Engineering Papers⌕ Search

Engineering topics

Deepto Chakrabarty

Publications and source records attributed to Deepto Chakrabarty.

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2021

A NICER Look at Thermonuclear X-ray Bursts from Aql X-1

We present spectral and temporal properties of all the thermonuclear X-ray bursts observed from Aql X-1 by the Neutron StarInterior and Composition Explorer (NICER) between 2017 July and 2021 April. This is the first systematic investigation of alarge sample of type I X-ray bursts from Aql X-1 with improved sensitivity at low energies. We detect 22 X-ray bursts includingtwo short recurrence burst events in which the separation was only 451 s and 496 s. We perform time resolved spectroscopy of thebursts using the fixed and scaled background (fa method) approaches. We show that the use of a scaling factor to the pre-burstemission is the statistically preferred model in about 68% of all the spectra compared to the fixed background approach. Typically the fa values are clustered around 1–3, but can reach up to 11 in a burst where photospheric radius expansion is observed. Such fa values indicate a very significant increase in the pre-burst emission especially at around the peak flux moments of the bursts.We show that the use of the fa factor alters the best fit spectral parameters of the burst emission. Finally, we employed a reflectionmodel instead of scaling the pre-burst emission. We show that reflection models also do fit the spectra and improve the goodnessof the fits. In all cases we see that the disc is highly ionized by the burst emission and the fraction of the reprocessed emission tothe incident burst flux is typically clustered around 20%

Tolga Güver↗
NASA NTRS2021

NICER Discovery of Millisecond X-Ray Pulsations and an Ultracompact Orbit in IGR J17494-3030

We report the detection of 376.05 Hz (2.66 ms) coherent X-ray pulsations in NICER observations of a transient outburst of the low-mass X-ray binary IGR J17494−3030 in 2020 October/November. The system is an accreting millisecond X-ray pulsar in a 75-minute ultracompact binary. The mass donor is most likely a ≃0.02 Mꙩ finite-entropy white dwarf composed of He or C/O. The fractional rms pulsed amplitude is 7.4%, and the soft (1–3 keV) X-ray pulse profile contains a significant second harmonic. The pulsed amplitude and pulse phase lag (relative to our mean timing model) are energy dependent, each having a local maximum at 4 and 1.5 keV, respectively. We also recovered the X-ray pulsations in archival 2012 XMM-Newton observations, allowing us to measure a long-term pulsar spin-down rate of ύ= -2.1(7) x (10)^(-14) Hz/s and to infer a pulsar surface dipole magnetic field strength of ≃10^(9)G. We show that the mass transfer in the binary is likely nonconservative, and we discuss various scenarios for mass loss from the system.

Mason Ng↗
NASA NTRS2021

The X-ray Bursts of XTE J1739-285: A NICER Sample

In this work, we report on observations with the Neutron Star Interior Composition Explorer of the known neutron star X-ray transient XTE J1739–285. We observed the source in 2020 February and March, finding it in a highly active bursting state. Across a 20 day period, we detected 32 thermonuclear X-ray bursts, with an average burst recurrence time of-+2.0 hr0.30.4. A timing and spectral analysis of the ensemble of X-ray bursts reveals homogeneous burst properties, evidence for short-recurrence time bursts, and the detection of a 386.5 Hz burst oscillation candidate. The latter is especially notable, given that a previous study of this source claimed a 1122 Hz burst oscillation candidate. We did not find any evidence of variability near 1122 Hz and instead find that the 386.5 Hz oscillation is the more prominent signal of the two burst oscillation candidates. Hence, we conclude it is unlikely that XTE J1739–285 has a submillisecond rotation period.

X-ray burst↗
NASA NTRS2020

Timing the Pulsations of The Accreting Millisecond Pulsar SAX J1808.4-3658 During Its 2019 Outburst

In this paper we present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst. Using observations collected with the Neutron Star Interior Composition Explorer (NICER), we establish the pulsar spin frequency and orbital phase during its latest epoch. We find that the 2019 outburst shows a pronounced evolution in pulse phase over the course of the outburst. These phase shifts are found to correlate with the source flux, and are interpreted in terms of hot-spot drift on the stellar surface, driven by changes in the mass accretion rate. Additionally, we find that the long-term evolution of the pulsar spin frequency shows evidence for a modulation at the Earth's orbital period, enabling pulsar timing based astrometry of this accreting millisecond pulsar.

Peter Bult↗
NASA NTRS2019

Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set

We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437−4715, J0030+0451, J1231−1411, and J2124−3358, selected as targets for constraining the mass–radius relation of neutron stars and the dense matter equation of state (EoS) via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass–radius relation and the dense matter EoS. We find that the long-term timing and flux behavior and the Fourier domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437−4715, for which multiple temperatures are required.

Neutron stars↗
NASA NTRS2019

NICER X-ray Observations of Seven Nearby Rotation-powered Millisecond Pulsars

The Neutron star Interior Composition Explorer observed several rotation-powered millisecond pulsars (MSPs) to search for or confirm the presence of X-ray pulsations. When broad and sine-like, these pulsations may indicate thermal emission from hot polar caps at the magnetic poles on the neutron star surface. We report confident detections (≥4.7σ after background filtering) of X-ray pulsations for five of the seven pulsars in our target sample: PSR J0614−3329, PSR J0636+5129, PSR J0751+1807, PSR J1012+5307, and PSR J2241−5236, while PSR J1552+5437 and PSR J1744−1134 remain undetected. Of those, only PSR J0751+1807 and PSR J1012+5307 had pulsations previously detected at the 1.7σ and almost 3σ confidence levels, respectively, in XMM-Newton data. All detected sources exhibit broad sine-like pulses, which are indicative of surface thermal radiation. As such, these MSPs are promising targets for future X-ray observations aimed at constraining the neutron star mass–radius relation and the dense matter equation of state using detailed pulse profile modeling. Furthermore, we find that three of the detected MSPs exhibit a significant phase offset between their X-ray and radio pulses.

Sebastien Guillot↗
NASA NTRS2019

An Evolving Broad Iron Line from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017–2018 giant outburst. The 1.2–10 keV energy spectrum of the source can be approximated with an absorbed cutoff power-law model. We detect strong, luminosity-dependent emission lines in the 6–7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex. An asymmetric broad-line profile, peaking at 6.67 keV, is possibly due to Doppler effects and gravitational redshift. The 1.2–79 keV broadband spectrum from Nuclear Spectroscopic Telescope Array (NuSTAR) and NICER observations at the outburst peak is well described by an absorbed cutoff power law plus multiple Gaussian lines and a blackbody component. Physical reflection models are also tested to probe the broad iron-line feature. Depending on the mass accretion rate, we found emission sites that are evolving from ~5000 km to a range closer to the surface of the neutron star. Our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultrafast outflow expected at super-Eddington accretion rates. We do not detect any signatures of a cyclotron absorption line in the NICER or NuSTAR data.

Gaurava K. Jaisawal↗
NASA NTRS2019

NICER Observes a Secondary Peak in the Decay of a Thermonuclear Burst from 4U 1608-52

We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608–52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of ≈3 keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and reburning in the cooling tail of the burst.

Gaurava K. Jaisawal↗
NASA NTRS2019

Arcus: The Soft X-ray Grating Explorer

Arcus provides high-resolution soft X-ray spectroscopy in the 12-50 Å bandpass with unprecedented sensitivity, including spectral resolution > 2500 and effective area > 250 cm2. The three top science goals for Arcus are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback, and (3) to explore how stars form and evolve. Arcus uses the same 12 m focal length grazing-incidence Silicon Pore X-ray Optics (SPOs) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. Combined with the high-heritage NGIS LEOStar-2 spacecraft and launched into 4:1 lunar resonant orbit, Arcus provides high sensitivity and high efficiency observing of a wide range of astrophysical sources.

Randall K Smith↗
NASA NTRS2019

Anti-Glitches in the Ultraluminous Accreting Pulsar NGC 300 ULX-1 Observed with NICER

We present evidence for three spin-down glitches (or “anti-glitches”) in the ultraluminous accreting X-ray pulsar NGC 300 ULX-1, in timing observations made with the Neutron Star Interior Composition Explorer. Our timing analysis reveals three sudden spin-down events of magnitudes Δν=−23, −30, and −43 μHz (fractional amplitudes Δν/ν=−4.4, −5.5, and −7.7×10−4). We determined fully phase-coherent timing solutions through the first two glitches, giving us high confidence in their detection, while the third candidate glitch is somewhat less secure. These are larger in magnitude (and opposite in sign) than any known radio pulsar glitch. This may be caused by the prolonged rapid spin up of the pulsar, causing a sudden transfer of angular momentum between the superfluid and non-superfluid components of the star. We find no evidence for profile or spectral changes at the epochs of the glitches, supporting the conclusion that these are due to the same process as in normal pulsar glitches, but in reverse.

Paul S Ray↗
NASA NTRS2018

Dense matter with eXTP

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

Anna L. Watts↗
NASA NTRS2014

NuSTAR Results and Future Plans for Magnetar and Rotation-Powered Pulsar Observations

The Nuclear Spectroscopic Telescope Array (NuSTAR) is the first focusing hard X-ray mission in orbit and operates in the 3-79 keV range. NuSTAR's sensitivity is roughly two orders of magnitude better than previous missions in this energy band thanks to its superb angular resolution. Since its launch in 2012 June, NuSTAR has performed excellently and observed many interesting sources including four magnetars, two rotation-powered pulsars and the cataclysmic variable AE Aquarii. NuSTAR also discovered 3.76-s pulsations from the transient source SGR J1745−29 recently found by Swift very close to the Galactic Center, clearly identifying the source as a transient magnetar. For magnetar 1E 1841−045, we show that the spectrum is well fit by an absorbed blackbody plus broken power-law model with a hard power-law photon index of approximately 1.3. This is consistent with previous results by INTEGRAL and RXTE. We also find an interesting double-peaked pulse profile in the 25-35 keV band. For AE Aquarii, we show that the spectrum can be described by a multi-temperature thermal model or a thermal plus non-thermal model; a multi-temperature thermal model without a non-thermal component cannot be ruled out. Furthermore, we do not see a spiky pulse profile in the hard X-ray band, as previously reported based on Suzaku observations. For other magnetars and rotation-powered pulsars observed with NuSTAR, data analysis results will be soon available.

stars: neutron aEuro" telescope aEuro" X-ray: star↗