arXiv subjects

P. Bult

Publications and source records attributed to P. Bult.

16 recordsLinked to original sources

arXiv2023

Thermonuclear Type-I X-ray Bursts and Burst Oscillations from the Eclipsing AMXP Swift J1749.4-2807

Swift J1749.4-2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this paper, we report on 7 thermonuclear (Type-I) X-ray bursts observed by NICER during its 2021 outburst. The first 6 bursts show slow rises and long decays, indicative of mixed H/He fuel, whereas the last burst shows fast rise and decay, suggesting He-rich fuel. Time-resolved spectroscopy of the bursts revealed typical phenomenology (i.e., an increase in black body temperature during the burst rise, and steady decrease in the decay), however they required a variable $N_\mathrm{H}$. We found that the values of $N_\mathrm{H}$ during the bursts were roughly double those found in the fits of the persistent emission prior to each burst. We interpret this change in absorption as evidence of burst-disc interaction, which we observe due to the high inclination of the system. We searched for burst oscillations during each burst and detected a signal in the first burst at the known spin frequency of the neutron star (517.92 Hz). This is the first time burst oscillations have been detected from Swift J1749.4-2807. We further find that each X-ray burst occurs on top of an elevated persistent count rate. We performed time-resolved spectroscopy on the combined data of the bursts with sufficient statistics (i.e., the clearest examples of this phenomenon) and found that the black body parameters evolve to hotter temperatures closer to the onset of the bursts. We interpret this as a consequence of an unusual marginally stable burning process similar to that seen through mHz QPOs.

astro-ph.HE↗

arXiv2023

Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER

We report the discovery of millihertz quasi-periodic oscillations (mHz QPOs) from the neutron star (NS) low-mass X-ray binary 4U 1730--22 using the Neutron Star Interior Composition Explorer (NICER). After being inactive for almost 50 years, 4U 1730--22 went into outburst twice between June and August 2021, and between February and July 2022. We analyse all the NICER observations of this source, and detect mHz QPOs with a significance > $4\sigma$ in 35 observations. The QPO frequency of the full data set ranged between ~4.5 and ~8.1 mHz with an average fractional rms amplitude of the order of ~2%. The X-ray colour analysis strongly suggests that 4U 1730--22 was in a soft spectral state during the QPO detections. Our findings are consistent with those reported for other sources where the mHz QPOs have been interpreted as the result of a special mode of He burning on the NS surface called marginally stable nuclear burning (MSNB). We conclude that the mHz QPOs reported in this work are also associated with the MSNB, making 4U 1730--22 the eighth source that shows this phenomenology. We discuss our findings in the context of the heat flux from the NS crust.

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arXiv2022

MAXI J1957+032: a new accreting millisecond X-ray pulsar in an ultra-compact binary

The detection of coherent X-ray pulsations at ~314 Hz (3.2 ms) classifies MAXI J1957+032 as a fast-rotating, accreting neutron star. We present the temporal and spectral analysis performed using NICER observations collected during the latest outburst of the source. Doppler modulation of the X-ray pulsation revealed the ultra-compact nature of the binary system characterised by an orbital period of ~1 hour and a projected semi-major axis of 14 lt-ms. The neutron star binary mass function suggests a minimum donor mass of 1.7e-2 Msun, assuming a neutron star mass of 1.4 Msun and a binary inclination angle lower than 60 degrees. This assumption is supported by the lack of eclipses or dips in the X-ray light curve of the source. We characterised the 0.5-10 keV energy spectrum of the source in outburst as the superposition of a relatively cold black-body-like thermal emission compatible with the emission from the neutron star surface and a Comptonisation component with photon index consistent with a typical hard state. We did not find evidence for iron K-alpha lines or reflection components.

astro-ph.HE↗

arXiv2022

Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4-2807 with NICER, XMM-Newton and NuSTAR

The neutron star low-mass X-ray binary SWIFT J1749.4-2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this manuscript we perform a spectral characterization of the system throughout its 2021, two-week-long outburst, analyzing 11 NICER observations and quasi-simultaneous XMM-Newton and NuSTAR single observations at the outburst peak. The broadband spectrum is well-modeled with a black body component with a temperature of $\sim$0.6 keV, most likely consistent with a hot spot on the neutron star surface, and a Comptonisation spectrum with power-law index $\Gamma \sim 1.9$, arising from a hot corona at $\sim$12 keV. No direct emission from the disc was found, possibly due to it being too cool. A high truncation radius for the disc, i.e., at $\sim$20--30 R$_{G}$ , was obtained from the analysis of the broadened profile of the Fe line in the reflection component. The significant detection of a blue-shifted Fe XXVI absorption line at $\sim$7 keV indicates weakly relativistic X-ray disc winds, which are typically absent in the hard state of X-ray binaries. By comparing the low flux observed during the outburst and the one expected in a conservative mass-transfer, we conclude that mass-transfer in the system is highly non-conservative, as also suggested by the wind detection. Finally, using the Nicer spectra alone, we followed the system while it was fading to quiescence. During the outburst decay, as the spectral shape hardened, the hot spot on the neutron star surface cooled down and shrank, a trend which could be consistent with the pure power-law spectrum observed during quiescence.

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arXiv2022

On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4-2807

We present the pulsar timing analysis of the accreting millisecond X-ray pulsar SWIFT J1749.4-2807 monitored by NICER and XMM-Newton during its latest outburst after almost eleven years of quiescence. From the coherent timing analysis of the pulse profiles, we updated the orbital ephemerides of the system. Large phase jumps of the fundamental frequency phase of the signal are visible during the outburst, consistent with what was observed during the previous outburst. Moreover, we report on the marginally significant evidence for non-zero eccentricity ($e\simeq 4\times 10^{-5}$) obtained independently from the analysis of both the 2021 and 2010 outbursts and we discuss possible compatible scenarios. Long-term orbital evolution of SWIFT J1749.4-2807 suggests a fast expansion of both the NS projected semi-major axis $(x)$, and the orbital period $(P_{\rm orb})$, at a rate of $\dot{x}\simeq 2.6\times 10^{-13}\,\text{lt-s}\,\text{s}^{-1}$ and $\dot{P}_{\rm orb}\simeq 4 \times 10^{-10}\,\text{s}\,\text{s}^{-1}$, respectively. SWIFT J1749.4-2807 is the only accreting millisecond X-ray pulsar, so far, from which the orbital period derivative has been directly measured from appreciable changes on the observed orbital period. Finally, no significant secular deceleration of the spin frequency of the compact object is detected, which allowed us to set a constraint on the magnetic field strength at the polar caps of $B_{PC}<1.3\times 10^{8}~\text{G}$, in line with typical values reported for AMXPs.

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arXiv2021

Dips and eclipses in the X-ray binary Swift J1858.6-0814 observed with NICER

We present the discovery of eclipses in the X-ray light curves of the X-ray binary Swift J1858.6-0814. From these, we find an orbital period of $P=76841.3_{-1.4}^{+1.3}$ s ($\approx21.3$ hours) and an eclipse duration of $t_{\rm ec}=4098_{-18}^{+17}$ s ($\approx1.14$ hours). We also find several absorption dips during the pre-eclipse phase. From the eclipse duration to orbital period ratio, the inclination of the binary orbit is constrained to $i>70^\circ$. The most likely range for the companion mass suggests that the inclination is likely to be closer to this value than $90^\circ$. The eclipses are also consistent with earlier data, in which strong variability ('flares') and the long orbital period prevent clear detection of the period or eclipses. We also find that the bright flares occurred preferentially in the post-eclipse phase of the orbit, likely due to increased thickness at the disc-accretion stream interface preventing flares being visible during the pre-eclipse phase. This supports the notion that variable obscuration is responsible for the unusually strong variability in Swift J1858.6-0814.

astro-ph.HE↗

arXiv2020

Discovery of Thermonuclear Type-I X-ray Bursts from the X-ray binary MAXI J1807+132

MAXI J1807+132 is a low-mass X-ray binary (LMXB) first detected in outburst in 2017. Observations during the 2017 outburst did not allow for an unambiguous identification of the nature of the compact object. MAXI J1807+132 was detected in outburst again in 2019 and was monitored regularly with NICER. In this paper we report on five days of observations during which we detected three thermonuclear (Type-I) X-ray bursts, identifying the system as a neutron star LMXB. Time-resolved spectroscopy of the three Type-I bursts revealed typical characteristics expected for these phenomena. All three Type-I bursts show slow rises and long decays, indicative of mixed H/He fuel. We find no strong evidence that any of the Type-I bursts reached the Eddington Luminosity; however, under the assumption that the brightest X-ray burst underwent photospheric radius expansion, we estimate a <12.4kpc upper limit for the distance. We searched for burst oscillations during the Type-I bursts from MAXI J1807+132 and found none (<10% amplitude upper limit at 95% confidence level). Finally, we found that the brightest Type-I burst shows a ~1.6sec pause during the rise. This pause is similar to one recently found with NICER in a bright Type-I burst from the accreting millisecond X-ray pulsar SAX J1808.4-3658. The fact that Type-I bursts from both sources can show this type of pause suggests that the origin of the pauses is independent of the composition of the burning fuel, the peak luminosity of the Type-I bursts, or whether the NS is an X-ray pulsar.

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arXiv2020

NICER observations reveal that the X-ray transient MAXI J1348-630 is a Black Hole X-ray binary

We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348-630 as observed with NICER. We produced the fundamental diagrams commonly used to trace the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348-630 is similar to that commonly observed in black hole transients. The source evolved from the hard state, through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the hard state in reverse during the outburst decay. At the end of the outburst, MAXI J1348-630 underwent two reflares with peak fluxes ~1 and ~2 orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the hard state only, without undergoing any state transitions, which is similar to the so-called "failed outbursts". Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348-630 is a black hole candidate.

astro-ph.HE↗

arXiv2020

Discovery of thermonuclear (Type I) X-ray bursts in the X-ray binary Swift J1858.6-0814 observed with NICER and NuSTAR

Swift J1858.6-0814 is a recently discovered X-ray binary notable for extremely strong variability (by factors $>100$ in soft X-rays) in its discovery state. We present the detection of five thermonuclear (Type I) X-ray bursts from Swift J1858.6-0814, implying that the compact object in the system is a neutron star. Some of the bursts show photospheric radius expansion, so their peak flux can be used to estimate the distance to the system. The peak luminosity, and hence distance, can depend on several system parameters; for the most likely values, a high inclination and a helium atmosphere, $D=12.8_{-0.6}^{+0.8}$ kpc, although systematic effects allow a conservative range of $9-18$ kpc. Before one burst, we detect a QPO at $9.6\pm0.5$ mHz with a fractional rms amplitude of $2.2\pm0.2$% ($0.5-10$ keV), likely due to marginally stable burning of helium; similar oscillations may be present before the other bursts but the light curves are not long enough to allow their detection. We also search for burst oscillations but do not detect any, with an upper limit in the best case of 15% fractional amplitude (over $1-8$ keV). Finally, we discuss the implications of the neutron star accretor and this distance on other inferences which have been made about the system. In particular, we find that Swift J1858.6-0814 was observed at super-Eddington luminosities at least during bright flares during the variable stage of its outburst.

astro-ph.HE↗

arXiv2020

X-ray Spectral and Timing evolution of MAXI J1727-203 with NICER

We present a detailed X-ray spectral and variability study of the full 2018 outburst of MAXI J1727-203 using NICER observations. The outburst lasted approximately four months. Spectral modelling in the 0.3-10 keV band shows the presence of both a soft thermal and a hard Comptonised component. The analysis of these components shows that MAXI J1727-203 evolved through the soft, intermediate and hard spectral states during the outburst. We find that the soft (disc) component was detected throughout almost the entire outburst, with temperatures ranging from ~0.4 keV, at the moment of maximum luminosity, to ~0.1 keV near the end of the outburst. The power spectrum in the hard and intermediate states shows broadband noise up to 20 Hz, with no evidence of quasi-periodic oscillations. We also study the rms spectra of the broadband noise at 0.3-10 keV of this source. We find that the fractional rms increases with energy in most of the outburst except during the hard state, where the fractional rms remains approximately constant with energy. We also find that, below 3 keV, the fractional rms follows the same trend generally observed at energies >3 keV, a behaviour known from previous studies of black holes and neutron stars. The spectral and timing evolution of MAXI J1727-203, as parametrised by the hardness-intensity, hardness-rms, and rms-intensity diagrams, suggest that the system hosts a black hole, although we could not rule out a neutron star.

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arXiv2020

A NICER look at the state transitions of the black hole candidate MAXI J1535-571 during its reflares

The black hole candidate and X-ray binary MAXI J1535-571 was discovered in September 2017. During the decay of its discovery outburst, and before returning to quiescence, the source underwent at least four reflaring events, with peak luminosities of $\sim$10$^{35-36}$ erg s$^{-1}$ (d/4.1 kpc)$^2$. To investigate the nature of these flares, we analysed a sample of NICER observations taken with almost daily cadence. In this work we present the detailed spectral and timing analysis of the evolution of the four reflares. The higher sensitivity of NICER at lower energies, in comparison with other X-ray detectors, allowed us to constrain the disc component of the spectrum at $\sim$0.5 keV. We found that during each reflare the source appears to trace out a q-shaped track in the hardness-intensity diagram similar to those observed in black hole binaries during full outbursts. MAXI J1535-571 transits between the hard state (valleys) and softer states (peaks) during these flares. Moreover, the Comptonised component is undetected at the peak of the first reflare, while the disc component is undetected during the valleys. Assuming the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions take place at the lowest luminosities ever observed in a black hole transient, while the soft-to-hard transitions occur at some of the lowest luminosities ever reported for such systems.

astro-ph.HE↗

arXiv2020

A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608-52

It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below $\sim 10^{-2}$ of the Eddington limit ($L_{\rm Edd}$). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608-52, to study changes in the reflection spectrum. We find that the broad Fe-K$\alpha$ line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is $\sim 10^{37}$ erg/s ($\sim 0.05$ $L_{\rm Edd}$), disappear during the decay of the outburst when the source luminosity drops to $\sim 4.5 \times 10^{35}$ erg/s ($\sim 0.002$ $L_{\rm Edd}$). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disk radius, disk ionisation, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disk ionisation ($\log \xi \geq 4.1$) or a much decreased reflection fraction. A changing disk truncation alone, however, cannot account for the lack of reprocessed Fe-K$\alpha$ emission. The required increase in ionisation parameter could occur if the inner accretion flow evaporates from a thin disk into a geometrically thicker flow, such as the commonly assumed formation of an radiatively inefficient accretion flow at lower mass accretion rates.

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arXiv2019

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

NICER observed several rotation-powered millisecond pulsars 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 ($\ge4.7\sigma$ 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 1.7$\sigma$ and almost 3$\sigma$ 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 millisecond pulsars exhibit a significant phase offset between their X-ray and radio pulses.

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arXiv2018

A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571

We present the discovery of a low-frequency $\approx 5.7$ Hz quasi-periodic oscillation (QPO) feature in observations of the black hole X-ray binary MAXI J1535-571 in its soft-intermediate state, obtained in September-October 2017 by the Neutron Star Interior Composition Explorer (NICER). The feature is relatively broad (compared to other low-frequency QPOs; quality factor $Q\approx 2$) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak harmonic and low-amplitude broadband noise. These characteristics identify it as a weak Type A/B QPO, similar to ones previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550-564. The lag-energy spectrum of the QPO shows increasing soft lags towards lower energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum). This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the Type B QPO in GX 339-4 pointed towards a precessing jet-like corona illuminating the accretion disk as the origin of the QPO signal. We suggest that this QPO in MAXI J1535-571 may have the same origin, with the different lag sign depending on the scale height of the emitting region and the observer inclination angle.

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arXiv2018

NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1

Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type I X-ray bursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impact on the disk during one of the first X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hard spectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits an excess of soft X-ray photons below 1 keV, where NICER's sensitivity peaks. We interpret the excess as a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burst persistent flux, possibly due to Poynting Robertson drag or coronal reprocessing. This is the first such detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER will be able to study how X-ray bursts affect the disk and corona for a range of accreting neutron star systems and disk states.

astro-ph.HE↗

arXiv2012

Accelerated Orbital Expansion And Secular Spin Down of the Accreting Millisecond Pulsar SAX J1808.4-3658

The accreting millisecond pulsar SAX J1808.4-3658 has shown a peculiar orbital evolution in the past with an orbital expansion much faster than expected from standard binary evolutionary scenarios. Previous limits on the pulsar spin frequency derivative during transient accretion outbursts were smaller than predicted by standard magnetic accretion torque theory, while the spin evolution between outbursts was consistent with magnetic dipole spin-down. In this paper we present the results of a coherent timing analysis of the 2011 outburst observed by the Rossi X-ray Timing Explorer and extend our previous long-term measurements of the orbital and spin evolution over a baseline of thirteen years. We find that the expansion of the 2 hr orbit is accelerating at a rate 1.6E-20 s/s^2 and we interpret this as the effect of short-term angular momentum exchange between the mass donor and the orbit. The gravitational quadrupole coupling due to variations in the oblateness of the companion can be a viable mechanism for explaining the observations. No significant spin frequency derivatives are detected during the 2011 outburst (<4E-13 Hz/s) and the long term spin down remains stable over thirteen years with a rate of approximately -1E-15 Hz/s.

astro-ph.HE↗