Phase-coherent Timing Solution Research Articles

Early X-ray and optical observations of the soft gamma-ray repeater SGR 0418+5729

Emission of two short hard X-ray bursts on 2009 June 5 disclosed the existence of a new soft gamma-ray repeater, now catalogued as SGR 0418+5729. After a few days, X-ray pulsations at a period of 9.1 s were discovered in its persistent emission. SGR 0418+5729 was monitored almost since its discovery with the Rossi X-ray Timing Explorer (2–10 keV energy range) and observed many times with Swift (0.2–10 keV). The source persistent X-ray emission faded by a factor of ∼10 in about 160 d, with a steepening in the decay about 19 d after the activation. The X-ray spectrum is well described by a simple absorbed blackbody, with a temperature decreasing in time. A phase-coherent timing solution over the ∼160 d time-span yielded no evidence for any significant evolution of the spin period, implying a 3σ upper limit of 1.1 × 10−13 s s−1 on the period derivative and of ∼3 × 1013 G on the surface dipole magnetic field. Phase-resolved spectroscopy provided evidence for a significant variation of the spectrum as a function of the stellar rotation, pointing to the presence of two emitting caps, one of which became hotter during the outburst. Finally, a deep observation of the field of SGR 0418+5729 with the new Gran Telescopio Canarias 10.4-m telescope allowed us to set an upper limit on the source optical flux of i′ > 25.1 mag, corresponding to an X-ray-to-optical flux ratio exceeding 104, consistent with the characteristics of other magnetars.

10 Years ofRXTEMonitoring of the Anomalous X‐Ray Pulsar 4U 0142+61: Long‐Term Variability

We report on 10 yr of monitoring of the 8.7-s Anomalous X-ray Pulsar 4U 0142+61 using the Rossi X-Ray Timing Explorer (RXTE). This pulsar exhibited stable rotation from 2000 until February 2006: the RMS phase residual for a spin-down model which includes ν,\(\dot{\nu}\), and\(\ddot{\nu}\)is 2.3%. We report a possible phase-coherent timing solution valid over a 10-yr span extending back to March 1996. A glitch may have occurred between 1998 and 2000, but it is not required by the existing data. We also report that the source’s pulse profile has been evolving in the past 6 years, such that the dip of emission between its two peaks has been getting shallower since 2000, almost as if the profile is recovering to its pre-2000 morphology, in which there was no clear distinction between the peaks. These profile variations are seen in the 2–4 keV band but not in 6–8 keV. Finally, we present the pulsed flux time series of the source in 2–10 keV. There is evidence of a slow but steady increase in the source’s pulsed flux since 2000. The pulsed flux variability and the narrow-band pulse profile changes present interesting challenges to aspects of the magnetar model.

Timing the Eccentric Binary Millisecond Pulsar in NGC 1851

We have used the Green Bank Telescope to observe the millisecond pulsar PSR J0514-4002A on 43 occasions spread over 2 years. This 5-ms pulsar is located in the globular cluster NGC 1851; it belongs to a binary system and has a highly eccentric (e = 0.888) orbit. We have obtained a phase-coherent timing solution for this object, including very precise position, spin and orbital parameters. The pulsar is located 4.6" (about 1.3 core radii) from the center of the cluster, and is likely to lie on its more distant half. The non-detection of eclipses at superior conjunction can be used, given the peculiar geometry of this system, to rule out the possibility of an extended companion. We have measured the rate of advance of periastron for this binary system to be $\dot{\omega}$ = 0.01289(4) degrees per year, which if due completely to general relativity, implies a total system mass of 2.453(14) solar masses. Given the known mass function, the pulsar mass has to be < 1.5 solar masses, and the mass of the companion has to be > 0.96 solar masses, implying that it is a heavy white dwarf. The 350-MHz flux density of this pulsar varies between 0.2 and 1.4 mJy; the origin of these variations is not known.

Evidence for a binary companion to the central compact object 1E 1207.4-5209

Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin period of 424 ms that contains at least two strong absorption features in its energy spectrum. This neutron star is positionally coincident with the supernova remnant PKS 1209-51/52 and has been identified as a member of the growing class of radio-quiet compact central objects in supernova remnants. From previous observations with Chandra and XMM-Newton, it has been found that the 1E 1207.4-5209 is not spinning down monotonically as is common for young, isolated pulsars. The spin frequency history requires either strong, frequent glitches, the presence of a fall-back disk, or a binary companion. Here, we report on a sequence of seven XMM-Newton observations of 1E 1207.4-5209 performed during a 40 day window between 2005 June 22 and July 31. Due to unanticipated variance in the phase measurements during the observation period that was beyond the statistical uncertainties, we could not identify a unique phase-coherent timing solution. The three most probable timing solutions give frequency time derivatives of +0.9, −2.6, and +1.6×10−12 Hz s−1 (listed in descending order of significance). We conclude that the local frequency derivative during our XMM-Newton observing campaign differs from the long-term spin-down rate by more than an order of magnitude. This measurement effectively rules out glitch models for 1E 1207.4-5209. If the long-term spin frequency variations are caused by timing noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than in other pulsars with similar period derivatives. Therefore, it is highly unlikely that the spin variations are caused by the same physical process that causes timing noise in other isolated pulsars. The most plausible scenario for the observed spin irregularities is the presence of a binary companion to 1E 1207.4-5209. We identified a family of orbital solutions that are consistent with our phase-connected timing solution, archival frequency measurements, and constraints on the companions mass imposed by deep IR and optical observations.

10 years of RXTE monitoring of anomalous X-ray pulsar 4U 0142+61: long-term variability

We report on 10 yr of monitoring of the 8.7-s Anomalous X-ray Pulsar 4U 0142+61 using the Rossi X-Ray Timing Explorer (RXTE). This pulsar exhibited stable rotation from 2000 until February 2006: the RMS phase residual for a spin-down model which includes ν, $\dot{\nu}$ , and $\ddot{\nu}$ is 2.3%. We report a possible phase-coherent timing solution valid over a 10-yr span extending back to March 1996. A glitch may have occurred between 1998 and 2000, but it is not required by the existing data. We also report that the source’s pulse profile has been evolving in the past 6 years, such that the dip of emission between its two peaks has been getting shallower since 2000, almost as if the profile is recovering to its pre-2000 morphology, in which there was no clear distinction between the peaks. These profile variations are seen in the 2–4 keV band but not in 6–8 keV. Finally, we present the pulsed flux time series of the source in 2–10 keV. There is evidence of a slow but steady increase in the source’s pulsed flux since 2000. The pulsed flux variability and the narrow-band pulse profile changes present interesting challenges to aspects of the magnetar model.

A Braking Index for the Young, High Magnetic Field, Rotation‐Powered Pulsar in Kesteven 75

We present the first phase-coherent measurement of a braking index for the young, energetic rotation-powered pulsar PSR J1846-0258. This 324 ms pulsar is located at the center of the supernova remnant Kesteven 75 and has a characteristic age of τc = 723 yr, a spin-down energy of Ė = 8.3 × 1036 ergs s-1, and an inferred magnetic field of 4.9 × 1013 G. Two independent phase-coherent timing solutions are derived, which together span 5.5 yr of data obtained with the Rossi X-Ray Timing Explorer. In addition, a partially phase-coherent timing analysis confirms the fully phase-coherent result. The measured value of the braking index, n = 2.65 ± 0.01, is significantly less than 3, the value expected from magnetic dipole radiation, implying another physical process must contribute to the pulsar's rotational evolution. Assuming the braking index has been constant since birth, we place an upper limit on the spin-down age of PSR J1846-0258 of 884 yr, the smallest age estimate of any rotation-powered pulsar.

The Millisecond Pulsars in NGC 6760

We present the results of recent Arecibo and Green Bank observations of the globular cluster NGC 6760. Using Arecibo, a phase-coherent timing solution has been obtained for the previously known binary pulsar in this cluster, PSR J1911+0102A. We have also discovered a new millisecond pulsar in NGC 6760, PSR J1911+0101B, an isolated object with a rotational period of 5.38 ms and a dispersion measure DM = 196.7 cm-3 pc. Both pulsars are located within 1.3 core radii of the cluster center and have negative period derivatives. The resulting lower limits for the accelerations of the pulsars are within the range expected given a simple model of the cluster. A search for eclipses in the PSR J1911+0102A binary system using both telescopes yielded negative results. The corresponding limits on the extra gas column density at superior conjunction are consistent with the hypothesis that the observational properties of ultra-low-mass binary pulsars like PSR J1911+0102A are strongly affected by the inclination of the orbital plane of the system. Among globular cluster pulsar populations, that of NGC 6760 exhibits one of the largest known spreads in DM. This quantity seems to be roughly proportional to a cluster's central DM; this suggests that the observed spread is caused by a turbulent interstellar medium at spatial scales of 1 pc.

Long-term Radio Timing Observations of PSR B1509–58

We present an updated phase-coherent timing solution for the young, energetic pulsar B1509–58 from twenty years of data. Using a partially phase-coherent timing analysis, we show that the second frequency derivative is changing in time, implying a third frequency derivative of This value is consistent with the simple power law model of pulsar rotation.

Phase-coherent timing of PSR J1811-1925: the pulsar at the heart of G11.2-0.3

The X-ray Pulsar PSR J1811-1925 in the historic supernova remnant G11.2-0.3 has a characteristic age over 10 times the age of the remnant. This likely implies that its current spin period, 65 ms, is close to its birth spin period. Alternatively, the pulsar may have an unusually high braking index. We report here on regular Rossi X-ray Timing Explorer/Proportional Counting Array (RXTE/PCA) timing observations of the pulsar that were designed to measure its braking index. We provide a preliminary phase-coherent timing solution which includes a significant ν ̈ . The braking index we measure is ≫3, likely a result of conventional timing noise. We also report on a preliminary analysis of the pulsar's hard spectrum: we determine a photon index of Γ=1.78±0.74, for the pulsed component of the spectrum, consistent within the uncertainties with previous ASCA and Chandra observations. The pulsed emission of PSR J1811-1925 is seen beyond 30 keV, and the pulsations remain sinusoidal up to and beyond this energy.

Discovery of Radio Pulsations from the X-Ray Pulsar J0205+6449 in Supernova Remnant 3C 58 with the Green Bank Telescope

We report the discovery with the 100m Green Bank Telescope of 65 ms radio pulsations from the X-ray pulsar J0205+6449 at the center of supernova remnant 3C58, making this possibly the youngest radio pulsar known. From our observations at frequencies of 820 and 1375 MHz, the free electron column density to USSR J0205+6449 is found to be 140.7 +/- 0.3/cc pc. The barycentric pulsar period P and P(dot) determined from a phase-coherent timing solution are consistent with the values previously measured from X-ray observations. The averaged radio profile of USSR J0205+6449 consists of one sharp pulse of width = 3 ms = 0.05 P. The pulsar is an exceedingly weak radio source, with pulse-averaged flux density in the 1400 MHz band of approximately 45 micro-Jy and a spectral index of approximately -2.1. Its radio luminosity of approximately 0.5 may kpc(exp 2) at 1400 MHz is lower than that of approximately 99% of known pulsar and is the lowest among known young pulsars.

Arecibo Timing Observations of 17 Pulsars along the Galactic Plane

We present phase-coherent timing solutions obtained for the first time for 17 pulsars discovered at Arecibo by Hulse & Taylor in a 430 MHz survey of the Galactic plane. This survey remains the most sensitive of the Galactic plane at 430 MHz and has comparable equivalent sensitivity to the 1400 MHz Parkes multibeam survey. Comparing both surveys we find that, as expected, the one at 430 MHz is limited in depth by interstellar dispersion and scattering effects and that the detection rate of pulsars with high spin-down luminosity ( > 1034 ergs s-1) at the low frequency is a factor of 5 smaller than at high frequency. We also present scatter-broadening measurements for two pulsars and pulse-nulling and mode-changing properties for two others.

Precision Timing of Two Anomalous X-Ray Pulsars.

We report on long-term X-ray timing of two anomalous X-ray pulsars, 1RXS J170849.0-400910 and 1E 2259+586, using the Rossi X-Ray Timing Explorer. In monthly observations made over 1.4 and 2.6 yr for the two pulsars, respectively, we have obtained phase-coherent timing solutions which imply that these objects have been rotating with great stability throughout the course of our observations. For 1RXS J170849.0-400910, we find a rotation frequency of 0.0909169331(5) Hz and frequency derivative -15.687&parl0;4&parr0;x10-14 Hz s-1 for epoch MJD 51215.931. For 1E 2259+586, we find a rotation frequency of 0.1432880613(2) Hz and frequency derivative -1.0026&parl0;7&parr0;x10-14 Hz s-1 for epoch MJD 51195.583. The rms phase residuals from these simple models are only approximately 0.01 cycles for both sources. We show that the frequency derivative for 1E 2259+586 is inconsistent with that inferred from incoherent frequency observations made over the last 20 yr. Our observations are consistent with the magnetar hypothesis and make binary accretion scenarios appear unlikely.