arXiv subjects

Deepto Chakrabarty

Publications and source records attributed to Deepto Chakrabarty.

At least 145 records · Page 8Linked to original sources

arXiv2002

Chandra High-Resolution Spectrum of the Anomalous X-ray Pulsar 4U 0142+61

We report on a 25 ks observation of the 8.7 s anomalous X-ray pulsar 4U~0142+61 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. The continuum spectrum is consistent with previous measurements and is well fit by an absorbed power-law + blackbody with photon index Gamma=3.3+/-0.4 and kT=0.418+/-0.013 keV. No evidence was found for emission or absorption lines, with an upper limit of ~50 eV on the equivalent width of broad features in the 2.5-13 A (0.95-5.0 keV) range and an upper limit of ~10 eV on the equivalent width of narrow features in the 4.1-17.7 A (0.7-3.0 keV) range. If the source is a magnetar, then the absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. We find no strong features that are indicative of cyclotron absorption for magnetic field strengths of (1.9-9.8)x10^{14} G. This is still consistent with the dipole field strength of B=1.3x10^{14} G (at the polar cap) estimated from the pulsar's spindown.

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arXiv2001

Search for the Optical Counterparts of Southern Anomalous X-Ray Pulsars and Radio-Quiet Neutron Stars in Young Supernova Remnants

We report on our search for the optical counterparts of the Southern Hemisphere anomalous X-ray pulsar 1E1048.1-5937 and the radio-quiet neutron stars in supernova remnants Puppis A, RCW 103, and PKS 1209-52. The observations were carried out with the new MIT/CfA MagIC camera on the Magellan-I 6.5 m telescope in Chile. We present deep multiband optical images of the X-ray error circles for each of these targets and discuss the resulting candidates and limits.

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arXiv2001

15 Years Going Steady - Timing the Magnetar 1E 1841-045

We report on a long-term monitoring campaign of 1E 1841-045, the 12-s anomalous X-ray pulsar and magnetar candidate at the center of the supernova remnant Kes 73. We have obtained approximately monthly observations of the pulsar with the Rossi X-ray Timing Explorer spanning over two years, during which time 1E 1841-045 is found to be rotating with sufficient stability to derive a phase-connected timing solution. A linear ephemeris is consistent with observations of the pulse period made over the last 15 yrs with the Ginga, ASCA, XTE, & SAX observatories. Phase residuals suggest the presence of "timing noise", as is typically observed from young radio pulsars. These results confirm a rapid, constant spin-down for the pulsar, which continues to maintain a steady flux; this is inconsistent with most accretion scenarios.

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arXiv2001

Long-Term Monitoring of Anomalous X-ray Pulsars

We report on long-term monitoring of anomalous X-ray pulsars (AXPs) using the Rossi X-ray Timing Explorer. Using phase-coherent timing, we find a wide variety of behaviors among the sources, ranging from high stability (in 1E 2259.1+586 and 4U 0142+61), to instabilities so severe that phase-coherent timing is not possible (in 1E 1048.1-5937). We note a correlation in which timing stability in AXPs decreases with increasing spin-down rate. The timing stability of soft gamma repeaters in quiescence is consistent with this trend, which is similar to one seen in radio pulsars. We consider high signal-to-noise ratio average pulse profiles as a function of energy for each AXP, and find a variety of behaviors. We find no large variability in pulse morphology nor in pulsed flux as a function of time.

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arXiv2001

Chandra/HETGS Spectroscopy of the Anomalous X-ray Pulsar 4U 0142+61

We report on a 25 ks observation of the anomalous X-ray pulsar 4U 0142+61 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. The continuum spectrum is consistent with previous measurements and is well fit by an absorbed power-law + blackbody model with photon index Gamma=3.3+/-0.4 and blackbody temperature kT=0.418+/-0.013 keV. The pulsar frequency was 0.1150966+/-0.0000017 Hz and the pulse fractions were between 8.7% and 21%, which are also consistent with past measurements. No evidence was found for emission or absorption lines with an upper limit of ~50 eV on the equivalent width of a broad feature in the 2.5-13 A (0.95-4.96 keV) range. The absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. For the energy range given above, the allowed magnetic field strengths of 4U 0142+61 are B<1.9x 10^14 G and B>9.8x 10^14 G.

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arXiv2001

Double-Peaked X-Ray Lines from the Oxygen/Neon-Rich Accretion Disk in 4U1626-67

We report on a 39 ks observation of the 7.7-s low-mass X-ray binary pulsar 4U1626-67 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-Ray Observatory. This ultracompact system consists of a disk-accreting magnetic neutron star and a very low mass, hydrogen-depleted companion in a 42-min binary. We have resolved the previously reported Ne/O emission line complex near 1 keV into Doppler pairs of broadened (2500 km/s FWHM) lines from highly ionized Ne and O. In most cases, the blue and red line components are of comparable strength, with blueshifts of 1550-2610 km/s and redshifts of 770-1900 km/s. The lines appear to originate in hot (10^6 K), dense material just below the X-ray-heated skin of the outer Keplerian accretion disk, or else possibly in a disk wind driven from the pulsar's magnetopause. The observed photoelectric absorption edges of Ne and O appear nearly an order of magnitude stronger than expected from interstellar material and are likely formed in cool, metal-rich material local to the source. Based on the inferred local abundance ratios, we argue that the mass donor in this binary is probably the 0.02 M_sun chemically fractionated core of a C-O-Ne or O-Ne-Mg white dwarf which has previously crystallized.

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arXiv2001

The Optical Counterpart of the Accreting Millisecond Pulsar SAX J1808.4-3658 in Outburst: Constraints on the Binary Inclination

We present multiband optical/infrared photometry of V4580 Sgr, the optical counterpart of the accretion-powered millisecond pulsar SAX J1808.4-3658, taken during the 1998 X-ray outburst of the system. The optical flux is consistent with emission from an X-ray-heated accretion disk. Self-consistent modeling of the X-ray and optical emission during the outburst yields best-fit extinction Av=0.68 and inclination cos(i)=0.65, assuming a distance of 2.5 kpc. The allowed inclination range requires that the pulsar's stellar companion has extremely low mass, 0.05-0.10 Msun. Some of the infrared observations are not consistent with disk emission and are too bright to be from either the disk or the companion, even in the presence of X-ray heating.

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arXiv2001

Keck Measurement of the XTE J2123-058 Radial Velocity Curve

We measured the radial velocity curve of the companion of the neutron star X-ray transient XTE J2123-058. Its semi-amplitude (K_2) of 298.5 +/- 6.9 km/s is the highest value that has been measured for any neutron star LMXB. The high value for K_2 is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function (f_2) of 0.684 +/- 0.047 solar masses, along with our constraints on the companion's spectral type (K5V-K9V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8 solar masses. We also derive a source distance of 8.5 +/- 2.5 kpc, indicating that XTE J2123-058 is unusually far, 5.0 +/- 1.5 kpc, from the Galactic plane. Our measurement of the systemic radial velocity is -94.5 +/- 5.5 km/s, which is significantly different from what would be observed if this object corotates with the disk of the Galaxy.

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arXiv2001

Ultracompact X-Ray Binaries with Neon-Rich Degenerate Donors

There are three low-mass X-ray binaries (4U 0614+091, 2S 0918-549, and 4U 1543-624) for which broad line emission near 0.7 keV was previously reported. A recent high-resolution observation of 4U 0614+091 with the Chandra/LETGS found evidence for an unusually high Ne/O abundance ratio along the line of sight but failed to detect the previously reported 0.7 keV feature. We have made a search of archival ASCA spectra and identified a fourth source with the 0.7 keV feature, the 20-min ultracompact binary 4U 1850-087. In all four of these sources, the 0.7 keV residual is eliminated from the ASCA spectra by allowing excess photoelectric absorption due to a non-solar relative abundance of neon, just as in the LETGS spectrum of 4U 0614+091. The optical properties of these systems suggest that all four are ultracompact (P_orb<80 min) binaries. We propose that there is excess neon local to each of these sources, as also found in the ultracompact binary pulsar 4U 1626-67. We suggest that the mass donor in these systems is a low-mass, neon-rich degenerate dwarf and that the binaries are all ultracompact.

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arXiv2001

Millihertz Optical/UV Oscillations in 4U 1626-67: Evidence for a Warped Accretion Disk

We have detected large-amplitude 0.3-1.2 mHz quasi-periodic oscillations (QPOs) from the low-mass X-ray binary pulsar 4U 1626-67/KZ TrA, using UV photometry from HST and ground-based optical photometry. These 1 mHz QPOs, which have coherence (Nu/DeltaNu)=8, are entirely distinct from the 130 mHz pulsar spin frequency, a previously known 48 mHz QPO, and the 42 min binary period (independently confirmed here). Unlike the 48 mHz and 130 mHz oscillations which are present in both the optical/UV and the X-ray emission, the 1 mHz QPOs are not detected in simultaneous observations with RXTE. The rms amplitude of the mHz QPO decreases from 15% in the far UV to 3% in the optical, while the upper limit on a corresponding X-ray QPO is as low as 0.8%. We suggest that the mHz oscillations are due to warping of the inner accretion disk. We also report the detection of coherent upper and lower sidebands of the 130 mHz optical pulsations, with unequal amplitude and a spacing of 1.93 mHz around the main pulsation. The origin of these sidebands remains unclear.

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arXiv2001

Long-Term RXTE Monitoring of the Anomalous X-ray Pulsar 1E 1048.1-5937

We report on long-term monitoring of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 using the Rossi X-ray Timing Explorer (RXTE). The timing behavior of this pulsar is different from that of other AXPs being monitored with RXTE. In particular, we show that the pulsar shows significant deviations from simple spin-down such that phase-coherent timing has not been possible over time spans longer than a few months. We find that the deviations from simple spin down are not consistent with single "glitch" type events, nor are they consistent with radiative precession. We show that in spite of the rotational irregularities, the pulsar exhibits neither pulse profile changes nor large pulsed flux variations. We discuss the implications of our results for AXP models. In the context of the magnetar model, we suggest that 1E 1048.1-5937 may be a transition object between the soft gamma-ray repeater and AXP populations, and the AXP most likely to one day undergo an outburst.

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arXiv2001

The erratic luminosity behavior of SAX J1808.4-3658 during its 2000 outburst

We report on the highly variable and erratic long-term X-ray luminosity behavior of the only known accretion-driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst, as observed with RXTE. The maximum observed luminosity is ~2.5 x 10^35 erg s^-1 (3-25 keV; for a distance of 2.5 kpc), which is a factor of ~10 lower than that observed during the 1996 and 1998 outbursts. Due to solar constraints, the source could not be observed for several months with RXTE before 21 Jan. 2000. Therefore, the exact moment of the outburst onset is unknown and the peak luminosity could have been significantly higher. On some occasions SAX J1808.4-3658 was observed with luminosities of ~10^35 erg s^-1, but on other occasions it could not be detected with RXTE resulting in upper limits of a few times 10^33 erg s^-1 (3-25 keV). The non-detections of the source during its 2000 outburst obtained with BeppoSAX demonstrate that its luminosity was at times <10^32 erg s^-1 (0.5-10 keV). However, only a few days after these BeppoSAX observations, we detected the source again with RXTE at high luminosities, giving a factor of >1000 of luminosity swings in this system on time scales of days. The last detection of SAX J1808.4-3658 with RXTE was on 2000 May 13, almost 4 months after the first detection. Due to the lack of sensitivity and observations during the 1996 and 1998 outbursts, it cannot be excluded that after those outbursts the source remained active for months and that the source behavior during the 2000 outburst is not unique. Long duration activity at low luminosities has been observed in other transients (both neutron stars and black holes), although not with such extreme variability which might point to a different origin for this behavior for the pulsar.

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arXiv2001

Millisecond Oscillations and Photospheric Radius Expansion in Thermonuclear X-Ray Bursts

We use archival data from the Rossi X-Ray Timing Explorer to examine 125 type I X-ray bursts from the 9 weakly magnetic accreting neutron stars where millisecond oscillations have been detected during some bursts. We find that oscillations from the 6 "fast" (approximately 600 Hz) sources are almost always observed during radius expansion bursts, whereas oscillations from the 3 "slow" (about 300 Hz) sources are about equally likely to be found in bursts both with and without photospheric radius expansion. This strongly suggests that the distinction between these two source groups cannot be an observational selection effect, but must instead arise from some physical mechanism.

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arXiv2001

A Brown Dwarf Companion for the Accreting Millisecond Pulsar SAX J1808.4-3658

BeppoSAX has revealed a population of faint neutron star X-ray transients in the Galactic bulge. King conjectured that these neutron stars are accreting from brown dwarfs (BD) with a time-averaged mass transfer rate <\dot M>\approx 10^{-11} M_sun/year. We show that the orbital parameters of the 401 Hz accreting millisecond pulsar SAX J1808.4-3658 support this hypothesis. A main-sequence mass donor requires a nearly face-on inclination and a higher <\dot M> than observed. However, the range of allowed inclinations is substantially relaxed and the predicted mass transfer rate is consistent with that observed if a hot 0.05 M_sun BD is the donor. We then explain the BD radius required (0.13 R_\sun) to fill the Roche lobe. All transiently accreting neutron stars have a minimum luminosity in quiescence set by the time-averaged mass transfer rate onto the neutron star. We show that the constant heating of the BD by this emission can maintain the higher entropy implied by a 0.13 R_\sun radius. Our work bolsters the case that SAX J1808.4-3658 is a progenitor to compact ms radio pulsar binaries (e.g. like those found in 47 Tuc). The very low <\dot M>'s implies that the progenitors to these radio pulsars are long-lived (Gyrs) transient systems, rather than short-lived (Myrs) Eddington-limited accretors. Hence, the accreting progenitor population to millisecond radio pulsars in 47 Tuc could still be present and found in quiescence with Chandra.(abridged)

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arXiv2001

Anomalous X-ray Pulsars and Soft gamma-ray Repeaters: Spectral Fits and the Magnetar Model

The energy source powering the X-ray emission from anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is still uncertain. In one scenario, the presence of an ultramagnetized neutron star, or ``magnetar'', with B on the order of 10^{14} - 10^{15} G is invoked. To investigate this hypothesis, we have analyzed archival ASCA data for several known AXPs and SGRs, and fitted them with a model in which all or part of the X-ray flux originates as thermal emission from a magnetar. Our magnetar spectral model includes the effects of the anisotropy of the heat flow through an ultramagnetized neutron star envelope, reprocessing by a light element atmosphere, and general relativistic corrections to the observed spectrum. We obtain good fits to the data with radii for the emitting areas which are generally consistent with those expected for neutron stars, in contrast to blackbody (BB) fits, which imply much smaller radii. Furthermore, the inclusion of atmospheric effects results in inferred temperatures which are lower than those implied by BB fits, but however still too high to be accounted by thermal cooling alone. An extra source of heating (possibly due to magnetic field decay) is needed. Despite the harder tail in the spectrum produced by reprocessing of the outgoing flux through the atmosphere, spectral fits still require a considerable fraction of the flux to be in a power-law component.

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arXiv2000

Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper 4U 1916-053

We report the discovery of a highly coherent oscillation in a type-I X-ray burst observed from 4U 1916-053 by the Rossi X-ray Timing Explorer (RXTE). The oscillation was most strongly detected approx. 1 s after the burst onset at a frequency of 269.3 Hz, and it increased in frequency over the following 4 seconds of the burst decay to a maximum of around 272 Hz. The total measured drift of 3.58 +/- 0.41 Hz (1 sigma) represents the largest fractional change in frequency (1.32 +/- 0.15 %) yet observed in any burst oscillation. If the asymptotic frequency of the oscillation is interpreted in terms of a decoupled surface burning layer, the implied neutron star spin period is around 3.7 ms. However, the expansion of the burning layer required to explain frequency drift during the burst is around 80 m, substantially larger than expected theoretically (assuming rigid rotation). The oscillation was not present in the persistent emission before the burst, nor in the initial rise. When detected its amplitude was 6-12% (RMS) with a roughly sinusoidal profile. The burst containing the oscillation showed no evidence for photospheric radius expansion, while at least 5 of the other 9 bursts observed from the source by RXTE during 1996 and 1998 did. No comparable oscillations were detected in the other bursts. A pair of kilohertz quasi-periodic oscillations (QPOs) has been previously reported from this source with a mean separation of 348 +/- 12 Hz. 4U 1916-053 is the first example of a source where the burst oscillation frequency is significantly smaller than the frequency separation of the kHz QPOs.

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arXiv2000

The Central X-Ray Point Source in Cassiopeia A

The spectacular first light observation by the Chandra X-Ray Observatory revealed an X-ray point source near the center of the 300 yr old Cas A supernova remnant. We present an analysis of the public X-ray spectral and timing data. No coherent pulsations were detected in the Chandra/HRC data. The 3-sigma upper limit on the pulsed fraction is <35% for P>20 ms. The Chandra/ACIS spectrum of the point source may be fit with an ideal blackbody (kT=0.5 keV), or with BB models modified by the presence of a NS atmosphere (kT=0.25-0.35 keV), but the temperature is higher and the inferred emitting area lower than expected for a 300 yr old NS according to standard cooling models. The spectrum may also be fit with a power law model (photon index 2.8-3.6). Both the spectral properties and the timing limits of the point source are inconsistent with a young Crab-like pulsar, but are quite similar to the properties of the anomalous X-ray pulsars. The spectral parameters are also very similar to those of the other radio-quiet X-ray point sources in the supernova remnants Pup A, RCW 103, and PKS 1209-52. Current limits on an optical counterpart for the Cas A point source rule out models that invoke fallback accretion onto a compact object if fallback disk properties are similar to those in quiescent low-mass X-ray binaries. However, the optical limits are marginally consistent with plausible alternative assumptions for a fallback disk. In this case, accreting NS models can explain the X-ray data, but an accreting BH model is not promising.

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arXiv2000

ASCA Observation of the New Transient X-ray Pulsar XTE J0111.2-7317 in the Small Magellanic Cloud

The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days after its discovery with the Proportional Counter Array onboard the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6x10^-10 erg cm^-2 s^-1 in the 0.7--10.0 keV band, which corresponds to the X-ray luminosity of 1.8x10^38 erg s^-1, if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of 30.9497 +/- 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with average value of \~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted to a simple power-law type model. The soft excess is eliminated if the spectrum is fitted to an ``inversely broken power-law'' model, in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above ~2 keV is modeled by a power-law. In these models, however, the thermal soft component requires a very large emission zone, and hence it is difficult to explain the observed pulsations at energies below 2 keV. A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 +/- 14 eV. Pulse phase resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum.

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