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  • 1
    Publication Date: 2016-06-24
    Description: We report broad-band spectral properties of the high-mass X-ray binary pulsar SMC X-2 by using three simultaneous Nuclear Spectroscopy Telescope Array and Swift /XRT observations during its 2015 outburst. The pulsar was significantly bright, reaching a luminosity up to as high as ~5.5 x 10 38 erg s –1 in 1–70 keV range. Spin period of the pulsar was estimated to be 2.37 s. Pulse profiles were found to be strongly luminosity dependent. The 1–70 keV energy spectrum of the pulsar was well described with three different continuum models such as (i) negative and positive power law with exponential cutoff, (ii) Fermi -Dirac cutoff power law and (iii) cutoff power-law models. Apart from the presence of an iron line at ~6.4 keV, a model independent absorption like feature at ~27 keV was detected in the pulsar spectrum. This feature was identified as a cyclotron absorption line and detected for the first time in this pulsar. Corresponding magnetic field of the neutron star was estimated to be ~2.3 x 10 12  G. The cyclotron line energy showed a marginal negative dependence on the luminosity. The cyclotron line parameters were found to be variable with pulse phase and interpreted as due to the effect of emission geometry or complicated structure of the pulsar magnetic field.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
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  • 2
    Publication Date: 2016-02-14
    Description: We report the timing and spectral properties of the Be/X-ray binary pulsar GX 304-1 using two Suzaku observations during its 2010 August and 2012 January X-ray outbursts. Pulsations at ~275 s were clearly detected in the light curves from both observations. Pulse profiles were found to be strongly energy-dependent. During the 2010 observation, the prominent dips seen in soft X-ray (≤10 keV) pulse profiles were found to be absent at higher energies. However, during the 2012 observation, the pulse profiles were complex as a result of the presence of several dips. Significant changes in the shape of the pulse profiles were detected at high energies (〉35 keV). A phase shift of ~0.3 was detected while comparing the phase of the main dip in the pulse profiles below and above ~35 keV. The broad-band energy spectrum of the pulsar was well described by a partially absorbed negative and positive power law with exponential cut-off (NPEX) model with 6.4-keV iron line and a cyclotron absorption feature. The energy of the cyclotron absorption line was found to be ~53 and 50 keV for the 2010 and 2012 observations, respectively, indicating a marginal positive dependence on source luminosity. Based on the results obtained from phase-resolved spectroscopy, the absorption dips in the pulse profiles can be interpreted as due to the presence of additional matter at same phases. Observed positive correlation between the cyclotron line energy and luminosity, and the significant pulse-phase variation of cyclotron parameters are discussed from the perspective of theoretical models on the cyclotron absorption line in X-ray pulsars.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 3
    Publication Date: 2013-08-22
    Description: We report the detection of quasi-periodic oscillation (QPO) at ~41 mHz in the transient high-mass Be/X-ray binary pulsar 4U 0115+634 using data from the Rossi X-Ray Timing Explorer ( RXTE ) observatory. The observations used in the present work were carried out during X-ray outbursts in 1999 March–April, 2004 September–October and 2008 March–April. This frequency of the newly detected QPO was found to vary in 27–46 mHz range. This ~41 mHz QPO was detected in four of the 36 pointed RXTE Proportional Counter Array (PCA) observations during 1999 outburst where as during 2004 and 2008 outbursts, it was detected in four and three times out of 33 and 26 observations, respectively. Though QPOs at ~2 mHz and ~62 mHz were reported earlier, the ~41 mHz QPO and its first harmonic were detected for the first time in this pulsar. There are three RXTE /PCA observations where multiple QPOs were detected in the power-density spectrum of 4U 0115+634. Simultaneous presence of multiple QPOs is rarely seen in accretion-powered X-ray pulsars. Spectral analysis of all the pointed RXTE /PCA observations revealed that the 3–30 keV energy spectrum was well described by Negative and Positive power law with EXponential (NPEX) cut-off continuum model along with interstellar absorption and cyclotron absorption components. During the three X-ray outbursts, however, no systematic variation in any of the spectral parameters other than the earlier reported anti-correlation between cyclotron absorption energy and luminosity was seen. Presence of any systematic variation of QPO frequency and rms of QPO with source flux were also investigated yielding negative results.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 4
    Publication Date: 2015-02-08
    Description: We present the results obtained from broad-band spectroscopy of the high-mass X-ray binary 4U 1700–37 using data from a Suzaku observation in 2006 September 13–14 covering 0.29–0.72 orbital phase range. The light curves showed significant and rapid variation in source flux during entire observation. We did not find any signature of pulsations in the light curves. However, a quasi-periodic oscillation at ~20 mHz was detected in the power density spectrum of the source. The 1–70 keV spectrum was fitted with various continuum models. However, we found that the partially absorbed high-energy cut-off power law and Negative and Positive power law with Exponential cut-off (NPEX) models described the source spectrum well. Iron emission lines at 6.4 and 7.1 keV were detected in the source spectrum. An absorption-like feature at ~39 keV was detected in the residuals while fitting the data with NPEX model. Considering the feature as cyclotron absorption line, the surface magnetic field of the neutron star was estimated to be ~3.4 10 12 G. To understand the cause of rapid variation in the source flux, time-resolved spectroscopy was carried out by dividing the observation into 20 narrow segments. The results obtained from the time-resolved spectroscopy are interpreted as the accretion of inhomogeneously distributed matter in the stellar wind of the supergiant companion star as the cause of observed flux variation in 4U 1700–37. A sharp increase in column density after ~0.63 orbital phase indicates the presence of an accretion wake that blocks the continuum and produces the eclipse like low-flux segment.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 5
    Publication Date: 2015-08-12
    Description: We report the broad-band spectral properties of the X-ray pulsar Cep X-4 by using a Suzaku observation in 2014 July. The 0.8–70 keV spectrum was found to be well described by three continuum models – Negative and Positive power-law with Exponential cut-off (NPEX), high-energy cut-off power-law and CompTT models. Additional components such as a cyclotron line at ~28 keV and two Gaussian components for iron lines at 6.4 and 6.9 keV were required in the spectral fitting. Apart from these, an additional absorption feature at ~45 keV was clearly detected in residuals obtained from the spectral fitting. This additional feature at ~45 keV was clearly seen in phase-resolved spectra of the pulsar. We identified this feature as the first harmonic of the fundamental cyclotron line at ~28 keV. The ratio between the first harmonic and fundamental line energies (1.7) was found to be in disagreement with the conventional factor of 2, indicating that the heights of line-forming regions are different or viewed at larger angles. The phase-resolved spectroscopy of the fundamental and first harmonic cyclotron lines shows significant pulse-phase variation of the line parameters. This can be interpreted as the effect of viewing angle or the role of complicated magnetic field of the pulsar.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
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  • 6
    Publication Date: 2019-07-12
    Description: We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820 30. NICER observed 4U 182030 in 2017 August during a low-flux, hard spectral state, accumulating about 60 ks of exposure. Five thermonuclear X-ray bursts were detected, of which four showed clear signs of PRE. We extracted spectra during the PRE phases and fit each to a model that includes a Comptonized component to describe the accretion-driven emission, and a blackbody for the burst thermal radiation. The temperature and spherical emitting radius of the fitted blackbody are used to assess the strength of PRE in each burst. The two strongest PRE bursts (burst pair 1) had blackbody temperatures of 0.6 keV and emitting radii of 100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (0.67 keV) and smaller radii (75 km). All of the PRE bursts show evidence of narrow line emission near 1 keV. By coadding the PRE phase spectra of burst pairs 1 and, separately, 2, we find, in both coadded spectra, significant, narrow, spectral features near 1.0 (emission), 1.7, and 3.0 keV (both in absorption). Remarkably, all the fitted line centroids in the coadded spectrum of burst pair 1 appear systematically blueshifted by a factor of 1.0460.006 compared to the centroids of pair 2, strongly indicative of a gravitational shift, a wind-induced blueshift, or more likely some combination of both effects. The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii, and thus have weaker gravitational redshifts, and they generate faster outflows, yielding higher blueshifts. We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN70157 , The Astrophysical Journal Letters (ISSN 2041-8205) (e-ISSN 2041-8213); 2; 878; L27
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  • 7
    Publication Date: 2019-07-13
    Description: Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type I X-raybursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impacton the disk during one of the first X-ray burst observations with the Neutron Star Interior CompositionExplorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hardspectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits anexcess of soft X-ray photons below 1 keV, where NICER's sensitivity peaks. We interpret the excessas a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burstpersistent flux, possibly due to Poynting Robertson drag or coronal reprocessing. This is the firstsuch detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER willbe able to study how X-ray bursts affect the disk and corona for a range of accreting neutron starsystems and disk states.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN66162 , Astrophysical Journal Letters (ISSN 2041-8205) (e-ISSN 2041-8213); 855; 1; L4
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  • 8
    Publication Date: 2019-07-13
    Description: We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster" GS 1826238. NICER observed the source in the periods 2017 June 2029, July 1113, and September 915, for a total useful exposure of 34 ks. Two consecutive dwells obtained on 2017 September 9 revealed highly significant oscillations at a frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases from 0.7% at 1 keV to 2% at 6 keV. Similar oscillations were also detected at lower significance in three additional dwells. The oscillation frequency and amplitude are consistent with those of mHz QPOs reported in other accreting neutron star systems. A thermonuclear X-ray burst was also observed on 2017 June 22. The burst properties and X-ray colors are both consistent with GS 1826 being in a soft spectral state during these observations, findings that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that the mHz oscillations are associated with blackbody emission from the neutron star surface, modeling of the phase-resolved spectra shows that the oscillation is consistent with being produced by modulation of the temperature component of this emission. In this interpretation, the blackbody normalization, proportional to the emitting surface area, is consistent with being constant through the oscillation cycle. We place the observations in the context of the current theory of marginally stable burning and briefly discuss the potential for constraining neutron star properties using mHz oscillations.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN66108 , The Astrophysical Journal (ISSN 0004-637X) (e-ISSN 1538-4357); 865; 1; 63
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  • 9
    Publication Date: 2019-07-13
    Description: We present results of recent Neutron Star Interior Composition Explorer (NICER) observations of the accreting millisecond X-ray pulsar (AMXP) IGR J17062-6143 that show that it resides in a circular, ultracompact binary with a 38-minute orbital period. NICER observed the source for 26 kiloseconds over a 5.3-day span in 2017 August, and again for 14 and 11 kiloseconds in 2017 October and November, respectively. A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hertz in Rossi X-ray Timing Explorer (RXTE) data, and reveals phase modulation due to orbital motion of the neutron star. A coherent search for the orbital solution using the Z squared method finds a best-fitting circular orbit with a period of 2278.21 seconds (37.97 minutes), a projected semimajor axis of 0.00390 lt-s (Localization Test Statistic), and a barycentric pulsar frequency of 163.6561105 Hertz. This is currently the shortest known orbital period for an AMXP. The mass function is 9.12 times 10 (sup minus 8) solar mass, presently the smallest known for a stellar binary. The minimum donor mass ranges from approximately 0.005 to 0.007 times the solar mass for a neutron star mass from 1.2 to 2 times the solar mass. Assuming mass transfer is driven by gravitational radiation, we find donor mass and binary inclination bounds of 0.0175-0.0155 times the solar mass and 19 degrees less than i less than 27.5 degrees, where the lower and upper bounds correspond to 1.4 and 2 times the solar mass neutron stars, respectively. Folding the data accounting for the orbital modulation reveals a sinusoidal profile with fractional amplitude 2.04 plus or minus 0.11 percent (0.3-3.2 kiloelectronvolts).
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN61305 , Astrophysical Journal (ISSN 2041-8205) (e-ISSN 2041-8213); 858; 2; L13
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  • 10
    Publication Date: 2019-07-13
    Description: We present Neutron Star Interior Composition Explorer (NICER) observations of the neutron star (NS) low-mass X-ray binary Serpens X-1 during the early mission phase in 2017. With the high spectral sensitivity and low-energy X-ray passband of NICER, we are able to detect the Fe L line complex in addition to the signature broad, asymmetric Fe K line. We confirm the presence of these lines by comparing the NICER data to archival observations with XMM-Newton/Reflection Grating Spectrometer (RGS) and NuSTAR. Both features originate close to the innermost stable circular orbit (ISCO). When modeling the lines with the relativistic line model RELLINE, we find that the Fe L blend requires an inner disk radius of 1.4(sup 0.2, sub -0.1)R(sub ISCO) and Fe K is at 1.03(sup 0.13, sub -0.03)R(sub ISCO) (errors quoted at 90%). This corresponds to a position of 17(sup 2.5, sub -1.2)km and 12(sup 1.6, sub -0.4)km for a canonical NS mass (M(sub NS)=1.4 solar mass) and dimensionless spin value of a = 0. Additionally, we employ a new version of the RELXILL model tailored for NS(sub s) and determine that these features arise from a dense disk and supersolar Fe abundance.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN57931 , The Astrophysical Journal Letters (ISSN 2041-8205) (e-ISSN 2041-8213); 858; L5; No. 1
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