Abstract
We present temporal and time-resolved spectral analyses of all the thermonuclear X-ray bursts observed from the neutron star low-mass X-ray binary 4U 1728−34 with NICER from 2017 June to ...2019 September. In total, we detected 11 X-ray bursts from the source and performed time-resolved spectroscopy. Unlike some of the earlier results for other bursting sources from NICER, our spectral results indicate that the use of a scaling factor for the persistent emission is not statistically necessary. This is primarily a result of the strong interstellar absorption in the line of sight toward 4U 1728−34, which causes the count rates to be significantly lower at low energies. We also searched for burst oscillations and detected modulations in six different bursts at around the previously known burst oscillation frequency of 363 Hz. Finally, we report the detection of oscillations prior to two bursts at 356 and 359 Hz, respectively. This is the first time in the literature where burst oscillations are detected before the rapid rise in X-ray flux, from any known burster. These oscillations disappear as soon as the burst starts to rise and occur at a somewhat lower frequency than the oscillations we detect during the bursts.
Abstract We present the discovery, with the Neutron Star Interior Composition Explorer (NICER), that SRGA J144459.2−604207 is a 447.9 Hz accreting millisecond X-ray pulsar (AMXP), which underwent a 4 ...week long outburst starting on 2024 February 15. The AMXP resides in a 5.22 hr binary, orbiting a low-mass companion donor with M d > 0.1 M ⊙ . We report on the temporal and spectral properties from NICER observations during the early days of the outburst, from 2024 February 21 through 2024 February 23, during which NICER also detected a type I X-ray burst that exhibited a plateau lasting ∼6 s. The spectra of the persistent emission were well described by an absorbed thermal blackbody and power-law model, with blackbody temperature kT ≈ 0.9 keV and power-law photon index Γ ≈ 1.9. Time-resolved burst spectroscopy confirmed the thermonuclear nature of the burst, where an additional blackbody component reached a maximum temperature of nearly kT ≈ 3 keV at the peak of the burst. We discuss the nature of the companion as well as the type I X-ray burst.
ABSTRACT
R-mode oscillations have been shown to have a significant potential to constrain the composition of fast spinning neutron stars. Due to their high rotation rates, millisecond pulsars (MSPs) ...provide a unique platform to constrain the properties of such oscillations, if their surface temperatures can be inferred. We present the results of our investigations of archival X-ray data of a number of MSPs, as well as recent XMM–Newton observations of PSR J1810+1744 and PSR J2241−5236. Using the neutron star atmosphere model and taking into account various uncertainties, we present new bounds on the surface temperature of these sources. Thereby, we significantly strengthen previous bounds on the amplitude of the r-mode oscillations in MSPs and find rigorous values as low as α ≲ 3 × 10−9. This is by now about three orders of magnitude below what standard saturation mechanisms in neutron stars could provide, which requires very strong dissipation in the interior, strongly pointing towards a structurally complex or exotic composition of these sources. At such low temperatures, sources could even be outside of the instability region, and taking into account the various uncertainties, we obtain for an observed surface temperature a simple frequency bound below which r-modes are excluded in slower spinning pulsars.
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%
Abstract
r-mode asteroseismology provides a unique way to study the internal composition of compact stars. Due to their precise timing, recycled millisecond radio pulsars present a particularly ...promising class of sources. Although their thermal properties are still poorly constrained, X-ray data is very useful for asteroseismology since r-modes could strongly heat a star. Using known and new upper bounds on the temperatures and luminosities of several non-accreting millisecond radio pulsars, we derive bounds on the r-mode amplitude as low as α ≲ 10−8 and discuss the impact on scenarios for their internal composition.
Abstract
We present the detection of 51 thermonuclear X-ray bursts observed from 4U 1636–536 by the Neutron Star Interior Composition Explorer (NICER) over the course of a 3 yr monitoring campaign. ...We perform time-resolved spectroscopy for 40 of these bursts and show the existence of a strong soft excess in all the burst spectra. The excess emission can be characterized by the use of a scaling factor (the
f
a
method) to the persistent emission of the source, which is attributed to the increased mass accretion rate onto the neutron star due to Poynting–Robertson drag. The soft excess emission can also be characterized by the use of a model taking into account the reflection of the burst emission off the accretion disk. We also present time-resolved spectral analysis of five X-ray bursts simultaneously observed by NICER and AstroSat, which confirm the main results with even greater precision. Finally, we present evidence for Compton cooling using seven X-ray bursts observed contemporaneously with NuSTAR, by means of a correlated decrease in the hard X-ray lightcurve of 4U 1636–536 as the bursts start.
Abstract
We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608−52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the ...outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks 30 and 18 s after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608−52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates, did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.
r-mode oscillations have been shown to have a significant potential to constrain the composition of fast spinning neutron stars. Due to their high rotation rates, millisecond pulsars (MSPs) provide a ...unique platform to constrain the properties of such oscillations, if their surface temperatures can be inferred. We present the results of our investigations of archival X-ray data of a number of MSPs, as well as recent XMM-Newton observations of PSR J1810+1744 and PSR J2241-5236. Using the neutron star atmosphere (NSA) model and taking into account various uncertainties, we present new bounds on the surface temperature of these sources. Thereby we significantly strengthen previous bounds on the amplitude of the r-mode oscillations in millisecond pulsars and find rigorous values as low as \(\alpha\lesssim 3 \times 10^{-9}\). This is by now about three orders of magnitude below what standard saturation mechanisms in neutron stars could provide, which requires very strong dissipation in the interior, strongly pointing towards a structurally complex or exotic composition of these sources. At such low temperatures sources could even be outside of the instability region, and taking into account the various uncertainties we obtain for an observed surface temperature a simple frequency bound below which r-modes are excluded in slower spinning pulsars.
Rusya ve Türkiye arasında imzalanan işbirliği protokolü kapsamında Mikhail Pavlinsky ART$-$XC uzay teleskobu verileri ile araştırmalar yapmak üzere TUG$-$SRG çalışma grubu oluşturulmuştur. Bu ...bildiride, TUG$-$SRG çalışmaları hakkında bir özet sunulmuş ve Aktif Galaksi Çekirdeklerinin~(AGÇ), adaylarının optik gözlemlerinin elde edilmesi ve analizleri ile ilgili süreçler ve ön sonuçlar verilmiştir. Bu işbirliği kapsamında gözlenen SRGAJ03p76 kaynağının RTT150 tayf gözlemleri, ışık eğrisi ve Tayfsal Enerji Dağılımı (TED) üzerine çalışmaları özetlenmiştir. ART$-$XC teleskobu ile sert X-ışın kaynağı olarak belirlenen SRGAJ03p76'nın ilk kez optik tayfsal gözlemi yapılarak $z\approx0.2$ bir kuazar olduğu doğrulanmıştır. TUG$-$SRG grubunun sert X-ışın kaynağı olan AGÇ'ler için çalışmaları devem edecektir.
A protocol signed between Russia and Turkey endures a collaboration over the data from the Mikhail Pavlinsky ART-XC space telescope. The TUG-SRG workgroup has been built to organize collaborative studies among Turkish researchers. This proceeding briefs the procedures running within the TUG-SRG workgroup in a specific example of an Active Galactic Nuclei~(AGN) candidate. The proceeding precedes the analysis of the RTT-150 optical spectra, ZTF light curve, and SED modeling for SRGAJ03p76. The first spectroscopic confirmation of the optical counterpart of this hard X-ray source reveals SRGAJ03p76 to be a $z\approx0.2$ quasar. AGN studies will continue within the TUG$-$SRG group to pursue optical identification of the hard X-ray sources.
We present the discovery, with the Neutron Star Interior Composition Explorer (NICER), that SRGA J144459.2-604207 is a 447.9 Hz accreting millisecond X-ray pulsar (AMXP), which underwent a four-week ...long outburst starting on 2024 February 15. The AMXP resides in a 5.22 hr binary, orbiting a low-mass companion donor with \(M_d>0.1M_\odot\). We report on the temporal and spectral properties from NICER observations during the early days of the outburst, from 2024 February 21 through 2024 February 23, during which NICER also detected a type-I X-ray burst that exhibited a plateau lasting ~6 s. The spectra of the persistent emission were well described by an absorbed thermal blackbody and power-law model, with blackbody temperature \(kT\approx0.9{\rm\,keV}\) and power-law photon index \(\Gamma\approx1.9\). Time-resolved burst spectroscopy confirmed the thermonuclear nature of the burst, where an additional blackbody component reached a maximum temperature of nearly \(kT\approx3{\rm\,keV}\) at the peak of the burst. We discuss the nature of the companion as well as the type-I X-ray burst.
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