Radio Peak Research Articles

HIGH-MASS STAR FORMATION TOWARD SOUTHERN INFRARED BUBBLE S10

ABSTRACT An investigation in radio and infrared wavelengths of two high-mass star-forming regions toward the southern Galactic bubble S10 is presented here. The two regions under study are associated with the broken bubble S10 and Extended Green Object, G345.99-0.02, respectively. Radio continuum emission mapped at 610 and 1280 MHz using the Giant Metrewave Radio Telescope, India, is detected toward both of the regions. These regions are estimated to be ionized by early-B- to late-O-type stars. Spitzer GLIMPSE mid-infrared data is used to identify young stellar objects (YSOs) associated with these regions. A Class-I/II-type source, with an estimated mass of 6.2 M ⊙, lies ∼7″ from the radio peak. Pixel-wise, modified blackbody fits to the thermal dust emission using Herschel far-infrared data is performed to construct dust temperature and column density maps. Eight clumps are detected in the two regions using the 250 μm image. The masses and linear diameter of these range between ∼300–1600 M ⊙ and 0.2–1.1 pc, respectively, which qualifies them as high-mass star-forming clumps. Modeling of the spectral energy distribution of these clumps indicates the presence of high luminosity, high accretion rate, massive YSOs possibly in the accelerating accretion phase. Furthermore, based on the radio and MIR morphology, the occurrence of a possible bow wave toward the likely ionizing star is explored.

SKA weak lensing – III. Added value of multiwavelength synergies for the mitigation of systematics

In this third paper of a series on radio weak lensing for cosmology with the Square Kilometre Array, we scrutinise synergies between cosmic shear measurements in the radio and optical/near-IR bands for mitigating systematic effects. We focus on three main classes of systematics: (i) experimental systematic errors in the observed shear; (ii) signal contamination by intrinsic alignments; and (iii) systematic effects due to an incorrect modelling of non-linear scales. First, we show that a comprehensive, multi-wavelength analysis provides a self-calibration method for experimental systematic effects, only implying <50% increment on the errors on cosmological parameters. We also illustrate how the cross-correlation between radio and optical/near-IR surveys alone is able to remove residual systematics with variance as large as 0.00001, i.e. the same order of magnitude of the cosmological signal. This also opens the possibility of using such a cross-correlation as a means to detect unknown experimental systematics. Secondly, we demonstrate that, thanks to polarisation information, radio weak lensing surveys will be able to mitigate contamination by intrinsic alignments, in a way similar but fully complementary to available self-calibration methods based on position-shear correlations. Lastly, we illustrate how radio weak lensing experiments, reaching higher redshifts than those accessible to optical surveys, will probe dark energy and the growth of cosmic structures in regimes less contaminated by non-linearities in the matter perturbations. For instance, the higher-redshift bins of radio catalogues peak at z~0.8-1, whereas their optical/near-IR counterparts are limited to z<0.5-0.7. This translates into having a cosmological signal 2 to 5 times less contaminated by non-linear perturbations.

Shocks in nova outflows – II. Synchrotron radio emission

The discovery of GeV gamma-rays from classical novae indicates that shocks and relativistic particle acceleration are energetically key in these events. Further evidence for shocks comes from thermal keV X-ray emission and an early peak in the radio light curve on a timescale of months with a brightness temperature which is too high to result from freely expanding photo-ionized gas. Paper I developed a one dimensional model for the thermal emission from nova shocks. This work concluded that the shock-powered radio peak cannot be thermal if line cooling operates in the post-shock gas at the rate determined by collisional ionization equilibrium. Here we extend this calculation to include non-thermal synchrotron emission. Applying our model to three classical novae, we constrain the amplification of the magnetic field $\epsilon_B$ and the efficiency $\epsilon_e$ of accelerating relativistic electrons of characteristic Lorentz factor $\gamma \sim 100$. If the shocks are radiative (low velocity $v_{\rm sh} \lesssim 1000$ km s$^{-1}$) and cover a large solid angle of the nova outflow, as likely characterize those producing gamma-rays, then values of $\epsilon_e \sim 0.01-0.1$ are required to achieve the peak radio brightness for $\epsilon_B = 10^{-2}$. Such high efficiencies exclude secondary pairs from pion decay as the source of the radio-emitting particles, instead favoring the direct acceleration of electrons at the shock. If the radio-emitting shocks are instead adiabatic (high velocity), as likely characterize those responsible for the thermal X-rays, then much higher brightness temperatures are possible, allowing the radio-emitting shocks to cover a smaller outflow solid angle.

Constraining the mass of the photon with gamma-ray bursts

One of the cornerstones of modern physics is Einstein's special relativity, with its constant speed of light and zero photon mass assumptions. Constraint on the rest mass mγ of photons is a fundamental way to test Einstein's theory, as well as other essential electromagnetic and particle theories. Since non-zero photon mass can give rise to frequency- (or energy-) dependent dispersions, measuring the time delay of photons with different frequencies emitted from explosive astrophysical events is an important and model-independent method to put such a constraint. The cosmological gamma-ray bursts (GRBs), with short time scales, high redshifts as well as broadband prompt and afterglow emissions, provide an ideal testbed for mγ constraints. In this paper we calculate the upper limits of the photon mass with GRB early time radio afterglow observations as well as multi-band radio peaks, thus improve the results of Schaefer (1999) by nearly half an order of magnitude.

XMM-NEWTON AND CHANDRA OBSERVATIONS OF THE FILLED-CENTER SUPERNOVA REMNANT G63.7+1.1

ABSTRACT Pulsar wind nebulae (PWNe) provide unique opportunities to study relativistic pulsar winds and their interaction with the surrounding medium. G63.7+1.1 is an intriguing supernova remnant studied so far only at radio wavelengths and classified as a PWN candidate based on its nonthermal radio emission. We present the first X-ray study of G63.7+1.1, obtained with XMM-Newton and the Chandra X-ray Observatory, with the goal of detecting and studying the candidate PWN in X-rays and searching for the neutron star candidate. The peak of the X-ray emission lies approximately 1.′2 from the peak of the 21 cm continuum emission and is offset from the center of the X-ray nebula. The diffuse X-ray emission is irregularly shaped (4.′2 × 3.′2), unlike the nearly symmetric 8′-diameter radio nebula. Its X-ray spectrum is fit with a power-law model, yielding a photon index of , a column density of (1.6 ± 0.5) × 1022 cm−2, and an X-ray luminosity of ∼2 × 1033 erg s−1 (0.5–10.0 keV) at a kinematic distance of 6 kpc. Several point-like sources were detected within the limit of the X-ray nebula. We present a study of their spectra, identifying 3XMM J194753.4+274357 (CXO J194753.3+274351) as a candidate for the location of the neutron star powering the nebula. Located in a dense environment, we estimate that G63.7+1.1 is old enough (≥8 kyr) for the reverse shock to have interacted with the PWN. This, together with the large size estimated from radio studies (∼14 pc) and the offset between the radio and X-ray peaks, suggests an evolved nebula, with the radio nebula being a “relic” PWN.

A COMPREHENSIVE STATISTICAL DESCRIPTION OF RADIO-THROUGH-γ-RAY SPECTRAL ENERGY DISTRIBUTIONS OF ALL KNOWN BLAZARS

ABSTRACT We combined multi-wavelength data for blazars from the Roma-BZCAT catalog and analyzed hundreds of X-ray spectra. We present the fluxes and spectral energy distributions (SEDs), in 12 frequency bands from radio to γ-rays, for a final sample of 2214 blazars. Using a model-independent statistical approach, we looked for systematic trends in the SEDs; the most significant trends involved the radio luminosities and X-ray spectral indices of the blazars. We used a principal component analysis (PCA) to determine the basis vectors of the blazar SEDs and, in order to maximize the size of the sample, imputed missing fluxes using the K-nearest neighbors method. Using more than an order of magnitude more data than was available when Fossati et al. first reported trends of SED shape with blazar luminosity, we confirmed the anti-correlation between radio luminosity and synchrotron peak frequency, although with greater scatter than was seen in the smaller sample. The same trend can be seen between bolometric luminosity and synchrotron peak frequency. Finally, we used all of the available blazar data to determine an empirical SED description that depends only on the radio luminosity at 1.4 GHz and the redshift. We verified that this statistically significant relation was not a result of the luminosity–luminosity correlations that are natural in flux-limited samples (i.e., where the correlation is actually caused by the redshift rather than the luminosity).

Long‐term analysis between radio occultation and ionosonde peak electron density and height during geomagnetic storms

AbstractFor the first time, a long‐term comparative analysis of radio occultation (RO) maximum electron density and peak height of the F2 layer (NmF2 and hmF2) with ionosonde data is presented during geomagnetic storm periods. Using the optimum spatial resolution of 4.5° × 4.5° in both latitude and longitude space over Grahamstown, GR13L(33.3°S, 26.5°E), South Africa, RO NmF2 and hmF2 (from CHAMP and COSMIC/FORMOSAT‐3) are directly compared to ionosonde values within 15 min of ionosonde observational data from 2003 to end of May 2015. This study provides for the first time the deviation of RO data from ionosonde data on a long‐term scale during disturbed conditions in a midlatitude region. We have found that maximum deviations between RO and ionosonde hmF2/NmF2 occur during the high solar activity periods. For some storms, deviations between RO and ionosonde hmF2 can reach values just over 30 km and 85 km during 2005–2010 and 2011–2015, respectively. Overall, statistical results show that hmF2 and NmF2 from these independent data sets agree to within ∼9% and 21% (1 standard deviation, 1σ) from 2003 to 2015. While the deviation can be large during some storm events, statistically and based on ionosonde data, RO F2 peak parameters in midlatitudes are not degraded significantly during disturbed conditions and can therefore be reliably used to study ionospheric dynamics during extreme space weather events.

Photometric and Spectroscopic Survey of the Cluster [DBS2003] 156 Associated with the H II Region G331.1−0.5☆☆Based on observations obtained at the Southern Observatory for Astrophysical Research (SOAR), a joint project of the Ministério de Ciência, Tecnologia e Inovação (MCTI) of the República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill

The Norma section of the Milky Way is especially interesting because it crosses three spiral arms: Sagittarius–Carina, Scutum–Crux and the Norma arm itself. Distance determinations of embedded young stellar clusters can contribute to define the spiral structure in this part of the Galaxy. However, spectrophotometric distances were obtained for only a few of these clusters in Norma. We present a photometric and spectroscopic study in the NIR of the [DBS2003] 156 stellar cluster, associated with the H II region G331.1−0.5. We aim to find the ionizing sources of the H II region and determine its distance. The cluster was observed in the J, H, and bands and eight potential massive stars were chosen among the detected sources according to color criteria; subsequent spectroscopy of these candidates was performed with the Ohio State Infrared Imager/Spectrometer spectrograph attached to the Southern Observatory for Astrophysical Research 4.1 m telescope. We identified and classified spectroscopically four early-type stars: IRS 176 (O8 V), IRS 308 (O-type), IRS 310 (O6 V), and IRS 71 (B1 Iab). Based on the proximity of IRS 176 and 308 with the radio continuum emission peaks and their relative positions with respect to the warm dust mid-infrared emission, we concluded that these two stars are the main ionizing sources of the H ii region G331.1−0.5. The mean spectrophotometric distance of IRS 176 and 310 of 3.38 ± 0.58 kpc is similar to that obtained in a previous work for two early-type stars of the neighbor cluster [DBS2003] 157 of 3.29 ± 0.58 kpc. The narrow range of radial velocities of radio sources in the area of the clusters [DBS2003] 156 and 157 and their similar visual extinction indicate that these clusters are physically associated. A common distance of 3.34 ± 0.34 kpc is derived for the system [DBS2003] 156 and 157.

ALMA IMAGING OF THE CO (6-5) LINE EMISSION IN NGC 7130*

ABSTRACT In this paper, we report our high-resolution (0.″20 × 0.″14 or ∼70 × 49 pc) observations of the CO(6-5) line emission, which probes warm and dense molecular gas, and the 434 μm dust continuum in the nuclear region of NGC 7130, obtained with the Atacama Large Millimeter Array (ALMA). The CO line and dust continuum fluxes detected in our ALMA observations are 1230 ± 74 Jy km s−1 and 814 ± 52 mJy, respectively, which account for 100% and 51% of their total fluxes. We find that the CO(6-5) and dust emissions are generally spatially correlated, but their brightest peaks show an offset of ∼70 pc, suggesting that the gas and dust emissions may start decoupling at this physical scale. The brightest peak of the CO(6-5) emission does not spatially correspond to the radio continuum peak, which is likely dominated by an active galactic nucleus (AGN). This, together with our additional quantitative analysis, suggests that the heating contribution of the AGN to the CO(6-5) emission in NGC 7130 is negligible. The CO(6-5) and the extinction-corrected Pa-α maps display striking differences, suggestive of either a breakdown of the correlation between warm dense gas and star formation at linear scales of &lt;100 pc or a large uncertainty in our extinction correction to the observed Pa-α image. Over a larger scale of ∼2.1 kpc, the double-lobed structure found in the CO(6-5) emission agrees well with the dust lanes in the optical/near-infrared images.

Pulsed γ-ray properties of Crab pulsar in a retarded dipole with a current-induced magnetic field

Motivated by the Fermi observations of some γ-ray pulsars in which the phases of radio and γ-ray peaks are almost the same, we investigate the outer gap model in a retarded dipole with a current-induced magnetic field and apply it to explain pulsed γ-ray properties of the Crab pulsar. Our results show that the observed γ-ray energy-dependent light curves, which almost align with the radio light curve and phase averaged spectrum for the Crab pulsar, are reproduced well.

THE UNUSUAL RADIO AFTERGLOW OF THE ULTRA-LONG GAMMA-RAY BURST GRB 130925A

GRB 130925A is one of the recent additions to the growing family of ultra-long GRBs (T90$ \gtrsim 1000$ s). While the X-ray emission of ultra-long GRBs have been studied extensively in the past, no comprehensive radio dataset has been obtained so far. We report here the early discovery of an unusual radio afterglow associated with the ultra-long GRB 130925A. The radio emission peaks at low-frequencies ($\sim 7$ GHz) at early times, only $2.2$ days after the burst occurred. More notably, the radio spectrum at frequencies above $10$ GHz exhibits a rather steep cut-off, compared to other long GRB radio afterglows. This cut-off can be explained if the emitting electrons are either mono-energetic or originate from a rather steep, $dN/dE \propto E^{-4}$, power-law energy distribution. An alternative electron acceleration mechanism may be required to produce such an electron energy distribution. Furthermore, the radio spectrum exhibits a secondary underlying and slowly varying component. This may hint that the radio emission we observed is comprised of emission from both a reverse and a forward shock. We discuss our results in comparison with previous works that studied the unusual X-ray spectrum of this event and discuss the implications of our findings on progenitor scenarios.

TYPE II AND TYPE III RADIO BURSTS AND THEIR CORRELATION WITH SOLAR ENERGETIC PROTON EVENTS

Using the Wind/WAVES radio observations from 2010-2013, we present an analysis of the 123 decametric-hectometric (DH) type II solar radio bursts during this period, the associated type III burst properties, and their correlation with solar energetic proton (SEP) properties determined from analysis of the Geostationary Operational Environmental Satellite (GOES) observations. We present a useful catalog of the type II burst, type III burst, Langmuir wave, and proton flux properties for these 123 events, which we employ to develop a statistical relationship between the radio properties and peak proton flux that can be used to forecast SEP events. We find that all SEP events with a peak > 10 MeV flux above 15 pfu are associated with a type II burst and virtually all SEP events, 92%, are also associated with a type III radio burst. Based on a principal component analysis, the radio burst properties that are most highly correlated with the occurrence of gradual SEP events and account for the most variance in the radio properties are the type III burst intensity and duration. Further, a logistic regression analysis with the radio-derived principal component (dominated by the type III and type II radio burst intensity and type III duration) obtains SEP predictions approaching the human forecaster rates, with a false alarm rate of 22%, a probability of detection of 62%, and with 85% of the classifications correct. Therefore, type III radio bursts that occur along with a DH type II burst are shown to be an important diagnostic that can be used to forecast SEP events.

EXTRAGALACTIC SYNCHROTRON TRANSIENTS IN THE ERA OF WIDE-FIELD RADIO SURVEYS. I. DETECTION RATES AND LIGHT CURVE CHARACTERISTICS

The impending era of wide-field radio surveys has the potential to revolutionize our understanding of astrophysical transients. Here we evaluate the prospects of a wide range of planned and hypothetical radio surveys using the properties and volumetric rates of known and hypothetical classes of extragalactic synchrotron radio transients (e.g., on-axis and off-axis gamma-ray bursts (GRBs), supernovae, tidal disruption events, compact object mergers). Utilizing these sources and physically motivated considerations we assess the allowed phase space of radio luminosity and peak timescale for extragalactic transients. We also include for the first time effects such as redshift evolution of the rates, K-corrections, and non-Euclidean luminosity distance, which affect the detection rates of the most sensitive surveys. The number of detected events is calculated by means of a Monte Carlo method, using the various survey properties (depth, cadence, area) and realistic detection criteria that include a cut on the minimum variability of the transients during the survey and an assessment of host galaxy contamination. We find that near-term GHz frequency surveys (ASKAP/VAST, Very Large Array Sky Survey) will detect few events: on- and off-axis long GRBs (LGRBs) and off-axis tidal disruption events, and neutron star binary mergers if of the mergers result in a stable millisecond magnetar. Low-frequency surveys (e.g., LOFAR) are unlikely to detect any transients, while a hypothetical large-scale mm survey may detect ?40 on-axis LGRBs. On the other hand, we find that SKA1 surveys at GHz have the potential to uncover thousands of transients, mainly on-axis and off-axis LGRBs, on-axis short GRBs, off-axis TDEs, and neutron star binary mergers with magnetar remnants.

Radio afterglows and host galaxies of gamma-ray bursts

Considering the contribution of emission from the host galaxies of gamma-ray bursts (GRBs) to radio afterglows, we investigate the effect of host galaxies on observations statistically. For the three types of event, i.e. low-luminosity, standard and high-luminosity GRBs, it is found that a tight correlation exists between the ratio of the radio flux (RRF) of the host galaxy to the total radio peak emission and the observational frequency. Towards lower frequencies, in particular, the contribution from the host increases significantly. The correlation can be used to obtain a useful estimate for the radio brightness of those host galaxies that only have very limited radio afterglow data. Using this prediction, we reconsidered the theoretical radio afterglow light curves for four kinds of event: high-luminosity, low-luminosity, standard and failed GRBs, taking into account the contribution from host galaxies and aiming to explore the detectability of these events by the Five-hundred-metre Aperture Spherical radio Telescope (FAST). Lying at a typical redshift of z = 1, most of the events can be detected easily by FAST. For the less fierce low-luminosity GRBs, their radio afterglows are not strong enough to exceed the sensitivity limit of FAST at such distances. However, since a large number of low-luminosity bursts actually happen very near to us, it is expected that FAST will still be able to detect many of them.

SUPERNOVA REMNANT KESTEVEN 27: INTERACTION WITH A NEIGHBOR Hi CLOUD VIEWED BYFERMI

We report on the likely detection of gamma-ray emission from the supernova remnant (SNR) Kesteven 27 (Kes 27). We analyze 5.7 yr Fermi Large Area Telescope data of the SNR region and find an unresolved source at a position consistent with the radio brightness peak and the X-ray knot of Kes 27, which is located in the eastern region of the SNR and caused by the interaction with a nearby HI cloud. The source's emission is best fit with a power-law spectrum with a photon index of 2.5 +/- 0.1 and a >0.2 GeV luminosity of 5.8*10^34 erg s^-1 assuming a distance of 4.3 kpc, as derived from radio observations of the nearby HI cloud. Comparing the properties of the source with that of other SNRs that are known to be interacting with nearby high-density clouds, we discuss the origin of the source's emission. The spectral energy distribution of the source can be described by a hadronic model that considers the interaction of energetic protons, escaping from the shock front of Kes 27, with a high-density cloud.

MODELING MULTI-WAVELENGTH PULSE PROFILES OF THE MILLISECOND PULSAR PSR B1821–24

PSR B1821$-$24 is a solitary millisecond pulsar (MSP) which radiates multi-wavelength pulsed photons. It has complex radio, X-ray and $\gamma$-ray pulse profiles with distinct peak phase-separations that challenge the traditional caustic emission models. Using the single-pole annular gap model with suitable magnetic inclination angle ($\alpha=40^\circ$) and viewing angle ($\zeta=75^\circ$), we managed to reproduce its pulse profiles of three wavebands. It is found that the middle radio peak is originated from the core gap region at high altitudes, and the other two radio peaks are originated from the annular gap region at relatively low altitudes. Two peaks of both X-ray and $\gamma$-ray wavebands are fundamentally originated from annular gap region, while the $\gamma$-ray emission generated from the core gap region contributes somewhat to the first $\gamma$-ray peak. Precisely reproducing the multi-wavelength pulse profiles of PSR B1821$-$24 enables us to understand emission regions of distinct wavebands and justify pulsar emission models.

Accelerating a water maser face-on jet from a high mass young stellar object

Abstract We report on long-term single-dish and VLBI monitoring for intermittent flare activities of a dominant blue-shifted H2O maser associated with a southern high mass young stellar object, G353.273+0.641. Bi-weekly single-dish monitoring using the Hokkaido University Tomakomai 11 m radio telescope has shown that a systematic acceleration continues over four years beyond the lifetime of individual maser features. This fact suggests that the H2O maser traces a region where molecular gas is steadily accelerated. There were five maser flares during the five years of monitoring, and maser distributions in four of them were densely monitored by VLBI Exploration of Radio Astrometry (VERA). The overall distribution of the maser features suggests the presence of a bipolar jet, with the 3D kinematics indicating that it is almost face-on (inclination angle of ∼ 8°–17° from the line of sight). Most maser features were recurrently excited within a region of 100×100 au2 around the radio continuum peak, while their spatial distributions significantly varied between each flare. This confirms that episodic propagations of outflow shocks recurrently invoke intermittent flare activities. We also measured annual parallax, deriving a source distance of 1.70$^{+0.19}_{-0.16}$ kpc that is consistent with the commonly used photometric distance.

A TWO-COMPONENT JET MODEL FOR THE TIDAL DISRUPTION EVENT SWIFT J164449.3+573451

We analyze both the early and late time radio and X-ray data of the tidal disruption event Swift J1644+57. The data at early times ($\lesssim 5$ days) necessitates separation of the radio and X-ray emission regions, either spatially or in velocity space. This leads us to suggest a two component jet model, in which the inner jet is initially relativistic with Lorentz factor $\Gamma \approx 15$, while the outer jet is trans-relativistic, with $\Gamma \lesssim 1.2$. This model enables a self-consistent interpretation of the late time radio data, both in terms of peak frequency and flux. We solve the dynamics, radiative cooling and expected radiation from both jet components. We show that while during the first month synchrotron emission from the outer jet dominates the radio emission, at later times radiation from ambient gas collected by the inner jet dominates. This provides a natural explanation to the observed re-brightening, without the need for late time inner engine activity. After 100 days, the radio emission peak is in the optically thick regime, leading to a decay of both the flux and peak frequency at later times. Our model's predictions for the evolution of radio emission in jetted tidal disruption events can be tested by future observations.

CONSTRAINTS ON THE EMISSION GEOMETRIES AND SPIN EVOLUTION OF GAMMA-RAY MILLISECOND PULSARS

Millisecond pulsars (MSPs) are a growing class of gamma-ray emitters. Pulsed gamma-ray signals have been detected from more than 40 MSPs with the Fermi Large Area Telescope (LAT). The wider radio beams and more compact magnetospheres of MSPs enable studies of emission geometries over a broader range of phase space than non-recycled radio-loud gamma-ray pulsars. We have modeled the gamma-ray light curves of 40 LAT-detected MSPs using geometric emission models assuming a vacuum retarded-dipole magnetic field. We modeled the radio profiles using a single-altitude hollow-cone beam, with a core component when indicated by polarimetry; however, for MSPs with gamma-ray and radio light curve peaks occurring at nearly the same rotational phase we assume that the radio emission is co-located with the gamma rays and caustic in nature. The best-fit parameters and confidence intervals are determined using a maximum likelihood technique. We divide the light curves into three model classes, with gamma-ray peaks trailing (Class I), aligned (Class II) or leading (Class III) the radio peaks. Outer gap and slot gap (two-pole caustic) models best fit roughly equal numbers of Class I and II, while Class III are exclusively fit with pair-starved polar cap models. Distinguishing between the model classes based on typical derived parameters is difficult. We explore the evolution of magnetic inclination angle with period and spin-down power, finding possible correlations. While the presence of significant off-peak emission can often be used as a discriminator between outer gap and slot gap models, a hybrid model may be needed.

Shocks in nova outflows – I. Thermal emission

Growing evidence for shocks in nova outflows includes (1) multiple velocity components in the optical spectra; (2) hard X-ray emission starting weeks to months after the outburst; (3) an early radio flare on time-scales of months, in excess of that predicted from the freely expanding photoionized gas; and, perhaps most dramatically, (4) ∼ GeV gamma-ray emission. We present a one-dimensional model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated thermal X-ray, optical, and radio emission. The lower velocity DES could represent an earlier stage of mass-loss from the white dwarf or ambient material not directly related to the thermonuclear runaway. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. Our predicted radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray producing classical nova V1324 Sco, obtaining an adequate fit to the early radio maximum assuming that the DES possesses a characteristic velocity ∼103 km s−1 and mass ∼ few 10−4 M⊙; the former is consistent with the velocities of narrow-line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferred independently by modelling the late radio peak as uniformly expanding photoionized gas. Importantly, however, our thermal model can only explain the peak radio fluxes if line cooling of the post-shock gas at temperatures ∼106 K is suppressed below its collisional ionization equilibrium value for solar abundances due to photoionization; if this condition is not satisfied, this strongly suggests that the early radio peak is instead non-thermal (e.g. synchrotron) in origin. Rapid evolution of the early radio light curves requires the DES to possess a steep outer density profile, which may indicate that the onset of mass-loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled as the result of the thermonuclear runaway event. Re-processed X-rays from the shock absorbed by the DES at early times are found to contribute significantly to the optical/UV emission.