arXiv subjects

M. Hernanz

Publications and source records attributed to M. Hernanz.

At least 73 records · Page 4Linked to original sources

arXiv2008

Detectability of gamma-ray emission from classical novae with Swift/BAT

Classical novae are expected to emit gamma rays during their explosions. The most important contribution to the early gamma-ray emission comes from the annihilation with electrons of the positrons generated by the decay of 13N and 18F. The photons are expected to be down-scattered to a few tens of keV, and the emission is predicted to occur some days before the visual discovery and to last ~2 days. Despite a number of attempts, no positive detections of such emission have been made, due to lack of sensitivity and of sky coverage. Because of its huge field of view, good sensitivity, and well-adapted energy band, Swift/BAT offers a new opportunity for such searches. BAT data can be retrospectively used to search for prompt gamma-ray emission from the direction of novae after their optical discovery. We have estimated the expected success rate for the detection with BAT of gamma rays from classical novae using a Monte Carlo approach. Searches were performed for emission from novae occurring since the launch of Swift. Using the actual observing program during the first 2.3 years of BAT operations as an example, and sensitivity achieved, we estimate the expected rate of detection of classical novae with BAT as ~0.2-0.5/yr, implying that several should be seen within a 10 yr mission. The search for emission in the directions of the 24 classical novae discovered since the Swift launch yielded no positive results, but none of these was known to be close enough for this to be a surprise. Detections of a recurrent nova (RS Oph) and a nearby dwarf nova (V455 And) demonstrate the efficacy of the technique. The absence of detections is consistent with the expectations from the Monte Carlo simulations, but the long-term prospects are encouraging given an anticipated Swift operating lifetime of ~10 years.

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arXiv2008

Evidence for Nonlinear Diffusive Shock Acceleration of Cosmic Rays in the 2006 Outburst of RS Ophiuchi

Spectroscopic observations of the 2006 outburst of RS Oph at both infrared (IR) and X-ray wavelengths have shown that the blast wave has decelerated at a higher rate than predicted by the standard test-particle adiabatic shock-wave model. The observed blast-wave evolution can be explained, however, by the diffusive shock acceleration of particles at the forward shock and the subsequent escape of the highest energy ions from the acceleration region. Nonlinear particle acceleration can also account for the difference of shock velocities deduced from the IR and X-ray data. We discuss the evolution of the nova remnant in the light of efficient particle acceleration at the blast wave.

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arXiv2007

Evidence for nonlinear diffusive shock acceleration of cosmic-rays in the 2006 outburst of the recurrent nova RS Ophiuchi

Spectroscopic observations of the 2006 outburst of the recurrent nova RS Ophiuchi at both infrared (IR) and X-ray wavelengths have shown that the blast wave has decelerated at a higher rate than predicted by the standard test-particle adiabatic shock-wave model. Here we show that the observed evolution of the nova remnant can be explained by the diffusive shock acceleration of particles at the blast wave and the subsequent escape of the highest energy ions from the shock region. Nonlinear particle acceleration can also account for the difference of shock velocities deduced from the IR and X-ray data. The maximum energy that accelerated electrons and protons can have achieved in few days after outburst is found to be as high as a few TeV. Using the semi-analytic model of nonlinear diffusive shock acceleration developed by Berezhko & Ellison, we show that the postshock temperature of the shocked gas measured with RXTE/PCA and Swift/XRT imply a relatively moderate acceleration efficiency.

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arXiv2007

XMM-Newton observations of Nova Sgr 1998

We report on X-ray observations of Nova Sagittarius 1998 (V4633 Sgr), performed with XMM-Newton at three different epochs, 934, 1083 and 1265 days after discovery. The nova was detected with the EPIC cameras at all three epochs, with emission spanning the whole energy range from 0.2 to 10 keV. The X-ray spectra do not change significantly at the different epochs, and are well fitted for the first and third observations with a multi-temperature optically thin thermal plasma, while lower statistics in the second observations lead to a poorer fit. The thermal plasma emission is most probably originated in the shock heated ejecta, with chemical composition similar to that of a CO nova. However, we can not completely rule out reestablished accretion as the origin of the emission. We also obtain upper limits for the temperature and luminosity of a potential white dwarf atmospheric component, and conclude that hydrogen burning had already turned-off by the time of our observations.

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arXiv2005

Hydrogen Burning of 17-O in Classical Novae

We report on the observation of a previously unknown resonance at E=194.1+/-0.6 keV (lab) in the 17-O(p,alpha)14-N reaction, with a measured resonance strength omega_gamma(p,alpha)=1.6+/-0.2 meV. We studied in the same experiment the 17-O(p,gamma)18-F reaction by an activation method and the resonance-strength ratio was found to be omega_gamma(p,alpha)/omega_gamma(p,gamma)=470+/-50. The corresponding excitation energy in the 18-F compound nucleus was determined to be 5789.8+/-0.3 keV by gamma-ray measurements using the 14-N(alpha,gamma)18-F reaction. These new resonance properties have important consequences for 17-O nucleosynthesis and gamma-ray astronomy of classical novae.

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arXiv2003

Proton-decaying states in 22Mg and the nucleosynthesis of 22Na in novae

Populating states in 22Mg via the (p,t) reaction in inverse kinematics with a 55 MeV/nucleon 24Mg beam, we have measured the proton-decay branching ratios of the levels at 5.96 MeV and 6.05 MeV and obtained an experimental upper limit on the branching ratio of the 5.71 MeV state. On the basis of the present and previous measurements, we assign spins and parities to the 5.96 MeV and 6.05 MeV states. We combine our branching ratios with independent measurements of the lifetimes of these states or their 22Ne analogs to compute the resonance strengths and thereby the astrophysical rate of the 21Na(p,g)22Mg reaction. We perform hydrodynamic calculations of nova outbursts with this new rate and analyze its impact on 22Na yields.

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arXiv2003

A Chandra LETGS observation of V4743 Sagittarius: A Super Soft X-ray Source and a Violently Variable Light Curve

V4743 Sgr (Nova Sgr 2002 No. 3) was discovered on 20 September 2002. We obtained a 5ks ACIS-S spectrum in November 2002 and found that the nova was faint in X-rays. We then obtained a 25ks CHANDRA LETGS observation on 19 March 2003. By this time, it had evolved into the Super Soft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large amplitude oscillations with a period of 1325s (22min) followed by a decline in total count rate after ~13ks of observations. The count rate dropped from ~40cts/s to practically zero within ~6ks and stayed low for the rest of the observation (~6ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations, and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1-2MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400km/s. The spectrum obtained after the decline in count rate showed emission lines of CVI, NVI, and NVII suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton ToO observation, obtained on 4 April 2003 and a later LETGS observation from 18 July 2003 also showed oscillations, but with smaller amplitudes.

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arXiv2003

The frequency of occurrence of novae hosting an ONe white dwarf

In this paper, we revisit the problem of the determination of the frequency of occurrence of galactic nova outbursts which involve an oxygen-neon (ONe) white dwarf. The improvement with respect to previous work on the subject derives from the fact that we use the results that our evolutionary calculations provide for the final mass and for the chemical profiles of intermediate-to-massive primary components of close binary systems. In particular, the final evolutionary stages, such as the carbon burning phase, have been carefully followed for the whole range of masses of interest. The chemical profiles obtained with our evolutionary code are of interest in determining the chemical composition of the ejecta after being processed through the thermonuclear runaway, although such other factors as the efficiency of the mixing between the accreted material and that of the underlying white dwarf must also be considered. In our calculations of the frequency of occurrence of nova outbursts involving an ONe white dwarf, we also take into account the observational selection effects introduced by the different recurrence times of the outbursts and by the spatial distribution of novae. In spite of the very different evolutionary sequences, we find that approximately 1/3 of the novae observed in outburst should involve an oxygen-neon white dwarf, in agreement with previous theoretical estimates.

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arXiv2003

The astrophysical rate of 15O(alpha,gamma)19Ne via the (p,t) reaction in inverse kinematics

A recoil coincidence technique has been applied to measure the alpha-decay branching ratios of near-threshold states in 19Ne. Populating these states using the (p,t) reaction in inverse kinematics, we detected the recoils and their decay products with 100% geometric efficiency using a magnetic spectrometer. Combining our branching ratio measurements with independent determinations of the radiative widths of these states, we calculate the astrophysical rate of 15O(alpha,gamma)19Ne. Using this reaction rate, we perform hydrodynamic calculations of nova outbursts and conclude that no significant breakout from the hot CNO cycles into the rp process occurs in novae via 15O(alpha,gamma)19Ne.

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arXiv2003

The 21Na(p,gamma)22Mg Reaction and Oxygen-Neon Novae

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in Oxygen-Neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E$_{c.m.}$= 205.7 $\pm$ 0.5 keV with a resonance strength $ωγ= 1.03\pm0.16_{stat}\pm0.14_{sys}$ meV.

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arXiv2002

The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 #2) in Outburst: The Discovery of Pulsations and a "Burst"

V1494 Aql (Nova Aql 1999 No. 2) was discovered on 2 December 1999. We obtained Chandra ACIS-I spectra on 15 April and 7 June 2000 which appear to show only emission lines. Our third observation, on 6 August, showed that its spectrum had evolved to that characteristic of a Super Soft X-ray Source. We then obtained Chandra LETG+HRC-S spectra on 28 September (8 ksec) and 1 October (17 ksec). We analyzed the X-ray light curve of our grating observations and found both a short time scale ``burst'' and oscillations. Neither of these phenomena have previously been seen in the light curve of a nova in outburst. The ``burst'' was a factor of 10 rise in X-ray counts near the middle of the second observation, and which lasted about 1000 sec; it exhibited at least two peaks, in addition to other structure. Our time series analysis of the combined 25 ksec observation shows a peak at 2500 s which is present in independent analyses of both the zeroth order image and the dispersed spectrum and is not present in similar analyses of grating data for HZ 43 and Sirius B. Further analyses of the V1494 Aql data find other periods present which implies that we are observing non-radial g+ modes from the pulsating, rekindled white dwarf.

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arXiv2002

WSO/UV: World Space Observatory/Ultraviolet

We summarize the capabilities of the World Space Observatory (UV) Project (WSO/UV). An example of the importance of this project (with a planned launch date of 2007/8) for the study of Classical Novae is given.

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arXiv2001

The prompt gamma-ray emission of novae

Classical novae are potential gamma-ray emitters, because of the disintegration of some radioactive nuclei synthesized during the explosion. Some short-lived isotopes (such as 13N and 18F), as well as the medium-lived 22Na, decay emitting positrons, which annihilate with electrons and thus are responsible for the prompt emission of gamma-rays from novae. This emission consists of a 511 keV line plus a continuum between 20 and 511 keV, and is released before the maximum in visual luminosity, i.e., before the discovery of the nova. The main characteristics of this prompt emission, together with the related uncertainties (both of nuclear and hydrodynamical origin, with a particular emphasis on the influence of the envelope properties) and prospects for detectability are analyzed in this paper.

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arXiv2001

Gamma-ray signatures of classical novae

The role of classical novae as potential gamma-ray emitters is reviewed, on the basis of theoretical models of the gamma-ray emission from different nova types. The interpretation of the up to now negative results of the gamma-ray observations of novae, as well as the prospects for detectability with future instruments (specially onboard INTEGRAL) are also discussed.

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arXiv2001

Upper limits of the $^{22}$Na yield from O-Ne nova

The radioactive isotope $^{22}$Na is one of the primary tracer isotopes that may potentially be detectable by gamma-ray spectroscopy. This isotope is predicted to be produced in ONe nova explosions, yet no detection of its 1.275 MeV gamma-ray signature has been reported so far. In particular, 6 years of COMPTEL observations of the galactic bulge, a region that should be particularily rich in nova explosions, have only led to an upper-limit. In this paper we will present the implications of the COMPTEL upper flux limit on the permissible $^{22}$Na yield for ONe novae. We base our analysis on extensive Monte-Carlo simulations of galactic 1.275 MeV emission that were constrained by the most recent results of galactic nova rates and distributions. We demonstrate that the non-detection of the 1.275 MeV line from the galactic bulge implies a solid upper $^{22}$Na yield limit in agreement with current theoretical nucleosynthesis calculations.

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arXiv2001

Explosive nucleosynthesis

Many radioactive nuclei relevant for gamma-ray astrophysics are synthesized during explosive events, such as classical novae and supernovae. A review of recent results of explosive nucleosynthesis in these scenarios is presented, with a special emphasis on the ensuing gamma-ray emission from individual nova and supernova explosions. The influence of the dynamic properties of the ejecta on the gamma-ray emission features, as well as the still remaining uncertainties in nova and supernova models is also reviewed.

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arXiv2001

Prospects for detectability of classical novae with INTEGRAL

Classical novae are potential gamma-ray emitters, both in lines and in a continuum. Continuum emission (at energies between 20-30 and 511 keV) and line emission at 511 keV are related to positron annihilation and its Comptonization in the expanding shell; 18F is the main responsible of positron production. The lines at 478 and 1275 keV have their origin in the decay of the radioactive nuclei 7Be and 22Na. Updated models of nova explosions have been adopted for the computation of the gamma-ray emission. New yields of some radioactive isotopes directly translate into new detectability distances of classical novae with INTEGRAL.

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arXiv2001

Nucleosynthesis in classical novae

A general review of the relevance of classical novae for the chemical evolution of the Galaxy, as well as for Galactic radioactivity is presented. A special emphasis is put on the pioneering work done by Jim Truran in this field of research. The impact of recent developments in nuclear astrophysics on nova nucleosynthesis, together with the prospects for observability of novae radioactivities with the INTEGRAL satellite are discussed.

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