Nova GK Research Articles

Swift monitoring of GK Persei during the 2018 dwarf nova outburst

ABSTRACT The old nova and intermediate polar (IP) GK Persei underwent one of its recurrent dwarf nova (DN) outbursts in 2018. We proposed monitoring it in UV and X-rays with the Neil Gehrels Swift Observatory, starting less than six days after the eruption, until 16 days after the eruption ended. For the first time, we could follow the decay to the minimum light UV and X-rays. We present the timing and spectral analysis, comparing the results with the previous outbursts and with the quiescent status. We confirm the spin modulation in X-rays with a period PWD = 351.325(9) s, only in the 2–10 keV range. The period was not detected in the 0.3–2 keV range and in the UV band, suggesting that the soft portion of the X-ray spectrum in GK Per does not originate near the poles, but in a wind or circumstellar material. The amplitude of the modulation was less prominent than in 2015, a fact that seems correlated with a lower average mass accretion rate. The spectral fits are consistent with a mass accretion rate increasing by a factor of 2 from rise to maximum and decreasing during the return to minimum, following the trend of the modulation amplitude. The maximum plasma temperature is higher than the Swift XRT energy range of 0.3–10 keV, thus it is not well constrained, but our spectral fits indicate that it may have varied irregularly during the outburst.

V392 Persei: A γ-ray bright nova eruption from a known dwarf nova

ABSTRACT V392 Persei is a known dwarf nova (DN) that underwent a classical nova eruption in 2018. Here we report ground-based optical, Swift UV and X-ray, and Fermi-LAT γ-ray observations following the eruption for almost three years. V392 Per is one of the fastest evolving novae yet observed, with a t2 decline time of 2 d. Early spectra present evidence for multiple and interacting mass ejections, with the associated shocks driving both the γ-ray and early optical luminosity. V392 Per entered Sun-constraint within days of eruption. Upon exit, the nova had evolved to the nebular phase, and we saw the tail of the supersoft X-ray phase. Subsequent optical emission captured the fading ejecta alongside a persistent narrow line emission spectrum from the accretion disc. Ongoing hard X-ray emission is characteristic of a standing accretion shock in an intermediate polar. Analysis of the optical data reveals an orbital period of 3.230 ± 0.003 d, but we see no evidence for a white dwarf (WD) spin period. The optical and X-ray data suggest a high mass WD, the pre-nova spectral energy distribution (SED) indicates an evolved donor, and the post-nova SED points to a high mass accretion rate. Following eruption, the system has remained in a nova-like high mass transfer state, rather than returning to the pre-nova DN low mass transfer configuration. We suggest that this high state is driven by irradiation of the donor by the nova eruption. In many ways, V392 Per shows similarity to the well-studied nova and DN GK Persei.

Hard X-ray view on intermediate polars in theGaiaera

The hardness of the X-ray spectra of intermediate polars (IPs) is determined mainly by the white dwarf (WD) compactness (mass-radius ratio, M/R) and, thus, hard X-ray spectra can be used to constrain the WD mass. An accurate mass estimate requires the finite size of the WD magnetosphere R_m to be taken into the account. We suggested to derive it either directly from the observed break frequency in power spectrum of X-ray or optical lightcurves of a polar, or assuming the corotation. Here we apply this method to all IPs observed by NuSTAR (10 objects) and Swift/BAT (35 objects). For the dwarf nova GK Per we also observe a change of the break frequency with flux, which allows to constrain the dependence of the magnetosphere radius on the mass-accretion rate. For our analysis we calculated an additional grid of two-parameter (M and R_m/R) model spectra assuming a fixed, tall height of the accretion column H_sh/R=0.25, which is appropriate to determine WD masses in low mass-accretion IPs like EX\,Hya. Using the Gaia Data Release 2 we obtain for the first time reliable estimates of the mass-accretion rate and the magnetic field strength at the WD surface for a large fraction of objects in our sample. We find that most IPs accrete at rate of ~10^{-9} M_Sun/yr, and have magnetic fields in the range 1--10 MG. The resulting WD mass average of our sample is 0.79 +/- 0.16 M_Sun, which is consistent with earlier estimates.

An estimation of the white dwarf mass in the Dwarf Nova GK Persei with NuSTAR observations of two states

We report on X-ray observations of the Dwarf Nova GK Persei performed by {\it NuSTAR} in 2015. GK Persei, behaving also as an Intermediate Polar, exhibited a Dwarf Nova outburst in 2015 March--April. The object was observed with {\sl NuSTAR} during the outburst state, and again in a quiescent state wherein the 15--50 keV flux was 33 times lower. Using a multi-temperature plasma emission and reflection model, the highest plasma temperature in the accretion column was measured as $19.7^{+1.3}_{-1.0}$~keV in outburst and $36.2^{+3.5}_{-3.2}$~keV in quiescence. The significant change of the maximum temperature is considered to reflect an accretion-induced decrease of the inner-disk radius $R_{\rm in}$, where accreting gas is captured by the magnetosphere. Assuming this radius scales as $R_{\rm in} \propto \dot{M}^{-2/7}$ where $\dot{M}$ is the mass accretion rate, we obtain $R_{\rm in} = 1.9 ^{+0.4}_{-0.2}~R_{\rm WD}$ and $R_{\rm in} = 7.4^{+2.1}_{-1.2}~R_{\rm WD}$ in outburst and quiescence respectively, where $R_{\rm WD}$ is the white-dwarf radius of this system. Utilising the measured temperatures and fluxes, as well as the standard mass-radius relation of white dwarfs, we estimate the white-dwarf mass as $M_{\rm WD} = 0.87~\pm~0.08~M_{\rm \odot}$ including typical systematic uncertainties by 7%. The surface magnetic field is also measured as $B \sim 5 \times 10^{5}$~G. These results exemplify a new X-ray method of estimating $M_{\rm WD}$ and $B$ of white dwarfs by using large changes in $\dot{M}$.

Shell-type supernova remnants as sources of cosmic rays

The investigation of VHE gamma-ray sources by any methods, including mirror Cherenkov telescopes, touches on the problem of the cosmic ray origin and, accordingly, the role of the Galaxy in their generation. The SHALON observations have yielded the results on Galactic supernova remnants (SNR) of different ages. Among them are: the shell-type SNRs Tycho’s SNR (1572y), Cas A (1680y), IC 443 (age∼(3–30)×103y), γCygni SNR (age∼(5–7)×103y), G166.0+4.3 (age∼24×103y) and classical nova GK Per (Nova 1901). For each of SNRs the observation results are presented with spectral energy distribution by SHALON in comparison with other experiment data and images by SHALON in together with data from X-ray by Chandra and radio-data by CGPS. The collected experimental data have confirmed the prediction of the theory about the hadronic generation mechanism of very high energy 800GeV–100TeV gamma-rays in Tycho’s SNR, Cas A and IC 443. For the first time, unique data on GK Per (Nova 1901) TeV gamma-ray emission were obtained with SHALON experiment. The X-ray data shows that, the nova remnant of GK Per could be a younger remnant that will resemble older SNRs like IC 443 which interact with molecular clouds. GK Per is supposed to be a candidate for TeV gamma-ray emission due to the accelerated particles in the reverse shock region.

Multimission observations of the old nova GK Per during the 2015 outburst

GK Per, a classical nova of 1901, is thought to undergo variable mass accretion on to a magnetized white dwarf (WD) in an intermediate polar system (IP). We organized a multi-mission observational campaign in the X-ray and ultraviolet (UV) energy ranges during its dwarf nova (DN) outburst in 2015 March-April. Comparing data from quiescence and near outburst, we have found that the maximum plasma temperature decreased from about 26 to 16.2+/-0.4 keV. This is consistent with the previously proposed scenario of increase in mass accretion rate while the inner radius of the magnetically disrupted accretion disc shrinks, thereby lowering the shock temperature. A NuSTAR observation also revealed a high-amplitude WD spin modulation of the very hard X-rays with a single-peaked profile, suggesting an obscuration of the lower accretion pole and an extended shock region on the WD surface. The X-ray spectrum of GK Per measured with the Swift X-Ray Telescope varied on time-scales of days and also showed a gradual increase of the soft X-ray flux below 2 keV, accompanied by a decrease of the hard flux above 2 keV. In the Chandra observation with the High Energy Transmission Gratings, we detected prominent emission lines, especially of Ne, Mg and Si, where the ratios of H-like to He-like transition for each element indicate a much lower temperature than the underlying continuum. We suggest that the X-ray emission in the 0.8-2 keV range originates from the magnetospheric boundary.

Shell-type SNRs as sources of cosmic rays

Investigations of VHE gamma-ray sources by any methods, including mirror Cherenkov telescopes, touch on the problem of the cosmic ray origin and, accordingly, the role of the Galaxy in their generation. SHALON observations have yielded results on Galactic supernova remnants (SNR) of different ages. Among them are: the shell-type SNRs Tycho's SNR (1572y), Cas A (1680y), IC 443 (age ∼ (3 ÷ 30) × 10 3 y), Cygni SNR (age ∼ (5 ÷ 7) × 10 3 y), G166.0 + 4.3 (age ∼ 24 × 10 3 y) and the classical nova GK Per (Nova 1901). Observation results are presented for each of the SNRs with spectral energy distributions by SHALON in comparison with other experiment data and images by SHALON together with data from X-rays by Chandra and radio-data by CGPS. The collected experimental data have confirmed the prediction of the theory about the hadronic generation mechanism of very high energy 800 GeV–100 TeV gamma-rays in Tycho's SNR, Cas A and IC443. For the first time, unique data on GK Per (Nova1901) TeV gamma-ray emission were obtained with the SHALON experiment. The X-ray data shows that the nova remnant of GK Per could be a younger remnant that will resemble older SNRs like IC 443 which interact with molecular clouds. GK Per is supposed to be a candidate for TeV gamma-ray emission due to accelerated particles in the reverse shock region.

Critical temperature and accretion rate of outbursts in long-period dwarf novae

Dwarf nova outbursts are nonlinear phenomena, and a time-dependent disk model is necessary to account for observations in detail. However, it is also necessary to elaborate a simpler steady-state fit to interpret observations. To know in what condition the outburst is initiated, understanding of the dwarf nova outburst is important. The parameterized, steady-state fitting formulae are suggested by Smak (Acta Astron. 52, 429 (2002); ibid 60, 83 (2010)) for the critical disk temperature and mass accretion rate above which the disk becomes thermally unstable. The fits give a single-valued temperature and accretion rate and are radius-independent whereas the observations show that the outbursts are radius-dependent phenomena of the ionizaton propagating in the disk. The fits have been tested to account for the observed outbursts only for systems with orbital periods shorter than a half day. Therefore, we examine the fits for orbital period as long as 2 days and compare the fits to the time-dependent model of a long-period dwarf nova GK Per. The fits are not much different from the time-dependent result for the critical temperature. However, the fits for the critical mass accretion rate above which the disk enters the hot state overestimate the time-dependent model for a long-period system like GK Per. The critical mass accretion rate in the intermediate state is consistent with that from the time-dependent disk model. However, the fit value should be treated as a maximum possible value below which the disk maintains the intermediate state, which is consistent with an interpretation for the observations of the Z Cam stars.

Results of observations of shell supernova remnants at ultrahigh energies with the SHALON mirror Cherenkov telescopes

The investigation of VHE gamma-ray sources by any methods, including mirror Cherenkov telescopes, touches on the problem of the cosmic ray origin and, accordingly, the role of the Galaxy in their generation. The SHALON observations have yielded the results on Galactic shell-type supernova remnants (SNR) on different evolution stages. Among them are: SNRs Tycho's SNR, Cas A, IC 443, $\gamma$Cygni SNR and classical nova GK Per (Nova 1901). For each of SNRs the observation results are presented with spectral energy distribution by SHALON in comparison with other experiment data and images by SHALON. The collected experimental data have confirmed the prediction of the theory about the hadronic generation mechanism of very high energy 800 GeV - 100 TeV $\gamma$-rays in Tycho's SNR, Cas A and IC443.

CC Sculptoris: a superhumping intermediate polar

We present high speed optical, spectroscopic and Swift X-ray observations made during the dwarf nova superoutburst of CC Scl in November 2011. An orbital period of 1.383 h and superhump period of 1.443 h were measured, but the principal new finding is that CC Scl is a previously unrecognised intermediate polar, with a white dwarf spin period of 389.49 s which is seen in both optical and Swift X-ray light curves only during the outburst. In this it closely resembles the old nova GK Per, but unlike the latter has one of the shortest orbital periods among intermediate polars.

AT Cnc: A SECOND DWARF NOVA WITH A CLASSICAL NOVA SHELL

We are systematically surveying all known and suspected Z Cam-type dwarf novae for classical nova shells. This survey is motivated by the discovery of the largest known classical nova shell, which surrounds the archetypal dwarf nova Z Camelopardalis. The Z Cam shell demonstrates that at least some dwarf novae must have undergone classical nova eruptions in the past, and that at least some classical novae become dwarf novae long after their nova thermonuclear outbursts, in accord with the hibernation scenario of cataclysmic binaries. Here we report the detection of a fragmented "shell", 3 arcmin in diameter, surrounding the dwarf nova AT Cancri. This second discovery demonstrates that nova shells surrounding Z Cam-type dwarf novae cannot be very rare. The shell geometry is suggestive of bipolar, conical ejection seen nearly pole-on. A spectrum of the brightest AT Cnc shell knot is similar to that of the ejecta of the classical nova GK Per, and of Z Cam, dominated by [NII] emission. Galex FUV imagery reveals a similar-sized, FUV-emitting shell. We determine a distance of 460 pc to AT Cnc, and an upper limit to its ejecta mass of ~ 5 x 10^{-5} Msun, typical of classical novae.

A Universal Decline Law of Classical Novae. II. GK Persei 1901 and Novae in 2005

Optical and infrared light-curves of classical novae are approximately homologous among various white dwarf (WD) masses and chemical compositions when free-free emission from optically thin ejecta dominates the continuum flux of novae. Such a homologous template light-curve is called a ``universal decline law.'' The timescale of the light curve depends strongly on the WD mass but weakly on the chemical composition, so we are able to roughly estimate the WD mass from the light-curve fitting. We have applied the universal decline law to the old nova GK Persei 1901 and recent novae that outbursted in 2005. The estimated WD mass is 1.15 M_sun for GK Per, which is consistent with a central value of the WD mass determined from the orbital velocity variations. The other WD masses of 10 novae in 2005 are also estimated to be 1.05 M_sun (V2361 Cyg), 1.15 M_sun (V382 Nor), 1.2 M_sun (V5115 Sgr), 0.7 M_sun (V378 Ser), 0.9 M_sun (V5116 Sgr), 1.25 M_sun (V1188 Sco), 0.7 M_sun (V1047 Cen), 0.95 M_sun (V476 Sct), 0.95 M_sun (V1663 Aql), and 1.30 M_sun (V477 Sct), within a rough accuracy of +- 0.1 M_sun. Four (V382 Nor, V5115 Sgr, V1188 Sco, and V477 Sct) of ten novae in the year 2005 are probably neon novae on an O-Ne-Mg WD. Each WD mass depends weakly on the chemical composition (especially the hydrogen content X in mass weight), i.e., the obtained WD masses increase by +0.5(X-0.35) M_sun for the six CO novae and by +0.5(X-0.55) M_sun for the four neon novae above. Various nova parameters are discussed in relation to its WD mass.

A multiwavelength study of the remnant of nova GK Persei

We present new observations of the nebular remnant of the old nova GK Persei 1901 in the optical using the Himalayan Chandra Telescope (HCT) and at low radio frequencies using the Giant Metrewave Radio Telescope (GMRT). The dimensions of the shell in the optical emission lines of [NII], [OIII] and [OII] are arcsec2, arcsec2 and arcsec2, respectively. The evolution of the nova remnant indicates shock interaction with the ambient medium, especially in the southwest quadrant. Application of a simple model for the shock and its evolution to determine the time dependence of the radius of the shell in the southwest quadrant indicates that the shell is now expanding into an ambient medium that has a density of 0.12 cm-3, compared to the density of the ambient medium of 0.8 cm-3 ahead of the shock in 1987. There are indications of a recent interaction of the nova remnant with the ambient medium in the northeast quadrant. There is a distinct flattening of the shell, as well as an increase in the number and brightness of the knots in the region. The brightest optical knots in this region are also detected in the radio images. The nova remnant of GK Per is detected at all the observed radio frequencies and is of a similar extent to the optical remnant. Putting together our radio observations with VLA archival data on GK Per from 1997, we obtain three interesting results: 1. the spectrum above 1.4 GHz follows a power law with an index -0.7 () and below 1.4 GHz follows a power law with an index ∼-0.85. This could be due to the presence of at least two populations of electrons dominating the global emission at different frequencies; 2. we record an annual secular decrease of 2.1% in the flux density of the nova remnant at 1.4 and 4.9 GHz between 1984 and 1997 which has left the spectral index unchanged at -0.7. No such decrease is observed in the flux densities below 1 GHz; 3. we record an increase in the flux density at 0.33 GHz compared to the previous estimate in 1987. We conclude that the remnant of nova GK Per is similar to supernova remnants and in particular to the young supernova remnant Cas A.

Echoes of an Explosive Past: Solving the Mystery of the First Superluminal Source

The classical nova GK Persei rivaled the brightness of Vega at the peak of its outburst in 1901. Early observations showed it to possess optical nebulosities apparently expanding at super-light velocities, later explained as "light echoes." Indeed, it was the first astronomical source in which such motion was observed. Subsequently, the expanding nova remnant has turned out to be the longest lived and most energetic among the classical novae and appears more like a supernova remnant (SNR) in miniature but evolving on human timescales. Here we report the results of multifrequency observations and archival research that confirm the presence of an ancient planetary nebula (PN), with an apparent hourglass morphology, ejected by the nova in a previous phase of evolution and into which the nova ejecta are now running. The 1901 nova outburst is therefore the first of ultimately very many that this system will undergo. Furthermore, from measurements of the proper motion of the central binary we can now understand the asymmetries observed both in the outer (ancient planetary) and inner (1901 nova ejecta) nebulae and their relationship to the longest lived of the light echoes first observed around 100 years ago. GK Persei may therefore be an important key to our understanding of interacting binary, PN, and SNR evolution.

Evidence for Magnetic Accretion during the 2002 Optical Outburst of the Old Nova GK Persei (1901)

ABSTRACTSpectra of the old nova GK Per taken at the beginning and during the rise of the 2002 dwarf nova–like outburst reveal that halfway to light maximum, most of the accretion disk had already changed from the “low” to the “high state.” The evolution of emission‐line fluxes, equivalent widths, and FWHMs confirms the dwarf nova nature of the outburst. The emission‐line profiles at midrise show the presence of an asymmetric broad component that we interpret as the signature of increased mass accretion onto the magnetic white dwarf.

Superluminal light echoes in astronomy

The echo produced when a light ‘pulse’ from a stellar source (e.g. a nova orsupernova) is reflected by circumstellar or interstellar material can appear as aluminous ring expanding at a rate that can be superluminal, i.e. having anapparent motion within the source, transverse to the observer’s line of sight, at aspeed greater than that of light. ‘Light-echo optics’ applied to the star RS Puppisand its nebula suggests that when nebular features in peripheral regions ofcircumstellar-shell images are observed, superluminal effects are not evident;however, such observations can give the stellar distance from the observer.Light-echo optics for an interstellar plane sheet, inclined to the observer’s line ofsight, can be applied to Nova GK Persei 1901 and SN 1987A, which showsuperluminal effects. For SN 1987A, an intense thermal x-ray source should beproduced in AD 2003, when the advancing supernova ejecta interact with acircumstellar ring, 250 light days in radius: the arc-shaped x-ray image,while not actually a ‘radiation echo’, should expand at a superluminalrate for about 75 days from the time of its first appearance, and alsofor the same time before the completion of its ‘circuit’ around the ring.

Three-Dimensional Fabry-Perot Imaging Spectroscopy of the Crab Nebula, Cassiopeia A, and Nova GK Persei

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The extended dust emission around the nova GK persei

We have applied maximum entropy reconstruction methods to the IRAS observations of the nova GK Persei to examine the spatial distribution of the far-IR emission. We have discovered discrete regions of emission in a co-linear structure extending to 17 arcmin on either side of the binary system, supporting a stellar origin for the structure. We postulate that the evolved secondary is the progenitor of the circumbinary envelope.

The previous incarnation of the old nova GK Persei

We present the results of a survey of the immediate environment of the old nova GK Per in the J = 2 → 1 line of 12CO at 230 GHz, at higher spatial resolution than previous studies of this area. We find a bipolar emission region which is centred on the nova and whose lobes are expanding symmetrically. The morphology, expansion velocity, mass and dimensions of the bipolar lobes are all within the observed ranges of those associated with planetary nebulae. Our results therefore strongly support previous suggestions that GK Per is surrounded by a ‘fossil’ planetary nebula.

The nebular remnant and quiescent spectrum of Nova GK Persei

CCD images of the nebular shell of the old nova GK Per in [N II] and [O III] emission lines, and also the quiescent spectrum in the optical region, are presented. Proper motion measurements of the knots in [N II] over the baseline 1984–90 give a mean expansion rate of 0.31 ± 0.07 arcsec yr -1 . This value agrees with models of the expansion of the ejecta into the ambient circumstellar medium. A correction for projection effects indicates that the ratio of the mass of the shell to the density of the circumstellar medium is a factor of 2 – 3 higher than estimated previously. The optical spectrum is decomposed into those of the secondary and the accretion disc. The estimate of M V = + 4.9 for the accretion disc implies a mass transfer rate of 3.5 × 10 -9 M☉ yr -1 in the steady state. The H and He emission-line fluxes are used to infer that (i) the line-emitting region has a high density, (ii) the He/H abundance is near-solar, and (iii) the ionizing source has a Zanstra temperature of 1.3 × 10 5 K and an effective radius of 0.01 R☉, which are consistent with its being a white dwarf.