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Register a new userNICER Observation of Unusual Burst Oscillations in 4U 1728-34
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The Neutron Star Interior Composition Explorer (NICER) has observed seven thermonuclear X-ray bursts from the Low Mass X-ray Binary (LMXB) neutron star 4U 1728-34 from the start of the missions operations until February of 2019. Three of these bursts show oscillations in their decaying tail with frequencies that are within 1 Hz of the previously detected burst oscillations from this source. Two of these burst oscillations have unusual properties: They have large fractional rms amplitudes of $ 48 pm 9 %$ and $ 46 pm 9 %$, and they are detected only at photon energies above 6 keV. By contrast, the third detected burst oscillation is compatible with previous observations of this source, with a fractional rms amplitude of $7.7 pm 1.5%$ rms in the 0.3 to 6.2 keV energy band. We discuss the implications of these large-amplitude burst oscillations, finding they are difficult to explain with the current theoretical models for X-ray burst tail oscillations.
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Recent theoretical and observational studies have shown that ashes from thermonuclear burning may be ejected during radius-expansion bursts, giving rise to photoionisation edges in the X-ray spectra. We report a search for such features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34. We analysed the spectra from four radius-expansion bursts detected in 2006 July, and two in 2002 March, but found no evidence for discrete features. We estimate upper limits for the equivalent widths of edges of a few hundred eV, which for the moderate temperatures observed during the bursts, are comparable with the predictions. During the 2006 July observation 4U 1728-34 exhibited weak, unusually frequent bursts (separated by <2 hr in some cases), with profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence times as short as those measured are insufficient to exhaust the accreted hydrogen at solar composition, suggesting that the source accretes hydrogen deficient fuel, for example from an evolved donor. The detection for the first time of a 10.77 min periodic signal in the persistent intensity, perhaps arising from orbital modulation, supports this explanation, and suggests that this system is an ultracompact binary similar to 4U 1820-30.
It has recently been shown that the persistent emission of a neutron star low-mass X-ray binary (LMXB) evolves during a thermonuclear (type-I) X-ray burst. The reason of this evolution, however, is not securely known. This uncertainty can introduce significant systematics in the neutron star radius measurement using burst spectra, particularly if an unknown but significant fraction of the burst emission, which is reprocessed, contributes to the changes in the persistent emission during the burst. Here, by analyzing individual burst data of AstroSat/LAXPC from the neutron star LMXB 4U 1728--34 in the soft state, we show that the burst emission is not significantly reprocessed by a corona covering the neutron star. Rather, our analysis suggests that the burst emission enhances the accretion disk emission, possibly by increasing the accretion rate via disk. This enhanced disk emission, which is Comptonized by a corona covering the disk, can explain an increased persistent emission observed during the burst. This finding provides an understanding of persistent emission components, and their interaction with the thermonuclear burst emission. Furthermore, since burst photons are not significantly reprocessed, non-burst and burst emissions can be reliably separated, which is required to reduce systematic uncertainties in the stellar radius measurement.
We report on a simultaneous NuSTAR and Swift observation of the neutron star low-mass X-ray binary 4U 1728-34. We identified and removed four Type I X-ray bursts during the observation in order to study the persistent emission. The continuum spectrum is hard and well described by a black body with $kT=$ 1.5 keV and a cutoff power law with $Gamma=$ 1.5 and a cutoff temperature of 25 keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe K$alpha$ line. By modeling the spectrum with a relativistically blurred reflection model, we find an upper limit for the inner disk radius of $R_{rm in}leq2 R_{rm ISCO}$. Consequently we find that $R_{rm NS}leq23$ km, assuming $M=1.4{mbox{$rm,M_{mathordodot}$}}$ and $a=0.15$. We also find an upper limit on the magnetic field of $Bleq2times10^8$ G.
While kilohertz quasi-periodic oscillations (kHz QPOs) have been well studied for decades since their initial discovery, the cause of these signals remains unknown, as no model has been able to accurately predict all of their spectral and timing properties. Separately, X-ray reverberation lags have been detected in AGN and stellar-mass black hole binaries, and reverberation may be expected to occur in neutron star systems as well, producing lags of the same amplitude as the lags measured of the kHz QPOs. Furthermore, the detection of a relativistically reflected Fe K line in the time-averaged spectra of many neutron star systems provides an additional motivation for testing reverberation. While it has been shown that the lag-energy properties of the lower kHz QPOs are unlikely to be produced by X-ray reverberation, the upper kHz QPOs have not yet been explored. We therefore model the upper kHz QPO lag-energy spectra using relativistic ray-tracing functions and apply them to archival RXTE data on 4U 1728-34 where upper kHz QPOs have been detected. By modeling the time-averaged spectra in which upper kHz QPOs had been significantly detected, we determine the reflected flux fraction across all energies and produce a model for the lag-energy spectra from X-ray reverberation. We explore the dependence of the modeled lag properties on several different types of reflection models, but are unable to successfully reproduce the measured lags of 4U 1728-34. We conclude that reverberation alone does not explain the measured time lags detected in upper kHz QPOs.
We report on the first observations of neutron star low-mass X-ray binaries with the Atacama Large Millimeter/submillimeter Array (ALMA) at $sim$300 GHz. Quasi-simultaneous observations of 4U 1728-34 and 4U 1820-30 were performed at radio (ATCA), infrared (VLT) and X-ray (Swift) frequencies, spanning more than eight decades in frequency coverage. Both sources are detected at high significance with ALMA. The spectral energy distribution of 4U 1728-34 is consistent with synchrotron emission from a jet with a break from optically thick to optically thin emission at 1.3-11.0$times$10$^{13}$ Hz. This is the third time a jet spectral break has been reported for a neutron star X-ray binary. The radio to mm spectral energy distribution of 4U 1820-30 has significant detections at 5 and 300~GHz. This confirms the presence of radio emission during a soft state for this neutron star and represents the first detection of mm emission during such a state, unambiguously pointing to the presence of a jet. We also report on three additional unrelated sources - showing mm emission - in the ALMA fields of view of 4U 1728-34 and 4U 1820-30.
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