Joern Wilms

Magnetic Fields of Accreting X-Ray Pulsars with the Rossi X-Ray Timing Explorer

Using a consistent set of models, we parameterized the X-ray spectra of all accreting pulsars in the Rossi X-Ray Timing Explorer (RXTE) database that exhibit cyclotron resonance scattering features (CRSFs, or cyclotron lines). These sources in our sample are Her X-1, 4U 0115+63, Cen X-3, 4U 1626-67, XTE J1946-274, Vela X-1, 4U 1907+09, 4U 1538-52, GX 301-2, and 4U 0352+309 (X Per). We searched for correlations among the spectral parameters, concentrating on how the cyclotron line energy relates to the continuum and therefore how the neutron star B-field influences the X-ray emission. As expected, we found a correlation between the CRSF energy and the spectral cutoff energy. However, with our consistent set of fits we found that the relationship is more complex than what has been reported previously. Also, we found not only that the width of the cyclotron line correlates with the energy (as suggested by theory), but also that the width scaled by the energy correlates with the depth of the feature. We discuss the implications of these results, including the possibility that accretion directly affects the relative alignment of the neutron star spin and dipole axes. Finally, we comment on the current state of fitting phenomenological models to spectra in the RXTE/BeppoSAX era and the need for better theoretical models of the X-ray continua of accreting pulsars.

Low‐Luminosity States of the Black Hole Candidate GX 339−4. I. ASCA and Simultaneous Radio/RXTE Observations

We discuss a series of observations of the black hole candidate GX 339(4 in low-luminosity, spec- trally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmol- ogy and Astrophysics (ASCA) data sets and eight separate Rossi X-Ray T iming Explorer (RXT E) data sets. Three of the RXT E observations were strictly simultaneous with 843 MHz and 8.3¨9.1 GHz radio observations. All of these observations have (3¨9 keV) —ux ergs s~1 cm~2. The ASCA data show (10~9 evidence for an B6.4 keV Fe line with equivalent width B40 eV, as well as evidence for a soft excess that is well modeled by a power law plus a multicolor blackbody spectrum with peak temperature B150¨200 eV. The RXT E data sets also show evidence of an Fe line with equivalent widths B20¨140 eV. Re—ection models show a hardening of the RXT E spectra with decreasing X-ray —ux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law —ux and the solid angle subtended by the re—ector. ii Sphere)disk ˇˇ Comptonization models and advection- dominated accretion —ow (ADAF) models also provide reasonable descriptions of the RXT E data. The former models yield coronal temperatures in the range 20¨50 keV and optical depths of qB 3. The model —ts to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio —ux is synchrotron emission from an extended out—ow of size greater than O(107GM/c2). Subject headings: binaries: closeblack hole physicsradiation mechanisms: nonthermal ¨ stars: individual (GX 339(4) ¨ X-rays: stars

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Spinning up an extragalactic neutron star Ultraluminous x-ray sources (ULXs) are strange objects in other galaxies that cannot be explained by conventional accretion onto stellar-mass objects. This has led to exotic interpretations, such as the long-sought intermediate-mass black holes. Israel et al. observed a ULX in the nearby galaxy NGC 5907 and found that it is instead a neutron star. The spinning neutron star is accreting material so fast that its spin period is quickly accelerating. The only way that it can consume enough material to explain these properties is if it has a strong multipolar magnetic field. Science, this issue p. 817 An ultraluminous x-ray source in NGC 5907 is a spinning neutron star with a complex magnetic field. Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of ~1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥ 1041 erg second−1) might harbor NSs.

Low‐Luminosity States of the Black Hole Candidate GX 339−4. II. Timing Analysis

Here we present timing analysis of a set of eight Rossi X-Ray Timing Explorer (RXTE) observations of the black hole candidate GX 339-4 that were taken during its hard/low state. On long timescales, the RXTE All Sky Monitor data reveal evidence of a 240 day periodicity, comparable to timescales expected from warped, precessing accretion disks. On short timescales all observations save one show evidence of a persistent fQPO≈0.3 Hz quasi-periodic oscillation. The broadband (10-3 to 102 Hz) power appears to be dominated by two independent processes that can be modeled as very broad Lorentzian fits with Q1. The coherence function between soft and hard photon variability shows that if these are truly independent processes, then they are individually coherent, but they are incoherent with one another. This is evidenced by the fact that the coherence function between the hard and soft variability is near unity between 5 × 10−3 to 10 Hz but shows evidence of a dip at f≈1 Hz. This is the region of overlap between the broad Lorentzian fits to the power spectral density. As in Cyg X-1, the coherence also drops dramatically at frequencies 10 Hz. Also as in Cyg X-1, the hard photon variability is seen to lag the soft photon variability with the lag time increasing with decreasing Fourier frequency. The magnitude of this time lag appears to be positively correlated with the flux of GX 339-4. We discuss all of these observations in light of current theoretical models of both black hole spectra and temporal variability.

Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-Ray Spectrum of 4U 0115+63

We have discovered a third harmonic cyclotron resonance scattering feature (CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi X-Ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows CRSFs at 12.40, 21.45, and 33.56 keV. With centroid energy ratios to the fundamental of 1.73 ± 0.08 and 2.71 ± 0.13, the CRSFs are not harmonically spaced. Strong variability of the continuum and CRSFs with pulse phase indicate a complex emission geometry near the neutron star polar cap. In addition, one RXTE observation, which spanned periastron passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is slower by 2 orders of magnitude than the beat-frequency QPO expected in this system and slower by a factor of more than 5 compared with other QPOs seen in accreting X-ray pulsars.

Discovery of a Cyclotron Resonant Scattering Feature in the Rossi X-Ray Timing Explorer Spectrum of 4U 0352+309 (X Persei)

We have discovered a ~29 keV cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of 4U 0352+309 (X Per) using observations taken with the Rossi X-Ray Timing Explorer. The source 4U 0352+309 is a persistent low-luminosity (LX = 4.2 × 1034 ergs s-1) X-ray pulsar with a 837 s period, which accretes material from the Be star X Per. The X-ray spectrum, unusual when compared to brighter accreting pulsars, may be due to the low mass accretion rate and could be typical of the new class of persistent low-luminosity Be/X-ray binary pulsars. We attempted spectral fits with continuum models used historically for 4U 0352+309 and found that all were improved by the addition of a CRSF at ~29 keV. The model that best fitted the observations is a combination of a 1.45 ± 0.02 keV blackbody with a 5.4 × 108 cm2 area and a power law with a 1.83 ± 0.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6 keV, implying a magnetic field strength of 2.5(1 + z) × 1012 G in the scattering region (where z is the gravitational redshift). Phase-resolved analysis shows that the blackbody and cyclotron line energies are consistent with being constant through the pulse.

Long term variability of Cygnus X-1 - III. Radio–X-ray correlations

Long time scale radio-X-ray correlations in black holes during the hard state have been found in many sources and there seems to emerge a universal underlying relationship which quantitatively describes this behavior. Although it would appear only natural to detect short term emission patterns in the X-ray and - with a certain time lag - in the radio, there has been little evidence for this up to now. The most prominent source for radio-X-ray correlations on short time scales (minutes) so far remains GRS 1915+105 where a single mass ejection could be detected successively in X-ray, IR, and radio wavebands. We analyze a database of more than 4 years of simultaneous radio-X-ray data for Cygnus X-1 from the Ryle Telescope and RXTE PCA/HEXTE. We confirm the existence of a radio-X-ray correlation on long time scales, especially at hard energies. We show that apparent correlations on short time scales in the lightcurves of Cygnus X-1 are most likely the coincidental outcome of white noise statistics. Interpreting this result as a breakdown of radio-X-ray correlations on shorter time scales, this sets a limit to the speed of the jet.

Stability of the Cyclotron Resonance Scattering Feature in Hercules X-1 with RXTE

Five observations of the hard X-ray spectrum of Her X-1 from RXTE show that the ~41 keV energy of the cyclotron scattering line is constant within statistics of a few percent per observation. The overall spectral shape, on the other hand, varies somewhat, with an rms of 2%. If the 41 keV feature truly originates as cyclotron resonance scattering in an unchanging 3 × 1012 G dipole field not far above the neutron star surface, these observations constrain the average height of scattering to within a range of 180 m. This is consistent with models that put the radiating structure within meters of the surface of the neutron star. In other pulsars, observed line centroid changes have been correlated with luminosity changes, and if interpreted as variations of the height at which scattering takes place, many hundreds of meters are required. These RXTE data, which sample nearly a factor of 2 in unabsorbed luminosity, are in conflict with a particular model for such an extended radiating structure. Comparison with other observations over many years indicates strongly that the centroid energy of this absorption line has increased some time between 1991 and 1993 by 23%, from 34 to 41 keV. Moreover, the cutoff energy of the spectral continuum increased at the same time from 16 to 20 keV, which is, within the statistical error of 5%, in direct proportion to the centroid. This may be a sign that both these characteristics of the spectrum are controlled in the same way by the magnetic field strength in the region of scattering.

The 1999 Hercules X-1 Anomalous Low State

A failed main-on in the 35 day cycle of Her X-1 was observed with the Rossi X-Ray Timing Explorer on 1999 April 26. Exceptions to the normal 35 day cycle have been seen only twice before—in 1983 and again in 1993. We present timing and spectral results of this latest anomalous low state (ALS) along with comparisons to the main-on and normal low states. Pulsations were observed in the 3-18 keV band with a fractional rms variation of 0.037 ± 0.003. Spectral analysis indicates that the ALS spectrum has the same shape as the main-on but is modified by heavy absorption and scattering. We find that 70% of the observed emission has passed through a cold absorber (NH = 5.0 × 1023 cm-2). This partially absorbing spectral fit can be applied to the normal low state with similar results. We find that the ALS observations may be interpreted as a decrease in inclination of the accretion disk causing the central X-ray source to be obscured over the entire 35 day cycle.

ON THE ENIGMATIC X-RAY SOURCE V1408 AQUILAE (\4U 1957)11)

Models for the characteristically soft X-ray spectrum of the compact X-ray source V1408 Aql (\4U 1957)11) have ranged from optically thick Comptonization to multicolor accretion disk models. We critically examine the X-ray spectrum of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data, archival data, and recent Rossi X-Ray T iming Explorer Roe ntgensatellit (RXT E) data. Although we are able to —t a variety of X-ray spectral models to these data, we favor an interpretation of the X-ray spectrum as being due to an accretion disk viewed at large inclination angles. Evidence for this hypothesis includes long-term (117, 235, 352 day) periodicities seen by the RXT E All Sky Monitor, which we interpret as being due to a warped precessing disk, and a 1 keV feature in the ASCA data, which we interpret as being the blend of L —uorescence features from a disk atmosphere or wind. We also present a timing analysis of the RXT E data and —nd upper limits of 4% for the rms variability between f \ 10~3 and 16 Hz. The situation of whether the compact object is a black hole or neutron star is still ambiguous; however, it now seems more likely that an X-rayemitting, warped accre- tion disk is an important component of this system. Subject headings: accretion, accretion disksblack hole physicsstars: individual (V1408 Aquilae) ¨ stars: neutron