arXiv subjects

Deepto Chakrabarty

Publications and source records attributed to Deepto Chakrabarty.

At least 127 records · Page 7Linked to original sources

arXiv2005

The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338

We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray Observatory as well as optical observations conducted during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power-law (with photon index = 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broad-band spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star, and therefore very unlikely to be responsible for the soft excess, making synchroton emission a more reasonable candidate.

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arXiv2005

X-ray Spectroscopy of Candidate Ultracompact X-ray Binaries

We present high-resolution spectroscopy of the neutron star/low-mass X-ray binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four systems in which we had previously inferred an unusual Ne/O ratio in the absorption along the line of sight, most likely from material local to the binaries. However, our recent Chandra X-ray Observatory LETGS spectrum of 4U 1850-087 finds a Ne/O ratio by number of 0.22+/-0.05, smaller than previously measured and consistent with the expected interstellar value. We propose that variations in the Ne/O ratio due to source variability, as previously observed in these sources, can explain the difference between the low- and high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS observation of 4U 0513-40 also shows no unusual abundance ratios in the absorption along the line of sight. We also present spectral results from a third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by an absorbed power-law + blackbody model with absorption consistent with the expected interstellar value. Finally, we present the non-detection of a fourth candidate ultracompact binary, 4U 1905+000, with an upper limit on the source luminosity of < 1 x 10^{32} erg s^{-1}. Using archival data, we show that the source has entered an extended quiescent state.

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arXiv2004

Millisecond Pulsars in X-Ray Binaries

Despite considerable evidence verifying that millisecond pulsars are spun up through sustained accretion in low-mass X-ray binaries (LMXBs), it has proven surprisingly difficult to actually detect millisecond X-ray pulsars in LMXBs. There are only 5 accretion-powered millisecond X-ray pulsars known among more than 80 LMXBs containing neutron stars, but there are another 11 "nuclear powered" millisecond pulsars which reveal their spin only during brief, thermonuclear X-ray bursts. In addition, 2 of the accretion-powered pulsars also exhibit X-ray burst oscillations, and their unusual properties, along with the absence of persistent pulsations in most LMXBs, suggest that the magnetic fields in many LMXBs may be hidden by accreted material. Interestingly, the nuclear-powered pulsars offer a statistically unbiased probe of the spin distribution of recycled pulsars and show that this distribution cuts off sharply above 730 Hz, well below the breakup spin rate for most neutron star equations of state. This indicates that some mechanism acts to halt or balance spin-up due to accretion and that submillisecond pulsars must be very rare (and are possibly nonexistent). It is unclear what provides the necessary angular momentum sink, although gravitational radiation is an attractive possibility.

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arXiv2004

High-Resolution X-ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge

(Abbrev.) We present high-resolution spectroscopy of the oxygen K-shell interstellar absorption edge in 7 X-ray binaries using the HETGS onboard Chandra. Using the brightest sources as templates, we found a best-fit model of 2 absorption edges and 5 Gaussian absorption lines. All of these features can be explained by the recent predictions of K-shell absorption from neutral and ionized atomic oxygen. We identify the K alpha and K beta absorption lines from neutral oxygen, as well as the S=3/2 absorption edge. The expected S=1/2 edge is not detected in these data due to overlap with instrumental features. We also identify the K alpha absorption lines from singly and doubly ionized oxygen. The OI K alpha absorption line is used as a benchmark with which to adjust the absolute wavelength scale for theoretical predictions of the absorption cross-sections. We find that shifts of 30-50 mA are required, consistent with differences previously noticed from comparisons of the theory with laboratory measurements. Significant oxygen features from dust or molecular components, as suggested in previous studies, are not required by our HETGS spectra. With these spectra, we can begin to measure the large-scale properties of the ISM. We place a limit on the velocity dispersion of the neutral lines of <200 km s^{-1}, consistent with measurements at other wavelengths. We also make the first measurement of the oxygen ionization fractions in the ISM. We constrain the interstellar ratio of OII/OI to ~0.1 and the ratio of OIII/OI to <0.1.

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arXiv2004

The Effect of Neutron Star Rotation on the Properties of Thermonuclear X-ray Bursts

Previously, observations with the Rossi X-ray Timing Explorer showed that millisecond oscillations occur preferentially in thermonuclear X-ray bursts with photospheric radius expansion from sources rotating near 600 Hz, while they occur with equal likelihood in X-ray bursts with and without radius expansion for sources rotating near 300 Hz. With a larger sample of data than in previous studies, we find that the detectability of the oscillations is not directly determined by the properties of the X-ray bursts. Instead, we find that (1) the oscillations are observed almost exclusively when the accretion rate onto the neutron star is high, but that (2) radius expansion is only observed at high accretion rates from the 600 Hz sources, whereas it occurs only at low accretion rates in the 300 Hz sources. The persistent millisecond pulsars provide the only apparent exceptions to these trends. The first result might be explained if the oscillation amplitudes are attenuated at low accretion rates by an extended electron corona. The second result indicates that the rotation period of the neutron star determines how the burst properties vary with accretion rate, possibly through the differences in the effective surface gravity or the strength of the Coriolis force.

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arXiv2003

X-Ray Spectroscopy of the Low-Mass X-ray Binaries 2S 0918-549 and 4U1543-624: Evidence for Neon-Rich Degenerate Donors

We present high-resolution spectroscopy of the neutron-star/low-mass X-ray binaries 2S 0918-549 and 4U 1543-624 with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory and the Reflection Grating Spectrometer onboard XMM-Newton. Previous low-resolution spectra of both sources showed a broad line-like feature at 0.7 keV that was originally attributed to unresolved line emission. We recently showed that this feature could also be due to excess neutral Ne absorption, and this is confirmed by the new high-resolution Chandra spectra. The Chandra spectra are each well fit by an absorbed power-law + blackbody model with a modified Ne/O number ratio of 0.52+/-0.12 for 2S 0918-549 and 1.5+/-0.3 for 4U 1543-624, compared to the interstellar-medium value of 0.18. The XMM spectrum of 2S 0918-549 is best fit by an absorbed power-law model with a Ne/O number ratio of 0.46+/-0.03, consistent with the Chandra result. On the other hand, the XMM spectrum of 4U 1543-624 is softer and less luminous than the Chandra spectrum and has a best-fit Ne/O number ratio of 0.54+/-0.03. The difference between the measured abundances and the expected interstellar ratio, as well as the variation of the column densities of O and Ne in 4U 1543-624, supports the suggestion that there is absorption local to these binaries. We propose that the variations in the O and Ne column densities of 4U 1543-624 are caused by changes in the ionization structure of the local absorbing material. It is important to understand the effect of ionization on the measured absorption columns before the abundance of the local material can be determined. This work supports our earlier suggestion that 2S 0918-549 and 4U 1543-624 are ultracompact binaries with Ne-rich companions.

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arXiv2003

Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar

The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supranuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin.

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arXiv2003

Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars

Millisecond pulsars are neutron stars (NSs) that are thought to have been spun-up by mass accretion from a stellar companion. It is unknown whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many NSs that are accreting from a companion exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond brightness oscillations during bursts from ten NSs (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here, we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting NSs, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most NS models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.

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arXiv2003

Eddington-limited X-ray Bursts as Distance Indicators. I. Systematic Trends and Spherical Symmetry in Bursts from 4U 1728-34

We investigate the limitations of thermonuclear X-ray bursts as a distance indicator for the weakly-magnetized accreting neutron star 4U 1728-34. We measured the unabsorbed peak flux of 81 bursts in public data from the Rossi X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66 bursts exhibited photospheric radius expansion and were distributed about a mean bolometric flux of 9.2e-8 erg/cm^2/s, while the remaining (non-radius expansion) bursts reached 4.5e-8 erg/cm^2/s, on average. The peak fluxes of the radius-expansion bursts were not constant, exhibiting a standard deviation of 9.4% and a total variation of 46%. These bursts showed significant correlations between their peak flux and the X-ray colors of the persistent emission immediately prior to the burst. We also found evidence for quasi-periodic variation of the peak fluxes of radius-expansion bursts, with a time scale of approximately 40 d. The persistent flux observed with RXTE/ASM over 5.8 yr exhibited quasi-periodic variability on a similar time scale. We suggest that these variations may have a common origin in reflection from a warped accretion disk. Once the systematic variation of the peak burst fluxes is subtracted, the residual scatter is only approximately 3%, roughly consistent with the measurement uncertainties. The narrowness of this distribution strongly suggests that i) the radiation from the neutron star atmosphere during radius-expansion episodes is nearly spherically symmetric, and ii) the radius-expansion bursts reach a common peak flux which may be interpreted as a standard candle intensity.Adopting the minimum peak flux for the radius-expansion bursts as the Eddington flux limit, we derive a distance for the source of 4.4-4.8 kpc.

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arXiv2003

The Energy Dependence of Millisecond Oscillations in Thermonuclear X-ray Bursts

We examine the energy-resolved pulse profiles of 51 flux oscillations observed during the decline of thermonuclear X-ray bursts from accreting weakly- magnetized neutron stars with the Rossi X-ray Timing Explorer. We find that the fractional rms amplitudes of the oscillations increase as a function of energy by 0.25%/keV to 0.9%/keV between 3-20 keV, and are as large as 20% in the 13-20 keV band. We also show that the pulses observed in the higher energy bands generally lag behind those in lower energy bands by 0.002 cycles/keV to 0.007 cycles/keV between 3-20 keV. This amounts to total delays of 0.03-0.12 cycles between the lowest and highest energy bands, or time delays that range from 100-200 mus. We then model the oscillations as flux variations arising from temperature patterns on the surfaces of rapidly rotating neutron stars. In this framework, we find that the increase in the pulse amplitude with photon energy can be explained if the cooler regions on the neutron star emit in the lower energy bands, reducing the flux variations there. On the other hand, the Doppler shifts caused by the rapid rotation of the neutron star should cause the hard pulses to precede the soft pulses by about 0.05 cycles (100 mus), in contrast to the observations. This suggests that the photons originating from the stellar surface are reprocessed by a hot corona of electrons before they reach the observer.

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arXiv2002

X-ray Spectroscopy of the Accreting Millisecond Pulsar XTE J0929-314 in Outburst

We present the high-resolution spectrum of the accretion-powered millisecond pulsar XTE J0929-314 during its 2002 outburst, measured using the Low Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The 1.5-25.3 A (0.5-8.3 keV) Chandra spectrum is well fit by a power-law + blackbody model with photon index Gamma=1.55+/-0.03, blackbody temperature kT_bb=0.65+/-0.03 keV, and blackbody normalization R_bb,km/d_10kpc=7.6+/-0.8. No emission or absorption features are found in the high-resolution spectrum, with a 3 sigma equivalent width upper limit of <0.007 A at 1.5 A and <0.12 A at 24 A. The neutral absorption edge depths are consistent with the estimated interstellar absorption along the line of sight to the source. We found no orbital modulation of the 2-10 keV X-ray flux, to a 3 sigma upper limit of 1.1%, which implies an upper limit on the binary inclination angle of i<85deg for a Roche-lobe-filling companion. We also present the broadband spectrum measured over the course of the outburst by the Rossi X-ray Timing Explorer. The XTE spectrum of XTE J0929-314 is also well fit with a power-law + blackbody model, with average values of Gamma=1.76+/-0.03, kT_bb=0.66+/-0.06 keV, and R_bb,km/d_10kpc=5.9+/-1.3 in the 2-50 keV energy range. The blackbody flux remained constant over the course of the outburst, while the power-law flux was strongly correlated to the (decreasing) flux of the source. We find that the difference in power-law photon indices measured from Chandra and XTE spectra can be explained by a change in the power-law photon index at low energies.

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arXiv2002

The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-ray Bursts

We present a comprehensive observational and theoretical analysis of the amplitudes and profiles of oscillations that occur during thermonuclear X-ray bursts from weakly-magnetized neutron stars in low mass X-ray binaries. Our sample contains 59 oscillations from six sources observed with the Rossi X-ray Timing Explorer. The oscillations that we examined occurred primarily during the decaying portions of bursts, and lasted for several seconds each. We find that the oscillations are as large as 15% during the declines of the bursts, and they appear and disappear due to intrinsic variations in their fractional amplitudes. However, the maxima in the amplitudes are not related to the underlying flux in the burst. We derive folded profiles for each oscillation train to study the pulse morphologies. The mean rms amplitudes of the oscillations are 5%, although the eclipsing source MXB 1659-298 routinely produces 10% oscillations in weak bursts. We also produce combined profiles from all of the oscillations from each source. Using these pulse profiles, we place upper limits on the fractional amplitudes of harmonic and half-frequency signals of 0.3% and 0.6%, respectively (95% confidence). We then compare the pulse morphologies to theoretical profiles from models with one or two antipodal bright regions on the surface of a rotating neutron star. We find that if one bright region is present on the star, it must either lie near the rotational pole or cover nearly half the neutron star in order to be consistent with the observed lack of harmonic signals. If an antipodal pattern is present, the hot regions must form very near the rotational equator. We discuss how these geometric constraints challenge current models for the production of brightness variations on the surface of a neutron star. (abridged)

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arXiv2002

The Likely Near-Infrared Counterpart to the Anomalous X-Ray Pulsar 1E 1048.1-5937

We report our discovery of the likely near-infrared counterpart to the anomalous X-ray pulsar (AXP) 1E1048.1-5937, using observations from the 6.5-m Baade (Magellan-I) telescope in Chile. We derived a precise position for the X-ray source using archival data from the Chandra X-Ray Observatory. This position is inconsistent with a position reported earlier from XMM-Newton, but we show that the originally reported XMM-Newton position suffered from attitude reconstruction problems. Only two of the infrared objects in a 17 arcsec x 17 arcsec field containing the target have unusual colors, and one of these has colors consistent with those of the identified counterparts of two other AXPs. The latter object is also the only source detected within the 0.6 arcsec Chandra error circle, and we identify it as the counterpart to 1E1048.1-5937. This is the first AXP counterpart detected in multiple infrared bands, with magnitudes J=21.7(3), H=20.8(3), and K=19.4(3). There is marginal evidence for spectral flattening at longer wavelengths.

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arXiv2002

Rotational Broadening Measurement for the Neutron Star X-Ray Transient XTE J2123-058

We used optical spectroscopy of the neutron star X-ray transient XTE J2123-058 to measure the rotational broadening of the binary companion's stellar absorption lines and determined that the companion's projected rotational velocity is vsini = 121 (+21)(-29) km/s. This value is considerably larger than the measurements of vsini obtained previously for three other neutron star systems where the values are between 34 and 65 km/s. The larger value is likely due to the combination of high binary inclination and short (6 hr) orbital period for XTE J2123-058. Along with previously measured parameters, the rotational broadening measurement allowed us to determine the binary parameters, including the ratio of the neutron star mass to the companion mass, q = M_1/M_2 = 2.7 (+1.3)(-0.9), the neutron star mass, M_1 = 1.46 (+0.30)(-0.39) solar masses, and the companion mass, M_2 = 0.53 (+0.28)(-0.39) solar masses, assuming a Roche lobe filling companion synchronously rotating with the binary orbit. These values are consistent with a previous measurement where the H alpha emission line was used to determine the semiamplitude of the neutron star's radial velocity curve (K_1). It is a significant result that the two methods give consistent values. We also report the first measurement of the XTE J2123-058 companion's radius. Assuming synchronous rotation, we obtain R_2 = 0.62 (+0.11)(-0.15) solar radii, which is in-line with that expected for a late K-type star on or close to the main sequence. Finally, we report the first precise spectroscopic determination of the binary orbital period (P_orb = 21442.3 +/- 1.8 seconds).

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arXiv2002

The Frequency Stability of Millisecond Oscillations in Thermonuclear X-Ray Bursts

We analyze the frequency evolution of millisecond oscillations observed during type I X-ray bursts with the Rossi X-ray Timing Explorer in order to establish the stability of the mechanism underlying the oscillations. Our sample contains 68 pulse trains detected in a search of 159 bursts from 8 accreting neutron stars. As a first step, we confirm that the oscillations usually drift upward in frequency by about 1% toward an apparent saturation frequency. Previously noted anomalies, such as drifts toward lower frequencies as the oscillations disappear (``spin-down'' episodes) and instances of two signals present simultaneously at frequencies separated by a few Hz, occur in 5% of oscillations. Having verified the generally accepted description of burst oscillations, we proceed to study the coherence of the oscillations during individual bursts, and the dispersion in the asymptotic frequencies in bursts observed over five years. On short time scales, we find that 30% of the oscillation trains do not appear to evolve smoothly in phase. This suggests either that two signals are present simultaneously with a frequency difference too small to resolve (< 1 Hz), that the frequency evolution is discontinuous, or that discrete phase jumps occur. On time scales of years, the maximum frequencies of the oscillations exhibit fractional dispersions of < 0.004. In the case of 4U 1636-536, this dispersion is uncorrelated with the known orbital phase, which indicates that a mechanism besides orbital Doppler shifts prevents the oscillations from appearing perfectly stable. We interpret these results under the assumption that the oscillations originate from anisotropies in the emission from the surfaces of these rotating neutron stars. (Abridged)

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arXiv2002

Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary

We have identified the third known accretion-powered millisecond pulsar, XTE J0929-314, with the Rossi X-Ray Timing Explorer. The source is a faint, high-Galactic-latitude X-ray transient (d >~ 5 kpc) that was in outburst during 2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude of 3-7% and was generally broad and sinusoidal, although occasionally double-peaked. The hard X-ray pulses arrived up to 770 microseconds earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of -(9.2 +/- 0.4) * 10^-14 Hz/s; the spin-down torque may arise from magnetic coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational radiation from the rapidly spinning pulsar. The pulsations were modulated by a 43.6 min ultracompact binary orbit, yielding the smallest measured mass function (2.7 * 10^-7 M_sun) of any stellar binary. The binary parameters imply an approximately 0.01 M_sun white dwarf donor and a moderately high inclination. We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates. This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries.

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arXiv2002

The X-Ray Position and Infrared Counterpart of the Eclipsing X-Ray Pulsar OAO 1657-415

We have measured the precise position of the 38-s eclipsing X-ray pulsar OAO 1657-415 with the Chandra X-Ray Observatory: RA = 17h00m48.90s, Dec = -41d39m21.6s, equninox J2000, error radius = 0.5 arcsec. Based on the previously measured pulsar mass function and X-ray eclipse duration, this 10.4-d high-mass X-ray binary is believed to contain a B supergiant companion. Deep optical imaging of the field did not detect any stars at the Chandra source position, setting a limit of V>23. However, near-IR imaging revealed a relatively bright star (J=14.1, H=11.9, K_s=10.7) coincident with the Chandra position, and we identify this star as the IR counterpart of OAO 1657-415. The IR colors and magnitudes and the optical non-detections for this star are all consistent with a highly reddened B supergiant (A_V= 20.4 +/- 1.3) at a distance of 6.4 +/- 1.5 kpc. This implies an X-ray luminosity of 3e36 erg/s (2-10 keV). IR spectroscopy can verify the spectral type of the companion and measure its radial velocity curve, yielding a neutron star mass measurement.

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arXiv2002

How Do Z and Atoll X-ray Binaries Differ?

Low-mass X-ray binaries containing weakly magnetized neutron stars may be divided into two classes, Z and atoll sources, based in large part upon the patterns which they trace in plots of two X-ray colors. In this paper, we examine color-color diagrams of nine atoll sources and five Z sources using data from the Rossi X-ray Timing Explorer. The five-year span of data we have examined is significantly longer than those of previous studies. We find that the previous clear distinction between atoll and Z sources may be an artifact of incomplete sampling, as those atoll sources which are sampled over a wide dynamic range in intensity (greater than a factor of 80) trace three-branched color-color patterns similar to the tracks for which Z sources are named. This suggests that the two classes are more intimately related than previously realized, which also has been suggested based upon recent timing studies. However, detailed differences in the spectral behavior of the two classes argue against any simple unification.

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