Soft gamma repeaters and anomalous X-ray pulsars are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields, B~10^(14)-10^(15) Gauss. The recent discovery of a soft gamma repeater with low magnetic field (< 7.5x10^(12) Gauss), SGR 0418+5729, which shows bursts similar to those of SGRs, implies that a high surface dipolar magnetic field might not be necessary for magnetar-like activity. We show that the quiescent and bursting properties of SGR 0418+5729 find natural explanations in the context of low-magnetic field Quark-Nova (detonative transition from a neutron star to a quark star) remnants, i.e. an old quark star surrounded by degenerate (iron-rich) Keplerian ring/debris ejected during the Quark-Nova explosion. We find that a 16 Myr old quark star surrounded by a ~ 10^(-10)xM_sun ring, extending in radius from ~ 30 km to 60 km, reproduces many observed properties of SGR 0418+5729. The SGR-like burst is caused by magnetic penetration of the inner part of the ring and subsequent accretion. Radiation feedback results in months-long accretion from the ring's non-degenerate atmosphere which matches well the observed decay phase. We make specific predictions (such as an accretion glitch of Delta P/P ~ - 2x10^(-11) during burst and a sub-keV proton cyclotron line from the ring) that can be tested by sensitive observations.
We present results of our dense spectroscopic monitoring of Nova V5668 Sgr. Starting on March 19 in 2015, only a few days after discovery, we have obtained a series of spectra with the TIGRE telescope and its HEROS echelle spectrograph which offers a resolution of R = 20,000 and covers the optical wavelength range from 3800 to 8800 Å. We performed a line identification of the discernible features for four spectra which are representative for the respective phases in the light curve evolution of that nova. By simultaneously analysing the variations in the visual light curve and the correspondin…
Nova Sagittarii 1943 (V1148 Sgr) was an 8th-mag optical transient that was unusual in having a late-type spectrum during its outburst, in striking contrast to the normal high-excitation spectra seen in classical novae. Unfortunately, only an approximate position was given in the discovery announcement, hampering follow-up attempts to observe its remnant. We have identified the nova on two photographic plates in the Harvard archive, allowing us to determine a precise astrometric position. Apart from these two plates, obtained in 1943 and 1944, none of the photographs in the Harvard collection, …
Resonance lines of $^7$Be are detected currently in five novae. Available abundances for this isotope estimated from equivalent widths of $^7$Be\,II and Ca\,II lines are significantly higher compared to predictions of models for the thermonuclear flash. In attempt to pinpoint the reason for this disparity we explore the possibility for the higher $^7$Be yield via computing kinetics of the thermonuclear burning in the framework of two-zone model and find that even for a favorable choice of parameters $^7$Be mass fraction does not exceed $3\cdot10^{-5}$. This is consistent with known theoretical…
We report the first detection of hard (>10 keV) X-ray emission simultaneous with gamma rays in a nova eruption. Observations of the nova V5855 Sgr carried out with the NuSTAR satellite on Day 12 of the eruption revealed faint, highly absorbed thermal X-rays. The extreme equivalent hydrogen column density towards the X-ray emitting region (~3 x 10$^{24}$ cm$^{-2}$) indicates that the shock producing the X-rays was deeply embedded within the nova ejecta. The slope of the X-ray spectrum favors a thermal origin for the bulk of the emission, and the constraints of the temperature in the shocked reg…
INTEGRAL observed the nova V5668 Sgr around the time of its optical maximum on March 21, 2015. Studies at UV wavelengths showed spectral lines of freshly produced Be-7. This could be measurable also in gamma-rays at 478 keV from the decay to Li-7. Novae are also expected to synthesise Na-22 which decays to Ne-22, emitting a 1275 keV photon. About one week before the optical maximum, a strong gamma-ray flash on time-scales of hours is expected from short-lived radioactive nuclei, such as N-13 and F-18. These beta-plus-unstable nuclei should yield emission up to 511 keV, but which has never been…
We report on the fast Nova Sgr 2016 N.4 being surprisingly trapped in a long-lasting and bright plateau (Delta I >= 10 mag above quiescence) six years past the nova eruption. Very few other novae experience a similar occurrence. We carried out an intensive observing campaign collecting daily BVRI photometry and monthly high-resolution optical spectroscopy, and observed the nova in ultraviolet and X-rays with Swift satellite at five distinct epochs. The bolometric luminosity radiated during the plateau is ~4200 Lsun (scaled to the distance of the Galactic Bulge), corresponding to stable nuclear…
V3890 Sgr is a recurrent nova which has been seen in outburst three times so far, with the most recent eruption occurring on 2019 August 27 UT. This latest outburst was followed in detail by the Neil Gehrels Swift Observatory, from less than a day after the eruption until the nova entered the Sun observing constraint, with a small number of additional observations after the constraint ended. The X-ray light-curve shows initial hard shock emission, followed by an early start of the super-soft source phase around day 8.5, with the soft emission ceasing by day 26. Together with the peak blackbody…
Since the Fermi discovery of $γ$-rays from novae, one of the biggest questions in the field has been how novae generate such high-energy emission. Shocks must be a fundamental ingredient. Six months of radio observations of the 2012 nova V5589 Sgr with the VLA and 15 weeks of X-ray observations with Swift/XRT show that the radio emission consisted of: 1) a shock-powered, non-thermal flare; and 2) weak thermal emission from $10^{-5}$ M$_\odot$ of freely expanding, photoionized ejecta. Absorption features in the optical spectrum and the peak optical brightness suggest that V5589 Sgr lies 4 kpc a…
Two XMM Newton observations of Nova V4743 Sgr (Nova Sgr 2002) were performed shortly after it returned to quiescence, 2 and 3.5 years after the explosion. The X-ray light curves revealed a modulation with a frequency of ~0.75 mHz, indicating that V4743 Sgr is most probably an intermediate polar (IP). The X-ray spectra have characteristics in common with known IPs, with a hard thermal plasma component that can be fitted only assuming a partially covering absorber. In 2004 the X-ray spectrum had also a supersoft blackbody-like component, whose temperature was close to that of the white dwarf (WD…
Nova V5116 Sgr 2005 No. 2, discovered on 2005 July 4, was observed with XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray spectrum showed that the nova had evolved to a pure supersoft X-ray source, indicative of residual H-burning on top of the white dwarf. The X-ray light-curve shows abrupt decreases and increases of the flux by a factor 8 with a periodicity of 2.97h, consistent with the possible orbital period of the system. The EPIC spectra are well fit with an ONe white dwarf atmosphere model, with the same temperature both in the low and the high flux periods. This…
We report K2 observations of the eclipsing cataclysmic variable V729 Sgr which covered nearly 80 days in duration. We find five short outbursts and two long outbursts, one of which shows a clear plateau phase in the rise to maximum brightness. The mean time between successive short outbursts is ~10 d while the time between the two long outbursts is ~38 d. The frequency of these outbursts are unprecedented for a CV above the orbital period gap. We find evidence that the mid-point of the eclipse occurs systematically earlier in outburst than in quiescence. During five of the six quiescent epochs…
Shocks in the Symbiotic Recurrent Nova V3890 Sgr: VLBI Radio Imaging and Fermi GeV Gamma-Rays
Michigan State University（密歇根州立大学）；Institute for Advanced Study, Princeton（普林斯顿高等研究院）；Columbia University（哥伦比亚大学）；National Radio Astronomy Observatory（美国国家射电天文台）；Flatiron Institute（平顿研究所）；Chalmers University of Technology（查尔姆斯理工大学）
The 30-year recurrent symbiotic nova V3890 Sgr exploded 2019 August 28 and was observed with multiple X-ray telescopes. An XMM-Newton observation during the SSS phase captured a high degree of X-ray variability including a deep dip in the middle of the observation, an initial rise of similar depth and shape and, after the deep dip, smaller dips of 10% amplitude, which might be periodic over 18.1-minutes. An eclipse model of the dips yields clump sizes and orbital radii of 0.5-8 and 5-150 white dwarf radii, respectively. The simultaneous UV light curve shows no significant variations beyond slo…
We present two further classical novae, V906 Car and V5668 Sgr, that show jets and accretion disc spectral signatures in their H-alpha complexes throughout the first 1000 days following their eruptions. From extensive densely time-sampled spectroscopy, we measure the appearance of the first high-velocity absorption component in V906 Car, and the duration of the commencement of the main H-alpha emission. We constrain the time taken for V5668 Sgr to transition to the nebular phase using [N II] 6584Å. We find these timings to be consistent with the jet and accretion disc model for explaining opti…
Two long AstroSat Soft X-ray Telescope observations were taken of the third recorded outburst of the Symbiotic Recurrent Nova, V3890 Sgr. The first observing run, 8.1-9.9 days after the outburst, initially showed a stable intensity level with a hard X-ray spectrum that we attribute to shocks between the nova ejecta and the pre-existing stellar companion. On day 8.57, the first, weak, signs appeared of Super Soft Source (SSS) emission powered by residual burning on the surface of the White Dwarf. The SSS emission was observed to be highly variable on time scales of hours. After day 8.9, the SSS…
Mergers of neutron star-neutron star (NS-NS) or neutron star-black hole (NS-BH) binaries are candidate sources of gravitational waves (GWs). At least a fraction of the merger remnant should be a stellar mass BH with a sub-relativistic ejecta. A collimated jet is launched via Blandford-Znajek mechanism from the central BH to trigger a short gamma-ray burst (SGRB). At the same time, a near-isotropic wind can be driven by the Blandford-Payne mechanism (BP). In previous work, additional energy injection to the ejecta from the BP mechanism was ignored, and radioactive decay has long been thought as…
AIMS: A few well studied cataclysmic variables (CVs) have shown discrete characteristic frequencies of fast variability; the most prominent ones are around log(f/Hz) $\simeq$ -3. Because we still have only small number statistics, we obtained a new observation to test whether this is a general characteristic of CVs, especially if mass transfer occurs at a high rate typical for dwarf nova in outbursts, in the so called "high state". METHODS: We analyzed optical Kepler data of the quiescent nova and intermediate polar V4743 Sgr. This system hosts a white dwarf accreting through a disk in the hig…
We present infrared spectroscopy of the 2019 eruption of the recurrent nova V3890 Sgr, obtained over the period 5.1-46.3 days after the eruption. The spectrum of the red giant became more prominent as the flux declined, and by day 46.3 dominated the spectrum. Hydrogen and helium emission lines consisted of a narrow component superposed on a broad pedestal. The full width at half maximum of the narrow components declined with time $t$ as the eruption progressed, as ${t}^{-0.74}$, whereas those of the broad components remained essentially constant. Conversely, the line fluxes of the narrow compo…
Nova Per 2018 (= V392 Per) halted the decline from maximum when it was 2mag brighter than quiescence and since 2019 has been stable at such a plateau. The ejecta have already fully diluted into the interstellar space. We obtained BVRIgrizY photometry and optical spectroscopy of V392 Per during the plateau phase and compared it with equivalent data gathered prior to the nova outburst. We find the companion star to be a G9 IV/III and the orbital period to be 3.4118 days, making V392 Per the longest known period for a classical nova. The location of V392 Per on the theoretical isochrones is inter…
Eight X-ray observations of V4743 Sgr (2002), observed with Chandra and XMM-Newton are presented. The nova turned off some time between days 301.9 and 371, and the X-ray flux subsequently decreased from day 301.9 to 526 following an exponential decline time scale of $(96 \pm 3)$ days. We use the absorption lines present in the SSS spectrum for diagnostic purposes, and characterize the physics and the dynamics of the expanding atmosphere during the explosion of the nova. The information extracted from this first stage is then used as input for computing full photoionization models of the ejecta…
We report extensive 3-yr multiwavelength observations of the WZ Sge-type dwarf nova SSS J122221.7-311525 during its unusual double superoutburst, the following decline and in quiescence. The second segment of the superoutburst had a long duration of 33 d and a very gentle decline with a rate of 0.02 mag/d, and it displayed an extended post-outburst decline lasting at least 500 d. Simultaneously with the start of the rapid fading from the superoutburst plateau, the system showed the appearance of a strong near-infrared excess resulting in very red colours, which reached extreme values (B-I~1.4)…
The classical nova YZ Reticuli was discovered in July 2020. Shortly after this we commenced a sustained, highly time-sampled coverage of its subsequent rapid evolution with time-resolved spectroscopy from the Global Jet Watch observatories. Its H-alpha complex exhibited qualitatively different spectral signatures in the following weeks and months. We find that these H-alpha complexes are well described by the same five Gaussian emission components throughout the six months following eruption. These five components appear to constitute two pairs of lines, from jet outflows and an accretion disc…
We show that old isolated neutron stars in groups and clusters of galaxies experiencing a Quark-Nova phase (QN: an explosive transition to a quark star) may be the source of FRBs. Each of the millions of fragments of the ultra-relativistic QN ejecta provides a collisionless plasma for which the ambient medium (galactic/halo, the intra-group/intra-cluster medium) acts as a relativistic plasma beam. The Buneman and the Weibel instabilities, successively induced by the beam in the fragment, generate particle bunching and observed coherent emission at GHz frequency with a corresponding fluence in …
Some intermediate polars show outbursts that are much shorter than those observed in normal dwarf novae, and whose origin has remained unelucidated for a long time. We examine here the case of V1223 Sgr, an intermediate polar that showed a short outburst in 1984, and compare the outburst characteristics with the predictions of the magnetospheric gating model. We use the archival data from the AAVSO from which we extract the outburst profiles. We use our code for computing the time-dependent evolution of an accretion disc truncated by the white dwarf magnetic field, using a simple description o…
