arXiv subjects

Victoria M. Kaspi

Publications and source records attributed to Victoria M. Kaspi.

At least 145 records · Page 8Linked to original sources

arXiv2007

Deep Searches for Radio Pulsations and Bursts from Four Southern Anomalous X-ray Pulsars

We have searched for persistent radio pulsations, bright single pulses, and bursts from four Southern anomalous X-ray pulsars (AXPs). Deep observations were conducted at 1.4 GHz in 1999 July and August with the Parkes 64-m telescope. For all of the target AXPs, the upper limits on integrated pulsed emission are ~0.02 mJy, while the limits on the flux of single pulses are ~1 Jy or better. The corresponding radio luminosity limits are significantly below the observed luminosities of the majority of the observed radio pulsars, and they are significantly below the radio luminosity of XTE J1810-197, the only AXP known thus far to emit radio pulsations, at the epoch when radio pulses were first detected from this source. Our null results support the hypothesis that pulsed radio emission from AXPs is only present in conjunction with X-ray outburst activity. However, we cannot rule out the possibility that pulsed radio emission is present, but that it is too weak to be detected in our observations, or that unfavorable viewing geometries prevent this emission from being seen by terrestrial observers. Given the possible association between the radio emission and transient X-ray behavior of XTE J1810-197, continued radio searches of these and other AXPs at different epochs are warranted, particularly after periods of X-ray burst activity.

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arXiv2007

New Phase-coherent Measurements of Pulsar Braking Indices

Pulsar braking indices offer insight into the physics that underlies pulsar spin-down. Only five braking indices have been measured via phase-coherent timing; all measured values are less than 3, the value expected from magnetic dipole radiation. Here we present new measurements for three of the five pulsar braking indices, obtained with phase-coherent timing for PSRs J1846-0258 (n=2.65+/-0.01), B1509-58 (n=2.839+/-0.001) and B0540-69 (n=2.140+/-0.009). We discuss the implications of these results and possible physical explanations for them.

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arXiv2006

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 nu, nudot, and nuddot 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 occured 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.

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arXiv2006

An XTE Archival Search for Coherent X-ray Pulsations in LMXB 4U 1820-30

As part of a large-scale search for coherent pulsations from LMXBs in the RXTE archive, we have completed a detailed series of searches for coherent pulsations of 4U 1820-30 -- an ultracompact LMXB with a binary period of 11.4 min, located in the globular cluster NGC6624. The short binary period implies any coherent signal would be highly accelerated, so we used phase modulation searches, orbital-parameter-fitting coherent searches, and standard acceleration searches to give significant sensitivity to millisecond pulsations. We searched, in four energy bands and at a range of luminosities, a total of 34 archival RXTE observations, 32 of which had on-source integration times longer than 10 ks, and some of which were made consecutively which allowed us to combine them. We found no pulsations. Using our phase modulation search technique, which we ran on all 34 observations, we have been able to place the first stringent (95% confidence) pulsed fraction limits of <~0.8% for all realistic spin frequencies (i.e. <~2kHz) and likely companion masses (0.02Msun <= M_c <= 0.3Msun). Using our orbital-parameter-fitting coherent search, which we ran on only 11 selected observations, we have placed a pulsed fraction limit of <~0.3% for spin frequencies <~1.25kHz and companion masses M_ <= 0.106Msun. By contrast, all five LMXBs known to emit coherent pulsations have intrinsic pulsed fractions in the range 3% to 7% when pulsations are observed. Hence, our searches rule out pulsations with significantly lower pulsed fractions than those already observed.

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arXiv2006

A Braking Index for the Young, High-Magnetic-Field, Rotation-Powered Pulsar in Kes 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 Kes 75 and has a characteristic age of tau_c = 723 years, a spin-down energy of 8.3x10^{36}erg/s, and inferred magnetic field of 4.9x10^{13} 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 J1846-0258 of 884 yr, the smallest age estimate of any rotation-powered pulsar.

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arXiv2006

A Radio Pulsar Spinning at 716 Hz

We have discovered a 716-Hz eclipsing binary radio pulsar in the globular cluster Terzan 5 using the Green Bank Telescope. It is the fastest-spinning neutron star ever found, breaking the 23-year-old record held by the 642-Hz pulsar B1937+21. The difficulty in detecting this pulsar, due to its very low flux density and high eclipse fraction (~40% of the orbit), suggests that even faster-spinning neutron stars exist. If the pulsar has a mass less than 2 Msun, then its radius is constrained by the spin rate to be < 16 km. The short period of this pulsar also constrains models that suggest gravitational radiation, through an r-mode instability, limits the maximum spin frequency of neutron stars.

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arXiv2005

Long-term Phase-coherent X-ray Timing of PSR B0540-69

We present a new phase-coherent timing analysis for the young, energetic pulsar PSR B0540-69 using 7.6 yr of data from the Rossi X-Ray Timing Explorer. We measure the braking index, n=2.140+/-0.009, and discuss our measurement in the context of other discordant values reported in the literature. We present an improved source position from the phase-coherent timing of the pulsar, to our knowledge, the first of its kind from X-ray pulsar timing. In addition, we detect evidence for a glitch which has been previously reported but later disputed. The glitch occured at MJD 51335+/-12 with a fractional change in frequency of (1.4+/-0.2)E{-9} and a fractional change in frequency derivative of (1.33+/-0.02)E{-4}. We calculate that the glitch activity parameter for PSR B0540-69 is two orders of magnitude smaller than that of the Crab pulsar which has otherwise very similar properties. This suggests that neutron stars of similar apparent ages, rotation properties and inferred dipolar B fields can have significantly different internal properties.

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arXiv2005

A Burst and Simultaneous Short-Term Pulsed Flux Enhancement from the Magnetar Candidate 1E 1048.1-5937

We report on the 2004 June 29 burst detected from the direction of the Anomalous X-ray Pulsar (AXP) 1E 1048.1-5937 using the Rossi X-ray Timing Explorer (RXTE). We find a simultaneous increase of ~3.5 times the quiescent value in the 2-10 keV pulsed flux of 1E 1048.1-5937 during the tail of the burst which identifies the AXP as the burst's origin. The burst was overall very similar to the two others reported from the direction of this source in 2001. The unambiguous identification of 1E 1048.1-5937 as the burster here confirms it was the origin of the 2001 bursts as well. The epoch of the burst peak was very close to the arrival time of 1E 1048.1-5937's pulse peak. The burst exhibited significant spectral evolution with the trend going from hard to soft. During the 11 days following the burst, the AXP was observed further with RXTE, XMM-Newton and Chandra. Pre- and post-burst observations revealed no change in the total flux or spectrum of the quiescent emission. Comparing all three bursts detected thus far from this source we find that this event was the most fluent (>3.3x10^-8 erg/cm^2 in the 2-20 keV band), had the highest peak flux (59+/-9x10^-10 erg/s/cm^2 in the 2-20 keV band), and the longest duration (>699 s). The long duration of the burst differentiates it from Soft Gamma Repeater (SGR) bursts which have typical durations of ~0.1 s. Bursts that occur preferentially at pulse maximum, have fast-rises and long X-tails containing the majority of the total burst energy have been seen uniquely from AXPs. The marked differences between AXP and SGRs bursts may provide new clues to help understand the physical differences between these objects.

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arXiv2005

Twenty-One Millisecond Pulsars in Terzan 5 Using the Green Bank Telescope

We have discovered 21 millisecond pulsars (MSPs) in the globular cluster Terzan 5 using the Green Bank Telescope, bringing the total of known MSPs in Terzan 5 to 24. These discoveries confirm fundamental predictions of globular cluster and binary system evolution. Thirteen of the new MSPs are in binaries, of which two show eclipses and two have highly eccentric orbits. The relativistic periastron advance for the two eccentric systems indicates that at least one of these pulsars has a mass >1.68 Msun at 95% confidence. Such large neutron star masses constrain the equation of state of matter at or beyond the nuclear equilibrium density.

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arXiv2004

21 years of Timing PSR B1509-58

We present an updated timing solution for the young, energetic pulsar PSR B1509-58 based on 21.3 years of radio timing data and 7.6 years of X-ray timing data. No glitches have occurred in this time span, in contrast to other well-studied young pulsars, which show frequent glitches. We report a measurement of the third frequency derivative of (-1.28+/-0.21)x10^(-31) s^(-4). This value is 1.65 standard deviations from, i.e. consistent with, that predicted by the simple constant magnetic dipole model of pulsar spin-down. We measured the braking index to be n=2.839+/-0.003 and show that it varies by 1.5% over 21.3 yr due to contamination from timing noise. Results of a low-resolution power spectral analysis of the significant noise apparent in the data yield a spectral index of alpha=-4.6+/-1.0 for the red noise component.

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arXiv2004

Orbital Modulation of the Apparent Dispersion Measure Towards PSR J0737-3039A

This paper has been withdrawn from astro-ph and from the submission process to ApJ Letters since the variations "observed" are not real. As pointed out to me first by Willem van Straten and shortly thereafter by Matthew Bailes and Dick Manchester, the "Orbital Modulation of the Apparent Dispersion Measure Towards PSR J0737-3039A" is, in fact, an artifact of the data reduction. The problem occurs only for observations using very wide bandwidths of fast-moving and compact-orbit binary pulsars when reduced in a "traditional" manner (for folding of data, this means using a single set of polycos to fold the full bandwidth of data -- as was done for this paper). To quote Dick Manchester: "I believe I know the reason for the apparent DM variation in your data. We saw a similar effect as soon as we started taking the dual-freq [i.e. simultaneous 10cm/50cm data from Parkes] data. I showed that the effect was due to calling the polyco for folding the data with an incorrect RF frequency. The differential delay due to the dispersion moves the apparent orbital phase by a significant amount for these very short-period binaries. This leads to a sinusoidal modulation at the orbital period in the residuals, the amplitude (and sign) of which is a function of the difference between the prediction frequency and the observed frequency." and Willem Van Straten: "We were using a polyco created with a constant centre frequency [...] to fold data observed at all other frequencies. The canonical thinking behind this was to leave the relative dispersion delays in the data, so that tempo could later fit for DM. However, because the position of PSR J0737-3039A changes significantly during the dispersion delay time, the binary phase predicted at the [...] barycentric proper time [for the observing frequency used for the polycos] is significantly different to that at the proper time of other frequencies. For most pulsars, this doesn't matter much. But v/c for PSR J0737-3039A is about 10^-3. Another way to describe the problem is that, although the dispersion time delay does not evolve with binary phase, the corresponding pulsar _phase_ varies strongly as a function of dispersion delay (and therefore frequency)." Matthew Bailes pointed out that if this Doppler-like effect is not accounted for, than for any binary pulsar, an orbital variation in DM will occur of amplitude: dDM = V_orb/c * DM. This implies that for binaries where this effect is significant, an orbital phase dependent de-dispersion methodology is required. For pulsar folding, that can be accomplished by using a different set of polycos for _each_ frequency channel (or subband) in your data. I have confirmed that the DM variations do in fact go away when the data is processed correctly. Hopefully this mistake on my part did not cause too many people to waste significant amounts of time trying to explain the "effect"! Sincerely, Scott Ransom

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arXiv2004

Anomalous X-ray Pulsar 1E 1048.1-5937: Pulsed Flux Flares and Large Torque Variations

We report on continued monitoring of the Anomalous X-ray pulsar (AXP) 1E 1048.1-5937 using the Rossi X-ray Timing Explorer. We confirm that this pulsar has exhibited significant pulsed flux variability. The principal features of this variability are two pulsed X-ray flares. Both flares lasted several months and had well-resolved few-week-long rises. The long rise times of the flares are a phenomenon not previously reported for this class of object. The epochs of the flare peaks were MJD 52,218.8+/-4.5 and 52,444.4+/-7.0. Both flares had shorter rise than fall times. The flares had peak fluxes of 2.21+/-0.16 and 3.00+/-0.13 times the quiescent value. We estimate a total 2-10 keV energy release of ~2.7x10^40 ergs and ~2.8x10^41 ergs for the flares, assuming a distance of 5 kpc. We also report large (factor of ~12) changes to the pulsar's spin-down rate on time scales of weeks to months, shorter than has been reported previously. We find marginal evidence for correlation between the flux and spin-down rate variability, with probability of nonrandom correlation 6%. We discuss the implications of our findings for AXP models.

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arXiv2004

X-ray Sources and their Optical Counterparts in the Globular Cluster M4

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

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arXiv2004

The X-ray Bursts from the Magnetar Candidate 1E 2259+586

We present a statistical analysis of the X-ray bursts observed from the 2002 June 18 outburst of the Anomalous X-ray Pulsar (AXP) 1E 2259+586, observed with the Proportional Counter Array aboard the Rossi X-ray Timing Explorer. We show that the properties of these bursts are similar to those of Soft Gamma-Repeaters (SGRs). The similarities we find are: the burst durations follow a log-normal distribution which peaks at 99 ms, the differential burst fluence distribution is well described by a power law of index -1.7, the burst fluences are positively correlated with the burst durations, the distribution of waiting times is well described by a log-normal distribution of mean 47 s, and the bursts are generally asymmetric with faster rise than fall times. However, we find several quantitative differences between the AXP and SGR bursts. Specifically, there is a correlation of burst phase with pulsed intensity, the AXP bursts we observed exhibit a wider range of durations, the correlation between burst fluence and duration is flatter than for SGRs, the observed AXP bursts are on average less energetic than observed SGR bursts, and the more energetic AXP bursts have the hardest spectra - the opposite of what is seen for SGRs. We conclude that the bursts are sufficiently similar that AXPs and SGRs can be considered united as a source class yet there are some interesting differences that may help determine what physically differentiates the two closely related manifestations of neutron stars.

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arXiv2004

Long Term Timing Observations of PSR B1509-58

We present an updated phase-coherent timing solution for the young, energetic pulsar PSR B1509-58 for 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 (-9+/-1)x10^-32 s^(-4). This value is consistent with the simple power law model of pulsar rotation.

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arXiv2004

A 20 cm Search for Pulsars in Globular Clusters with Arecibo and the GBT

We are conducting deep searches for radio pulsations at L-band (~ 20cm) towards more than 30 globular clusters using the 305m Arecibo telescope in Puerto Rico and the 100m Green Bank Telescope in West Virginia. With roughly three quarters of our search data analyzed, we have discovered 12 new millisecond pulsars, 11 of which are in binary systems, and at least three of which eclipse. We have timing solutions for several of these systems.

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arXiv2004

PSR J2021+3651: A New Gamma-Ray Pulsar Candidate

The COS B high energy gamma-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716, has avoided identification with a low energy counterpart for over twenty years. We present a likely identification with the discovery and subsequent timing of a young and energetic pulsar, PSR J2021+3651, with the Wideband Arecibo Pulsar Processor at the Arecibo Observatory. PSR J2021+3651 has a rotation period P = 104ms and Pdot = 9.6 x 10^{-14}, implying a characteristic age T_c = 17kyr and a spin-down luminosity Edot = 3.4 x 10^{36} erg s^{-1}. The pulsar is also coincident with the ASCA source AX J2021.1+3651. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA. The pulsar's unexpectedly high dispersion measure DM = 371 pc cm^{-3} and the d > 10kpc DM distance pose a new question: is PSR J2021+3651 an extremely efficient gamma-ray pulsar at the edge of the Galaxy? This is a question for AGILE and GLAST to answer.

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arXiv2004

The Pulsed Spectra of Two Extraordinary Pulsars

We report on X-ray monitoring of two isolated pulsars within the same RXTE field of view. PSR J1811-1925 in the young supernova remnant G11.2-0.3 has a nearly sinusoidal pulse profile with a hard pulsed spectrum (photon index \~1.2). The pulsar is a highly efficient (~ 1% of spin-down energy) emitter of 2-50 keV pulsed X-rays despite having a fairly typical B ~ 2e12 G magnetic field. PSR J1809-1943/XTE J1810-197 is a newly discovered slow (P=5.54 s), apparently isolated X-ray pulsar which increased in flux by a factor of ~100 in 2003 January. Nine months of monitoring observations have shown a decrease in pulsed flux of ~ 30% without a significant change in its apparently thermal spectrum (kT ~0.7 keV) or pulse profile. During this time, the spin-down torque has fluctuated by a factor of ~ 2. Both the torque and the flux have remained steady for the last 3 months, at levels consistent with a magnetar interpretation.

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