Altan Baykal

Recent X-Ray Measurements of the Accretion-powered Pulsar 4U 1907+09

X-ray observations of the accreting X-ray pulsar 4U 1907)09 obtained during 1996 February with the Proportional Counter Array onboard the Rossi X-Ray T iming Explorer (RXT E) have made possible the -rst measurement of the intrinsic pulse period since 1984: s. 4U (P pulse )

X-Ray Spectra and Pulse Frequency Changes in SAX J2103.5+4545

The 1999 November outburst of the transient pulsar SAX J2103.5+4545 was monitored with the large area detectors of the Rossi X-Ray Timing Explorer until the pulsar faded after a year. The 358 s pulsar was spun up for 150 days, at which point the flux dropped quickly by a factor of ≈7, the frequency saturated, and, as the flux continued to decline, a weak spin-down began. The pulses remained strong during the decay and the spin-up/flux correlation can be fitted to the Ghosh and Lamb derivations for the spin-up caused by accretion from a thin, pressure-dominated disk, for a distance of ≈3.2 kpc and a surface magnetic field of ≈1.2 ×1013 G. During the bright spin-up part of the outburst, the flux was subject to strong orbital modulation, peaking ≈3 days after periastron of the eccentric 12.68 day orbit, while during the faint part, there was little orbital modulation. The X-ray spectra were typical of accreting pulsars, describable by a cutoff power law, with an emission line near the 6.4 keV of Kα fluorescence from cool iron. The equivalent width of this emission did not share the orbital modulation, but nearly doubled during the faint phase, despite little change in the column density. The outburst could have been caused by an episode of increased wind from a Be star, such that a small accretion disk was formed during each periastron passage. A change in the wind and disk structure apparently occurred after 5 months such that the accretion rate was no longer modulated or the diffusion time was longer. The distance estimate implies that the X-ray luminosity observed was between 1 × 1036 and 6 × 1034 ergs s-1, with a small but definite correlation with the intrinsic power-law spectral index.

Pulsar braking indices, glitches and energy dissipation in neutron stars

Almost all pulsars with anomalous positive ¨ � measurements (corresponding to anomalous braking indices in the range 5 10 −7 ) as well as post-glitch or interglitch ¨ � measurements, obey the scaling between ¨ � and glitch parameters originally noted in the Vela pulsar. Negative second derivative values can be understood in terms of glitches that were missed or remained unresolved. We discuss the glitch rates and a priori probabilities of positive and negative braking indices according to the model developed for the Vela pulsar. This behaviour supports the universal occurrence of a non-linear dynamical coupling between the neutron star crust and an interior superfluid component. The implied lower limit to dynamical energy dissipation in a neutron star with spindown rate ˙ � is ˙ Ediss > 1.7× 10 −6 ˙ Erot. Thermal luminosities and surface temperatures due to dynamical energy dissipation are estimated for old neutron stars which are spinning down as rotating magnetic dipoles beyond the pulsar death line.

Dipping Activity in the X-Ray Pulsar 4U 1907+09

New observations of 4U 1907+09 with the Rossi X-Ray Timing Explorer show a kind of behavior never recognized before from this X-ray pulsar: intermittently, the X-ray intensity fades below the detection threshold during intervals ranging from a few minutes to perhaps 1.5 hours. The largest decrease of the 2-15 keV intensity is at least 98%. The observations span 70% of the orbit of the binary system of which the pulsar is a part. The dips were found to have a broad range of orbital phases. Given the strong variability of the source outside the dip periods with moderate changes in the spectrum, the occasional variable weak emission during dip periods with timescales and spectrum similar to that outside the dip periods, and the absence of strong enhancements in the column density of cold circumstellar matter (i.e., NH remains below 1023 cm-2), it is suggested that the dips are caused by cessations of the mass accretion by the neutron star rather than occultations by circumstellar gas. An inhomogeneous wind from the companion star may be responsible.

DISCOVERY OF A SOFT SPECTRAL COMPONENT AND TRANSIENT 22.7 SECOND QUASI-PERIODIC OSCILLATIONS OF SAX J2103.5+4545

XMM-Newton observed SAX J2103.5+4545 on 2003 January 6, while the Rossi X-Ray Timing Explorer (RXTE) was also monitoring the source. Using the RXTE Proportional Counter Array data set between 2002 December 3 and 2003 January 29, the spin period and average spin-up rate during the XMM-Newton observations were found to be 354.7940 ± 0.0008 s and (7.4 ± 0.9) × 10-13 Hz s-1, respectively. In the power spectrum of the 0.9-11 keV EPIC PN light curve, we found quasi-periodic oscillations (QPOs) around 0.044 Hz (22.7 s) with an rms fractional amplitude of ~6.6%. We interpreted this QPO feature as the Keplerian motion of inhomogeneities through the inner disk. In the X-ray spectrum, in addition to the power-law component with high-energy cutoff and the ~6.4 keV fluorescent iron emission line, we discovered a soft component consistent with blackbody emission with kT ~ 1.9 keV. The pulse phase spectroscopy of the source revealed that the blackbody flux peaked at the peak of the pulse with an emission radius of ~0.3 km, suggesting the polar cap on the neutron star surface as the source of the blackbody emission. The flux of the iron emission line at ~6.42 keV was shown to peak at the off-pulse phase, supporting the idea that this feature arises from fluorescent emission of the circumstellar material around the neutron star rather than the hot region in the vicinity of the neutron star polar cap.

Timing studies on RXTE observations of SAX J2103.5+4545

SAX J2103.5+4545 has been continuously monitored for ∼900 d by the Rossi X-ray Timing Explorer (RXTE) since its outburst in 2002 July. Using these observations and previous archival RXTE observations of SAX J2103.5+4545, we refined the binary orbital parameters and find the new orbital period as P = (12.665 36 ± 0.000 88) d and the eccentricity as 0.4055 ± 0.0032. With these new orbital parameters, we constructed the pulse frequency and pulse frequency derivative histories of the pulsar and confirmed the correlation between X-ray flux and pulse frequency derivative presented by Baykal, Stark & Swank. We constructed the power spectra for the fluctuations of pulse frequency derivatives and found that the power-law index of the noise spectra is 2.13 ± 0.6. The power-law index is consistent with random walk in pulse frequency derivative and is the steepest among the HMXRBs. X-ray spectra analysis confirmed the inverse correlation trend between power-law index and X-ray flux found by Baykal et al.

The timing noise of PSR 0823+26, 1706-16, 1749-28, 2021+51 and the anomalous braking indices

We have investigated the stability of the pulse frequency second derivatives (ν) of PSR 0823+26, 1706-16, 1749-28 and 2021+51 that show significant quadratic trends in their pulse-frequency histories in order to determine whether the observed second derivatives are secular or if they arise as part of noise processes. We have used time-of-arrival (TOA) data extending to more than three decades, which are the longest time-spans ever taken into account in pulse-timing analyses. We investigated the stability of the pulse-frequency second derivative in the framework of low-resolution noise power spectra estimated from the residuals of pulse frequency and TOA data. We have found that the ν terms of these sources arise from the red torque noise in the fluctuations of pulse-frequency derivatives, which may originate from the external torques from the magnetosphere of the pulsar.

The steady spin-down rate of 4U 1907+09

Using X-ray data from the Rossi X-ray Timing Explorer, we report the pulse timing results of the accretion-powered, high-mass X-ray binary pulsar 4UU 1907+09, covering a time-span of almost two years. We measured three new pulse periods in addition to the previously measured four pulse periods. We are able to connect pulse arrival times in phase for more than a year. The source has been spinning down almost at a constant rate, with a spin-down rate of v = (- 3.54 +/- 0.02) x 10(exp -14) Hz s(exp -1) for more than 15 yr. Residuals of pulse arrival times yield a very low level of random-walk noise, with a strength of approximately 2 x 10(exp -20) rad(exp 2) s(exp -3) on a time-scale of 383 d, which is 40 times lower than that of the high-mass X-ray binary pulsar Vela X-1. The noise strength is only a factor of five greater than that of the low-mass X-ray binary pulsar 4U 1626-67. The low level of the timing noise and the very stable spin-down rate of 4U 1907+09 make this source unique among the high-mass X-ray binary pulsars, providing another example, in addition to 4U 1626-67, of long-term quiet spin down from an accruing source. These examples show that the extended quiet spin-down episodes observed in the anomalous X-ray pulsars 1RXS J170849.0-400910 and 1E 2259+586 do not necessarily imply that these sources are not accreting pulsars.

A comprehensive study of RXTE and INTEGRAL observations of the X-ray pulsar 4U 1907+09

We analyse observations of the accretion-powered pulsar 4U 1907+09 from the International Gamma-Ray Astrophysics Laboratory (INTEGRAL; between 2005 October and 2007 November) and from the Rossi X-ray Timing Explorer (RXTE; between 2007 June and 2011 March). Using observations from the Imager on-board the INTEGRAL satellite (IBIS), the INTEGRAL Soft Gamma-Ray Imager (ISGRI) and the RXTE Proportional Counter Array (PCA), we update the pulse period history of the source. We construct power spectrum density of pulse frequencies, and we find that fluctuations in the pulse frequency derivatives are consistent with the random walk model with a noise strength of 1.27 × 10−21 Hz s−2. From the X-ray spectral analysis of RXTE/PCA observations, we find that the hydrogen column density is variable over the binary orbit, and that it tends to increase just after the periastron passage. We also show that the X-ray spectrum becomes hardened with decreasing X-ray flux. We discuss the pulse-to-pulse variability of the source near dipping ingress and egress. We find that the source is more likely to undergo dipping states after apastron until periastron when the accretion from clumpy wind might dominate. Thus, occasional transitions to a temporary propeller state might occur.

ROTSE Observations of the Young Cluster IC 348

CCD observations of stars in the young cluster IC 348 were obtained from 2004 August to 2005 January with the 0.45 m ROTSE-IIId robotic reflecting telescope at the Turkish National Observatory site, Bakirlitepe, Turkey. The timing analysis of selected stars whose X-ray counterparts were detected by the Chandra X-Ray Observatory were studied. The time series of stars were searched for rotational periodicity by using different period-search methods. Thirty-five stars were found to be periodic with periods ranging from 0.74 to 32.3 days. Eighteen of the 35 periodic stars were new detections. Four of the new detections were classical T Tauri stars (CTTSs), and the others were weak-line T Tauri stars (WTTSs) or G-type (or unknown spectral class) stars. In this study, we confirmed the stability of rotation periods of TTSs. The periods obtained by Cohen et al. and us were different by 1%. We also confirmed the 3.24 hr pulsation period of H254, which is a δ Scuti–type star as noted by Ripepi et al., but the other periods detected by them were not found. We examined the correlation between X-ray luminosity and rotation period for our sample of TTSs. There is a decline in the rotation period with X-ray luminosity for late-type TTSs.