Diana Marcu

A DOUBLE-PEAKED OUTBURST OF A 0535+26 OBSERVED WITH INTEGRAL, RXTE, AND SUZAKU

The Be/X-ray binary A 0535+26 showed a normal (type I) outburst in 2009 August. It is the fourth in a series of normal outbursts associated with the periastron, but is unusual because it presented a double-peaked light curve. The two peaks reached a flux of ~450 mCrab in the 15-50 keV range. We present results of the timing and spectral analysis of INTEGRAL, RXTE, and Suzaku observations of the outburst. The energy-dependent pulse profiles and their evolution during the outburst are studied. No significant differences with respect to other normal outbursts are observed. The centroid energy of the fundamental cyclotron line shows no significant variation during the outburst. A spectral hardening with increasing luminosity is observed. We conclude that the source is accreting in the sub-critical regime. We discuss possible explanations for the double-peaked outburst.

Spectral and Timing Nature of the Symbiotic X-Ray Binary 4U 1954+319: The Slowest Rotating Neutron Star in AN X-Ray Binary System

The symbiotic X-ray binary (SyXB) 4U 1954+319 is a rare system hosting a peculiar neutron star (NS) and an M-type optical companion. Its ~5.4 hr NS spin period is the longest among all known accretion-powered pulsars and exhibited large (~7%) fluctuations over 8 yr. A spin trend transition was detected with Swift/BAT around an X-ray brightening in 2012. The source was in quiescent and bright states before and after this outburst based on 60 ks Suzaku observations in 2011 and 2012. The observed continuum is well described by a Comptonized model with the addition of a narrow 6.4 keV Fe–Kα line during the outburst. Spectral similarities to slowly rotating pulsars in high-mass X-ray binaries, its high pulsed fraction (~60%–80%), and the location in the Corbet diagram favor high B-field (gsim 1012 G) over a weak field as in low-mass X-ray binaries. The observed low X-ray luminosity (10^(33)–10^(35) erg s^(−1)), probable wide orbit, and a slow stellar wind of this SyXB make quasi-spherical accretion in the subsonic settling regime a plausible model. Assuming a ~10^13 G NS, this scheme can explain the ~5.4 hr equilibrium rotation without employing the magnetar-like field (~10^16 G) required in the disk accretion case. The timescales of multiple irregular flares (~50 s) can also be attributed to the free-fall time from the Alfven shell for a ~10^13 G field. A physical interpretation of SyXBs beyond the canonical binary classifications is discussed.

THE 5 hr PULSE PERIOD AND BROADBAND SPECTRUM OF THE SYMBIOTIC X-RAY BINARY 3A 1954+319

We present an analysis of the highly variable accreting X-ray pulsar 3A 1954+319 using 2005-2009 monitoring data obtained with INTEGRAL and Swift. This considerably extends the pulse period history and covers flaring episodes in 2005 and 2008. In 2006 the source was identified as one of only a few known symbiotic X-ray binaries (SyXBs), i.e., systems composed of a neutron star accreting from the inhomogeneous medium around an M-giant star. The extremely long pulse period of {approx}5.3 hr is directly visible in the 2008 INTEGRAL-ISGRI outburst light curve. The pulse profile is double peaked and generally not significantly energy dependent although there is an indication of possible softening during the main pulse. During the outburst a strong spin-up of -1.8 Multiplication-Sign 10{sup -4} hr hr{sup -1} occurred. Between 2005 and 2008 a long-term spin-down trend of 2.1 Multiplication-Sign 10{sup -5} hr hr{sup -1} was observed for the first time for this source. The 3-80 keV pulse peak spectrum of 3A 1954+319 during the 2008 flare could be well described by a thermal Comptonization model. We interpret the results within the framework of a recently developed quasi-spherical accretion model for SyXBs.We present an analysis of the highly variable accreting X-ra y pulsar 3A 1954+319 using 2005–2009 monitoring data obtained with INTEGRAL andSwift. This considerably extends the pulse period history and cov ers flaring episodes in 2005 and 2008. In 2006 the source was ident ified as one of only a few known symbiotic X-ray binaries (SyXBs), i.e., systems composed of a neutron star accreting from the inhomogeneous medium around an M-giant star. The extremely long pulse period of ∼5.3 hr is directly visible in the 2008 INTEGRALISGRI outburst light curve. The pulse profile is double peake d and generally not significantly energy dependent although there is an indication of possible softening durin g the main pulse. During the outburst a strong spinup of−1.8× 10−4 hr hr−1 occurred. Between 2005 and 2008 a long-term spin-down trend of 2.1× 10−5 hr hr−1 was observed for the first time for this source. The 3–80 keV pu lse peak spectrum of 3A 1954 +319 during the 2008 flare could be well described by a thermal Comptonizatio n m del. We interpret the results within the framework of a recently developed quasi-spherical accreti on model for SyXBs. Subject headings: binaries: symbiotic — stars: individual (3A 1954 +319) — stars: neutron — X-rays: binaries

A Suzaku View of Cyclotron Line Sources and Candidates

Seventeen accreting neutron star pulsars, mostly high mass X-ray binaries with half of them Be-type transients, are known to exhibit Cyclotron Resonance Scattering Features (CRSFs) in their X-ray spectra, with characteristic line energies from 10 to 60 keV. To date about two thirds of them, plus a few similar systems without known CRSFs, have been observed with Suzaku. We present an overview of results from these observations, including the discovery of a CRSF in the transient 1A 1118-61 and pulse phase resolved spectroscopy of GX 301-2. These observations allow for the determination of cyclotron line parameters to an unprecedented degree of accuracy within a moderate amount of observing time. This is important since these parameters vary-e.g., with orbital phase, pulse phase, or luminosity-depending on the geometry of the magnetic field of the pulsar and the properties of the accretion column at the magnetic poles. We briefly introduce a spectral model for CRSFs that is currently being developed and that...

Spinning-Up: The Case of the Symbiotic X-Ray Binary 3A 1954+319

We present a timing and spectral analysis of the variable X-ray source 3A 1954+319, Our analysis is mainly based on an outburst serendipitously observed during INTEGRAL Key Program observations of the Cygnus region in 2008 fall and on the Swift/BAT longterm light curve, Previous observations, though sparse, have identified the source to be one of only nine known symbiotic X-ray binaries, i.e., systems composed of an accreting neutron star orbiting in a highly inhomogeneous medium around an M-giant companion. The spectrum of 3A 1954+319 above 20 keY can be best described by a broken power law model. The extremely long pulse period of approx.5.3 hours is clearly visible in the INTEGRAL/ISGRI light curve and confirmed through an epoch folding period search. Furthermore, the light curve allows us to determine a very strong spin up of -2x10(exp 4)h/h during the outburst. This spin up is confirmed by the pulse period evolution calculated from Swift/BAT data. The Swift/BAT data also show a long spin-down trend prior to the 2008 outburst. which is confirmed in archival INTEGRAL/ISGRI data. We discuss possible accretion models and geometries allowing for the transfer of such large amounts of angular momentum and investigate the harder spectrum of this outburst compared to previously published results.

Cygnus X-1: shedding light on the spectral variability of the hard state of black holes

E-mail: Victoria.Grinberg@sternwarte.uni-erlangen.de 1Remeis-Observatory/ECAP/FAU, Bamberg, Germany 2CRESST/NASA-GSFC, Greenbelt, MD, USA 3UMBC, Baltimore, MD, USA 4GMU, Fairfax, VA, USA 5ESAC, Madrid, Spain 6UMCP, College Park, MD, USA 7MIT Kavli Institute, Cambridge, MA, US 8UVA, Amsterdam, NL 9UCSD, La Jolla, CA, USA 10SSL, UC Berkeley, Berkeley, CA, USA 11DSM/DAPNIA/SAp, CEA Saclay, France 12University of Cambridge, Cambridge, UK

Timing Analysis with INTEGRAL: Comparing Different Reconstruction Algorithms

INTEGRAL is one of the few instruments capable of detecting X-rays above 20 keV. It is therefore in principle well suited for studying X-ray variability in this regime. Because INTEGRAL uses coded mask instruments for imaging, the reconstruction of light curves of X-ray sources is highly non-trivial. We present results from a comparison of two commonly employed algorithms, which primarily measure flux from mask deconvolution (ii_lc_extract) and from calculating the pixel illuminated fraction (ii_light). Both methods agree well for timescales above about 10 s, the highest time resolution for which image reconstruction is possible. For higher time resolution, ii light produces meaningful results, although the overall variance of the lightcurves is not preserved.