ABSTRACT
Near-infrared (NIR) observations of normal Type Ia supernovae (SNe Ia) obtained between 150 and 500 d past maximum light reveal the existence of an extended plateau. Here, we present ...observations of the underluminous, 1991bg-like SN 2021qvv. Early, ground-based optical and NIR observations show that SN 2021qvv is similar to SN 2006mr, making it one of the dimmest, fastest evolving 1991bg-like SNe to date. Late-time (170–250 d) Hubble Space Telescope observations of SN 2021qvv reveal no sign of a plateau. An extrapolation of these observations backwards to earlier-phase NIR observations of SN 2006mr suggests the complete absence of an NIR plateau, at least out to 250 d. This absence may be due to a higher ionization state of the ejecta, as predicted by certain sub-Chandrasekhar-mass detonation models, or to the lower temperatures of the ejecta of 1991bg-like SNe, relative to normal SNe Ia, which might preclude their becoming fluorescent and shifting ultraviolet light into the NIR. This suggestion can be tested by acquiring NIR imaging of a sample of 1991bg-like SNe that covers the entire range from slowly evolving to fast-evolving events (0.2 ≲ sBV ≲ 0.6). A detection of the NIR plateau in slower evolving, hotter 1991bg-like SNe would provide further evidence that these SNe exist along a continuum with normal SNe Ia. Theoretical progenitor and explosion scenarios would then have to match the observed properties of both SN Ia subtypes.
We present the detection of a variable star with an 18.05 min period in far-ultraviolet (FUV) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope (HST). A candidate optical ...counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster, but it is statistically possible that this is a chance superposition. This interpretation is supported by optical spectroscopy obtained with HST/Space Telescope Imaging Spectrograph (STIS): the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star (i.e. a compact accretor fed by a giant donor). We therefore consider two other possibilities for the nature of FUV variable: (i) an intermediate polar (i.e. a compact binary containing an accreting magnetic white dwarf), or (ii) an AM CVn star (i.e. an interacting double-degenerate system). In the intermediate polar scenario, the object is expected to be an X-ray source. However, no X-rays are detected at its location in ≃65 ksec of Chandra imaging, which limits the X-ray luminosity to L
X ≤ 1032 erg s−1. We therefore favour the AM CVn interpretation, but a FUV spectrum is needed to distinguish conclusively between the two possibilities. If the object is an AM CVn binary, it would be the first such system known in any globular cluster.
Abstract
To confirm the nature of the donor star in the ultracompact X-ray binary candidate 47 Tuc X9, we obtained optical spectra (3000–10 000 Å) with the Hubble Space Telescope / Space Telescope ...Imaging Spectrograph. We find no strong emission or absorption features in the spectrum of X9. In particular, we place 3σ upper limits on the H α and He ii λ4686 emission line equivalent widths − EWH α ≲ 14 Å and $\rm -EW_{He\, \small {II}} \lesssim 9$ Å, respectively. This is much lower than seen for typical X-ray binaries at a similar X-ray luminosity (which, for $L_{\rm 2\text{--}10\,keV} \approx 10^{33}\text{--}10^{34}$ erg s−1 is typically − EWH α ∼ 50 Å). This supports our previous suggestion, by Bahramian et al., of an H-poor donor in X9. We perform timing analysis on archival far-ultraviolet, V- and I-band data to search for periodicities. In the optical bands, we recover the 7-d superorbital period initially discovered in X-rays, but we do not recover the orbital period. In the far-ultraviolet, we find evidence for a 27.2 min period (shorter than the 28.2 min period seen in X-rays). We find that either a neutron star or black hole could explain the observed properties of X9. We also perform binary evolution calculations, showing that the formation of an initial black hole/ He-star binary early in the life of a globular cluster could evolve into a present-day system such as X9 (should the compact object in this system indeed be a black hole) via mass-transfer driven by gravitational wave radiation.
We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) wave band. Based on these observations, we ...identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062 c 0.004 mag semiamplitude, and we clearly detect a period of 22.5806 c 0.0002 minutes. We have carried out extensive Monte Carlo simulations, which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV-bright (FUV 17 mag) and is characterized by an extremely blue spectral energy distribution (F sub(l) a l super(-2.0)). We also find evidence for an excess of flux between 1500 and 1600 Ae and probably between 1600 and 2000 A that might be due to C IV 1550 Ae and He II 1640 Ae emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for mass transfer driven by gravitational waves at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is -34. We conclude that all observational characteristics of M 15 X-2 are consistent with its being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.
We present the first results of a deep Chandra survey of the inner 61 of the Fornax cluster of galaxies. Ten 50 ks pointings were obtained in a mosaic centered on the giant elliptical galaxy NGC 1399 ...at the nominal cluster center. Emission and temperature maps of Fornax are presented, and an initial study of more than 700 detected X-ray point sources is made. Regions as small as 100 pc are resolved. The intracluster gas in Fornax exhibits a highly asymmetric morphology and temperature structure, dominated by a 180 kpc extended "plume" of low surface brightness, cool (,1 keV) gas to the northeast of NGC 1399 with a sharper edge to the southwest. The elliptical galaxy NGC 1404 also exhibits a cool halo of X-ray gas within the cluster, with a highly sharpened leading edge as it presumably falls into the cluster and a comet-like tail. We estimate that some 6200-400 point sources are physically associated with Fornax. Confirming earlier works, we find that the globular cluster population in NGC 1399 is highly X-ray active, extending to globular clusters that may in fact be intracluster systems. We have also found a remarkable correlation between the location of giant and dwarf cluster galaxies and the presence of X-ray counterparts, such that systems inhabiting regions of low gas density are more likely to show X-ray activity. Not only does this correlate with the asymmetry of the intracluster gas, but it also correlates with the axis joining the center of Fornax to an infalling group 1 Mpc to the southwest. We suggest that Fornax may be experiencing either an intergalactic "headwind" due to motion relative to the surrounding large-scale structure or that the intracluster medium has been disturbed relative to the overall cluster gravitational potential by previous activity.
We have obtained deep FUV and NUV images of the inner region of the dense globular cluster M15 with the HST ACS. The FUV - NUV color-magnitude diagram shows a well-defined track of horizontal branch ...stars, as well as a trail of blue stragglers and white dwarfs. The main-sequence turnoff is clearly visible at FUV unk 23.5 mag and FUV - NUV unk 3 mag, and the main-sequence stars form a prominent track that extends at least 2 mag below the main-sequence turnoff. As such, this is the deepest FUV - NUV color-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colors and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variable candidates, one known and one probable SX Phoenicis star, and the well-known low-mass X-ray binary AC 211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyrae stars, and SX Phoenicis exhibit massive variability amplitudes in this wave band (several magnitudes).
We have searched for variable sources in the core region of M 80, using far-ultraviolet (FUV) data taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. We found three ...sources that exhibit strong signs of variability in our data. Among these is source TDK 1, which we believe to be an RR Lyrae star that reached maximum brightness during our observations. The light curve shows a ≳3 mag FUV brightening over the course of ≈5 h, with an estimated peak brightness of ≈16.7 mag, followed by a decrease to ≈20 mag. Archival optical data obtained with Wide-Field Planetary Camera 2 confirm that TDK 1 is variable in all wavebands. TDK 1's spectral energy distribution is reasonably fitted by a star with temperature K and radius , consistent with the suggestion that it is an RR Lyrae. Based on the photometric and variability characteristics of the other two variables, we suggest that TDK 2 is likely to be an SX Phoenicis star with min period, and TDK 3 is likely another RR Lyrae. Finally, we briefly discuss the FUV counterparts to two previously known variables in M 80, the classical nova T Sco and the dwarf nova DN 1.
Aims. Using new Chandra X-ray observations and existing XMM-Newton X-ray and Hubble far ultraviolet observations, we aim to detect and identify the faint X-ray sources belonging to the Galactic ...globular cluster NGC 2808 in order to understand their role in the evolution of globular clusters. Methods. We present a Chandra X-ray observation of the Galactic globular cluster NGC 2808. We classify the X-ray sources associated with the cluster by analysing their colours and variability. Previous observations with XMM-Newton and far ultraviolet observations with the Hubble Space Telescope are re-investigated to help identify the Chandra sources associated with the cluster. We compare our results to population synthesis models and observations of other Galactic globular clusters. Results. We detect 113 sources, of which 16 fall inside the half-mass radius of NGC 2808 and are concentrated towards the cluster core. From statistical analysis, these 16 sources are very likely to be linked to the cluster. We detect short-term (1 day) variability in X-rays for 7 sources, of which 2 fall inside the half-mass radius, and long-term (28 months) variability for 10 further sources, of which 2 fall inside the half-mass radius. Ultraviolet counterparts are found for 8 Chandra sources in the core, of which 2 have good matching probabilities and have ultraviolet properties expected for cataclysmic variables. We find one likely neutron star-quiescent low-mass X-ray binary and 8 cataclysmic variable candidates in the core of NGC 2808. The other sources are cataclysmic variable candidates, but some could possibly be active binaries or millisecond pulsars. We find a possible deficit of X-ray sources compared to 47 Tuc which could be related to the metallicity content and the complexity of the evolution of NGC 2808.
Decoherence and the Loschmidt echo Cucchietti, F M; Dalvit, D A R; Paz, J P ...
Physical review letters,
11/2003, Letnik:
91, Številka:
21
Journal Article
Recenzirano
Decoherence causes entropy increase that can be quantified using, e.g., the purity sigma=Trrho(2). When the Hamiltonian of a quantum system is perturbed, its sensitivity to such perturbation can be ...measured by the Loschmidt echo M(t). It is given by the squared overlap between the perturbed and unperturbed state. We describe the relation between the temporal behavior of sigma(t) and the average Mmacr;(t). In this way we show that the decay of the Loschmidt echo can be analyzed using tools developed in the study of decoherence. In particular, for systems with a classically chaotic Hamiltonian the decay of sigma and Mmacr; has a regime where it is dominated by the Lyapunov exponents.
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