based on spectral state, I analyze Rossi X-ray Timing Explorer (RXTE) spectra from all the sources, and use a Comptonization model to obtain measurements of their optical depths. I then discuss efforts to spatially resolve X-ray jets from the accretion-powered millisecond pulsar SAX J1808.4-3658 and the Z source XTE J1701-462. Each was observed by the Chandra X-ray Observatory to produce a high-spatial-resolution image. This work was motivated in part by my analysis of XTE J1814-338, which found an apparent excess of infrared flux which could be attributed to jet emission. Next, I discuss the measured temperatures of thermonuclear X-ray bursts. The detection of line features in these bursts, and hence from the surfaces of neutron stars, has been an important goal for high-resolution X-ray spectroscopy. A measurement of the wavelengths of identified line features would yield a measurement of the neutron star's gravitational redshift, which would help constrain current models for the neutron star equation of state.(cont.) Although such a measurement has been made for one source, other searches have not been able to repeat this measurement. I consider the effects of burst temperature on the formation of discrete spectral features, using a large sample of bursts observed by the RXTE PCA. Finally, I present analysis of high-resolution Chandra HETG spectra of a sample of Galactic LMXBs. High-resolution spectra are able to resolve line features, such as the prominent Ne and O emission lines in the ultracompact X-ray binary 4U 1626-67. They also allow for more precise measurements of photoelectric absorption edges, which can otherwise hinder the determination of continuum spectral components, particularly in the lower-energy spectral regions.by Miriam Ilana Krauss.Ph.D

The 2011 Outburst of Recurrent Nova T Pyx: X-ray Observations Expose the White Dwarf Mass and Ejection Dynamics

The recurrent nova T Pyx underwent its sixth historical outburst in 2011, and became the subject of an intensive multi-wavelength observational campaign. We analyze data from the Swift and Suzaku satellites to produce a detailed X-ray light curve augmented by epochs of spectral information. X-ray observations yield mostly non-detections in the first four months of outburst, but both a super-soft and hard X-ray component rise rapidly after Day 115. The super-soft X-ray component, attributable to the photosphere of the nuclear-burning white dwarf, is relatively cool (~45 eV) and implies that the white dwarf in T Pyx is significantly below the Chandrasekhar mass (~1 M_sun). The late turn-on time of the super-soft component yields a large nova ejecta mass (>~10^-5 M_sun), consistent with estimates at other wavelengths. The hard X-ray component is well fit by a ~1 keV thermal plasma, and is attributed to shocks internal to the 2011 nova ejecta. The presence of a strong oxygen line in this thermal plasma on Day 194 requires a significantly super-solar abundance of oxygen and implies that the ejecta are polluted by white dwarf material. The X-ray light curve can be explained by a dual-phase ejection, with a significant delay between the first and second ejection phases, and the second ejection finally released two months after outburst. A delayed ejection is consistent with optical and radio observations of T Pyx, but the physical mechanism producing such a delay remains a mystery.Comment: Re-submitted to ApJ after revision

The Radio Light Curve of the Gamma-Ray Nova in V407 Cyg: Thermal Emission from the Ionized Symbiotic Envelope, Devoured from Within by the Nova Blast

We present multi-frequency radio observations of the 2010 nova event in the symbiotic binary V407 Cygni, obtained with the Karl G. Jansky Very Large Array and spanning 1-45 GHz and 17-770 days following discovery. This nova---the first ever detected in gamma rays---shows a radio light curve dominated by the wind of the Mira giant companion, rather than the nova ejecta themselves. The radio luminosity grew as the wind became increasingly ionized by the nova outburst, and faded as the wind was violently heated from within by the nova shock. This study marks the first time that this physical mechanism has been shown to dominate the radio light curve of an astrophysical transient. We do not observe a thermal signature from the nova ejecta or synchrotron emission from the shock, due to the fact that these components were hidden behind the absorbing screen of the Mira wind. We estimate a mass loss rate for the Mira wind of Mdot_w ~ 10^-6 M_sun/yr. We also present the only radio detection of V407 Cyg before the 2010 nova, gleaned from unpublished 1993 archival VLA data, which shows that the radio luminosity of the Mira wind varies by a factor of >~20 even in quiescence. Although V407 Cyg likely hosts a massive accreting white dwarf, making it a candidate progenitor system for a Type Ia supernova, the dense and radially continuous circumbinary material surrounding V407 Cyg is inconsistent with observational constraints on the environments of most Type Ia supernovae.Comment: Resubmitted to ApJ after incorporating referee's comment

The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338

We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray Observatory as well as optical observations conducted during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power-law (with photon index = 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broad-band spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star, and therefore very unlikely to be responsible for the soft excess, making synchroton emission a more reasonable candidate.Comment: Accepted for publication in ApJ. 6 pages; 3 figure

Radio studies of novae: a current status report and highlights of new results

Novae, which are the sudden visual brightening triggered by runaway thermonuclear burning on the surface of an accreting white dwarf, are fairly common and bright events. Despite their astronomical significance as nearby laboratories for the study of nuclear burning and accretion phenomena, many aspects of these common stellar explosions are observationally not well-constrained and remain poorly understood. Radio observations, modeling and interpretation can potentially play a crucial role in addressing some of these puzzling issues. In this review on radio studies of novae, we focus on the possibility of testing and improving the nova models with radio observations, and present a current status report on the progress in both the observational front and theoretical developments. We specifically address the issues of accurate estimation of ejecta mass, multi-phase and complex ejection phenomena, and the effect of a dense environment around novae. With highlights of new observational results, we illustrate how radio observations can shed light on some of these long-standing puzzles.Comment: 19 pages, 4 figures. Review article published in the Bulletin of the Astronomical Society of India (BASI) special issue on nova

High-resolution X-ray spectroscopy of the ultracompact LMXB pulsar 4U 1626-67

[abridged] We report results from four recent observations of the ultracompact LMXB pulsar 4U 1626-67. All the observations obtained high-resolution X-ray spectra of the system, two from the Chandra X-ray Observatory using the HETGS, and two from the XMM-Newton Observatory using the RGS as well as the EPIC PN and MOS. These data allow us to study in detail the prominent Ne and O emission line complexes which make 4U 1626-67 unique among LMXBs. The observations were spaced over a period of 3 years for a total observing time of 238 ks, allowing us to monitor the line regions as well as the overall source flux, continuum spectrum, and timing properties. The structure of the emission lines and the ratios of the components of the helium-like Ne IX and O VII triplets support the hypothesis that they are formed in the high-density environment of the accretion disk. We do not find any significant changes in the line widths or ratios over this time period, though we note that the line equivalent widths decrease. We are able to place constraints on the strengths of the Ne K, Fe L, and O K photoelectric absorption edges, and find that the data do not require an overabundance of Ne or O in the system relative to the expected ISM values. We find that the pulsar is still spinning down, and note that the pulse profile has changed significantly from what was found prior to the torque reversal in 1990, suggesting that this event may be linked to a change in the geometry of the accretion column. The flux of 4U 1626-67 continues to decrease, in keeping with the trend of the last approximately 30 years over which it has been observed. Taking into consideration current theory on disk stability, we expect that 4U 1626-67 will enter a period of quiescence in 2-15 years.Comment: 10 pages, 7 figures, submitted to Ap

Chandra Observations of Associates of η\eta Car: I. Luminosities

The region around the $\eta$ Car nebula has three OB associations, which contain a Wolf-Rayet star and several massive O3 stars. An early Chandra ACIS-I image was centered on $\eta$ Car and includes Trumpler 16 and part of Trumpler 14. The Chandra image confirms the well-known result that O and very early B stars are X-ray sources with L$_X$ $\simeq$ 10$^{-7} \times$ L$_{bol}$ over an X-ray luminosity range of about 100. Two new anomalously strong X-ray sources have been found among the hot star population, Tr 16-244, a heavily-reddened O3 I star, and Tr 16-22, a heavily-reddened O8.5 V star. Two stars have an unusually large L$_X$/L$_{bol}$: HD 93162, a Wolf-Rayet star (and possible binary), and Tr 16-22, a possible colliding wind binary In addition, a population of sources associated with cool stars is detected. In the color-magnitude diagram, these X-ray sources sit above the sequence of field stars in the Carina arm. The OB stars are on average more X-ray luminous than the cool star X-ray sources. X-ray sources among A stars have similar X-ray luminosities to cooler stars, and may be due to cooler companions. Upper limits are presented for B stars which are not detected in X-rays. These upper limits are also the upper limits for any cool companions which the hot stars may have. Hardness ratios are presented for the most luminous sources in bands 0.5 to 0.9 keV, 0.9 to 1.5 keV, and 1.5 to 2.04 kev. The available information on the binary nature of the hot stars is discussed, but binarity does not correlate with X-ray strength in a simple way.Comment: accepted by Ap