The Be/X-ray transients V0332+53 and 4U 0115+63 exhibited giant, type-II outbursts in 2015. Here we present Swift/XRT follow-up observations at the end of those outbursts. Surprisingly, the sources ...did not decay back to their known quiescent levels but stalled at a (slowly decaying) meta-stable state with luminosities a factor ∼10 above that observed in quiescence. The spectra in these states are considerably softer than the outburst spectra and appear to soften in time when the luminosity decreases. The physical mechanism behind these meta-stable states is unclear and they could be due to low-level accretion (either directly on to the neutron stars or on to their magnetospheres) or due to cooling of the accretion-heated neutron star crusts. Based on the spectra, the slowly decreasing luminosities, and the spectral softening, we favour the crust cooling hypothesis but we cannot exclude the accretion scenarios. On top of this meta-stable state, weak accretion events were observed that occurred at periastron passage and may thus be related to regular type-I outbursts.
Kluyveromyces marxianus' high production of 2-phenylethyl acetate (2-PEA) via L-phenylalanine (Phe) catabolism makes it relevant for industries relying on the production of aroma compounds through ...fermentation processes. This study assessed the physiological impact of exogenous supplementation of Phe on cell viability, fermentation performance, and, by extension, on lipid and amino acid metabolism in a wine isolate of this yeast.
The data showed that Phe exerted cytotoxic effects on K. marxianus IWBT Y885, which were minimal on Saccharomyces cerevisiae and impacted amino acid metabolism and aroma production. We demonstrated that K. marxianus strains fermented sugars more effectively in the absence of Phe. While lipid supplementation did not mitigate any deleterious effects of Phe, it supported viability maintenance and fermentation performance in the absence of Phe. Phe supplementation succeeded in augmenting the production of 2-PE and 2-PEA.
The enhanced production of 2-PEA in K. marxianus suggests that this transesterification may be, at least in part, a compensatory detoxification mechanism for this yeast.
The transient neutron star low-mass X-ray binary EXO 1745−248, located in the globular cluster Terzan 5, was detected during its quiescent state with Chandra in 2003. The source displayed a 0.5-10 ...keV quiescent X-ray luminosity of L
q∼ 1033 (D/5.5 kpc)2 erg s−1, which was completely dominated by hard non-thermal emission. This is at odds with other non-pulsating neutron stars that typically show detectable soft thermal emission at such quiescent luminosities. Here we use three additional Chandra observations, performed in 2009 and 2011, to further study the quiescent properties of EXO 1745−248. We find that the power-law intensity varies considerably up to a factor of ∼3 within hours and by about one order of magnitude between the different epochs. We discuss the implications of the observed change in quiescent flux for the interpretation of the hard power-law emission. We constrain the neutron star surface temperature as seen by a distant observer to kT
∞≲ 42 eV and the thermal bolometric luminosity to L
th
q, bol≲ 7 × 1031 (D/5.5 kpc)2 erg s−1. This confirms that EXO 1745−248 harbours a relatively cold neutron star and suggests that, for example, enhanced cooling mechanisms are operating in the stellar core or that the binary on average resides in quiescence for hundreds of years.
Abstract
We present the first fully simultaneous fits to the near-infrared (NIR) and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A*, the supermassive black hole at ...the Milky Way's centre. Our study arises from ambitious multiwavelength monitoring campaigns with XMM–Newton, NuSTAR and SINFONI. The average multiwavelength spectrum is well reproduced by a broken power law with ΓNIR = 1.7 ± 0.1 and ΓX = 2.27 ± 0.12. The difference in spectral slopes (ΔΓ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 103 s, when a very steep X-ray spectrum (ΔΓ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high-energy cut-off in the electron distribution (γmax) induces an initial cut-off in the optical–UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γmax. We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A*'s flaring emission.
It is assumed that accreting neutron stars in low-mass X-ray binaries are heated due to the compression of the existing crust by the freshly accreted matter which gives rise to a variety of nuclear ...reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements (the deep-crustal heating scenario). In this paper we discuss if neutron stars in the so-called very-faint X-ray transients (VFXTs; those transients have outburst peak 2-10 keV X-ray luminosities <1 × 1036 erg s−1) can be used to test this deep-crustal heating model. We demonstrate that such systems would indeed be very interesting objects to test the deep-crustal heating model with, but that the interpretation of the results might be challenging because of the large uncertainties in our estimates of the accretion rate history of those VFXTs, both the short-term (less than a few tens of thousands of years) and the one throughout their lifetime. The latter is particularly important because it can be so low that the neutron stars might not have accreted enough matter to become massive enough that enhanced core cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processes have been inferred. However, the amount of matter can also not be too low because then the crust might not have been replaced significantly by accreted matter and thus a hybrid crust of partly accreted and partly original, albeit further compressed matter, might be present. This would inhibit the full range of pycnonuclear reactions to occur and therefore possibly decrease the amount of heat deposited in the crust. More detailed calculations of the heating and cooling properties of such hybrid crusts have to be performed to be conclusive. Furthermore, better understanding is needed about how a hybrid crust affects other properties such as the thermal conductivity. A potentially interesting way to observe the effects of a hybrid crust on the heating and cooling of an accreting neutron star is to observe the crust cooling of such a neutron star after a prolonged (years to decades) accretion episode and compare the results with similar studies performed for neutron stars with a fully accreted crust. We also show that some individual neutron-star low-mass X-ray binaries might have hybrid crusts as well as possibly many of the neutron stars in high-mass X-ray binaries. This has to be taken into account when studying the cooling properties of those systems when they are in quiescence. In addition, we show that the VFXTs are likely not the dominate transients that are associated with the brightest (∼1033 erg s−1) low-luminosity X-ray sources in globular clusters as was previously hypothesized.
AX J1754.2−2754, 1RXS J171824.2−402934 and 1RXH J173523.7−354013 are three persistent neutron star low-mass X-ray binaries that display a 2-10 keV accretion luminosity L
X of only (1-10) × 1034 erg ...s−1 (i.e. only 0.005-0.05 per cent of the Eddington limit). The phenomenology of accreting neutron stars which accrete at such low accretion rates is not yet well known and the reason why they have such low accretion rates is also not clear. Therefore, we have obtained XMM-Newton data of these three sources and here we report our analysis of the high-quality X-ray spectra we have obtained for them. We find that AX J1754.2−2754 has L
X ∼ 1035 erg s−1, while the other two have X-ray luminosities about an order of magnitude lower. However, all sources have a similar, relatively soft, spectrum with a photon index of 2.3-2.5, when the spectrum is fitted with an absorbed power-law model. This model fits the data of AX J1754.2−2754 adequately, but it cannot fit the data obtained for 1RXS J171824.2−402934 and 1RXH J173523.7−354013. For those sources, a clear soft thermal component is needed to fit their spectra. This soft component contributes 40-50 per cent to the 0.5-10 keV flux of the sources. The presence of this soft component might be the reason why the spectra of these two sources are soft. When including this additional spectral component, the power-law photon indices are significantly lower. It can be excluded that a similar component with similar contributions to the 2-10 keV X-ray flux is present for AX J1754.2−2754, indicating that the soft spectrum of this source is mostly due to the fact that the power-law component itself is not hard. We note that we cannot exclude that a weaker soft component is present in the spectrum of this source which only contributes up to ∼25 per cent to the 0.5-10 keV X-ray flux. We discuss our results in the context of what is known of accreting neutron stars at very low accretion rate.
We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known ...X-ray transients AX J1745.6–2901, CXOGC J174535.5–290124, GRS 1741–2853, XMM J174457–2850.3 and CXOGC J174538.0–290022. All these sources are known to undergo very faint X-ray outbursts with 2–10 keV peak luminosities of LX,peak ~ 1034−36 erg s-1, although the two confirmed neutron star low-mass X-ray binaries AX J1745.6–2901 and GRS 1741–2853 can also become brighter (LX,peak ~ 1036−37 erg s-1). We discuss the observed long-term lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6–2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741–2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2–10 keV luminosity of LX ~ 1 × 1037 (D / 7.2 kpc)2 erg s-1. This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457–2850.3 becomes bright only during short episodes (days) and is often found active in between quiescence (LX ~ 1032 erg s-1) and its maximum outburst luminosity of LX ~ 1036 erg s-1. CXOGC J174535.5–290124 and CXOGC J174538.0–290022, as well as three other very-faint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of ⟨Ṁ⟩long ≲ 2 × 10-12 M⊙ yr-1. Despite having obtained two years of new data in 2008 and 2009, no new X-ray transients were detected.
We report on a series of Swift/X-ray telescope observations, performed between 2012 February and 22 March, during the quiescent state of the neutron-star X-ray binary SAX J1750.8−2900. In these ...observations, the source was either just detected or undetected, depending on the exposure length (which ranged from ∼0.3 to ∼3.8 ks). The upper limits for the non-detections were consistent with the detected luminosities (when fitting a thermal model to the spectrum) of ∼1034 erg s−1 (0.5-10 keV). This level is consistent with what has been measured previously for this source in quiescence. However, on March 17 the source was found to have an order of magnitude larger count rate. When fitting the flare spectrum with an absorbed power-law model, we obtained a flare luminosity of (3-4) × 1034 erg s−1 (0.5-10 keV). Follow-up Swift observations showed that this flare lasted <16 d. This event was very likely due to a brief episode of low-level accretion on to the neutron star and provides further evidence that the quiescent state of neutron-star X-ray transients might not be as quiet as is generally assumed. The detection of this low-level accretion flare raises the question whether the quiescent emission of the source (outside the flare) could also be due to residual accretion, albeit continuous instead of episodic. However, we provide arguments which would suggest that the lowest intensity level might instead represent the cooling of the accretion-heated neutron star.
We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the transient X-ray binary IGR J16597–3704 using NuSTAR and Swift. The source was discovered by INTEGRAL in the globular cluster ...NGC 6256 at a distance of 9.1 kpc. The X-ray pulsations show a clear Doppler modulation that implies an orbital period of ~46 min and a projected semi-major axis of ~5 lt-ms, which makes IGR J16597–3704 an ultracompact X-ray binary system. We estimated a minimum companion mass of 6.5 × 10−10 M⊙, assuming a neutron star mass of 1.4 M⊙, and an inclination angle of <75° (suggested by the absence of eclipses or dips in its light curve). The broad-band energy spectrum of the source is well described by a disk blackbody component (kT ~ 1.4 keV) plus a comptonised power-law with photon index ~2.3 and an electron temperature of ~30 keV. Radio pulsations from the source were unsuccessfully searched for with the Parkes Observatory.
Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs
. It has been predicted
that localized thermonuclear bursts on white dwarfs can ...also take place, similar to type-I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately strong magnetized white dwarf from a low-mass companion, have been observed on several occasions in the past 40 years
. During these bursts, the optical/ultraviolet luminosity increases by a factor of more than three in less than an hour and fades in around ten hours. Fast outflows have been observed in ultraviolet spectral lines
, with velocities of more than 3,500 kilometres per second, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae and in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy of approximately 10
times than those of classical nova explosions (micronovae) and bear a strong resemblance to type-I X-ray bursts
. We exclude accretion or stellar magnetic reconnection events as their origin and suggest thermonuclear runaway events in magnetically confined accretion columns as a viable explanation.
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