Refined Neutron-Star Mass Determinations for Six Eclipsing X-Ray Pulsar Binaries

We present an improved method for determining the mass of neutron stars in eclipsing X-ray pulsar binaries and apply the method to six systems, namely Vela X-1, 4U 1538-52, SMC X-1, LMC X-4, Cen X-3, and Her X-1. In previous studies to determine neutron star mass, the X-ray eclipse duration has been approximated analytically by assuming the companion star is spherical with an effective Roche lobe radius. We use a numerical code based on Roche geometry with various optimizers to analyze the published data for these systems, which we supplement with new spectroscopic and photometric data for 4U 1538-52. This allows us to model the eclipse duration more accurately and thus calculate an improved value for the neutron star mass. The derived neutron star mass also depends on the assumed Roche lobe filling factor beta of the companion star, where beta = 1 indicates a completely filled Roche lobe. In previous work a range of beta between 0.9 and 1.0 was usually adopted. We use optical ellipsoidal lightcurve data to constrain beta. We find neutron star masses of 1.77 +/- 0.08 M_{sun} for Vela X-1, 0.87 +/- 0.07 M_{sun} for 4U 1538-52 (eccentric orbit), 1.00 +/- 0.10 M_{sun} for 4U 1538-52 (circular orbit), 1.04 +/- 0.09 M_{sun} for SMC X-1, 1.29 +/- 0.05 M_{sun} for LMC X-4, 1.49 +/- 0.08 M_{sun} for Cen X-3, and 1.07 +/- 0.36 M_{sun} for Her X-1. We discuss the limits of the approximations that were used to derive the earlier mass determinations, and we comment on the implications our new masses have for observationally refining the upper and lower bounds of the neutron star mass distribution.Comment: 10 figures, accepted for publication in The Astrophysical Journa

A Prediction Formula of Supersoft X-ray Phase of Classical Novae

On the basis of the recently developed universal decline law of classical novae, we propose prediction formulae for supersoft X-ray on and off times, i.e., t_{X-on} = (10 \pm 1.8) t_3 days and t_{X-off} = (5.3 \pm 1.4) (t_3)^{1.5} days for 8 < t_3 < 80 days. We have determined the absolute magnitude of our free-free emission model light curves and derived maximum magnitude vs. rate of decline (MMRD) relations. Our theoretical MMRD relations are governed by two parameters, one is the white dwarf (WD) mass and the other is the initial envelope mass at a nova outburst; this second parameter explains the scatter of MMRD points of individual novae. Our theoretical MMRD relations are also in good agreement with the well-known empirical formulae. We also show another empirical relation of M_V(15) ~ -5.7 \pm 0.3 based on the absolute magnitude of our model light curves, i.e., the absolute magnitude at 15 days after optical maximum is almost common among various novae. We analyzed ten nova light curves, in which a supersoft X-ray phase was detected, and estimated their WD masses. The models best reproducing simultaneously the optical and supersoft X-ray observations are ONeMg WDs with 1.28 \pm 0.04 M_\sun (V598 Pup), 1.23 \pm 0.05 M_\sun (V382 Vel), 1.15 \pm 0.06 M_\sun (V4743 Sgr), 1.13 \pm 0.06 M_\sun (V1281 Sco), 1.2 \pm 0.05 M_\sun (V597 Pup), 1.06 \pm 0.07 M_\sun (V1494 Aql), 1.04 \pm 0.07 M_\sun (V2467 Cyg), 1.07 \pm 0.07 M_\sun (V5116 Sgr), 1.05 \pm 0.05 M_\sun (V574 Pup), and a CO WD with 0.93 \pm 0.08 M_\sun (V458 Vul). The newly proposed relationships are consistent with the emergence or decay epoch of the supersoft X-ray phase of these ten novae. Finally, we discuss the mechanism of shock-origin hard X-ray component in relation to the emergence of companion star from the WD envelope.Comment: 36 pages, 29 figures, to appear in the Astrophysical Journa

On some universal sums of generalized polygonal numbers

For $m=3,4,\ldots$ those $p_m(x)=(m-2)x(x-1)/2+x$ with $x\in\mathbb Z$ are called generalized $m$-gonal numbers. Sun [13] studied for what values of positive integers $a,b,c$ the sum $ap_5+bp_5+cp_5$ is universal over $\mathbb Z$ (i.e., any $n\in\mathbb N=\{0,1,2,\ldots\}$ has the form $ap_5(x)+bp_5(y)+cp_5(z)$ with $x,y,z\in\mathbb Z$). We prove that $p_5+bp_5+3p_5\,(b=1,2,3,4,9)$ and $p_5+2p_5+6p_5$ are universal over $\mathbb Z$, as conjectured by Sun. Sun also conjectured that any $n\in\mathbb N$ can be written as $p_3(x)+p_5(y)+p_{11}(z)$ and $3p_3(x)+p_5(y)+p_7(z)$ with $x,y,z\in\mathbb N$; in contrast, we show that $p_3+p_5+p_{11}$ and $3p_3+p_5+p_7$ are universal over $\mathbb Z$. Our proofs are essentially elementary and hence suitable for general readers.Comment: Final published versio

Multiwavelength modelling the SED of supersoft X-ray sources III. RS Ophiuchi: The supersoft X-ray phase and beyond

I modelled the 14 \AA - 37 $\mu$m SED of the recurrent symbiotic nova RS Oph during its supersoft source (SSS) phase and the following quiescent phase. During the SSS phase, the model SEDs revealed the presence of a strong stellar and nebular component of radiation in the spectrum. The former was emitted by the burning WD at highly super-Eddington rate, while the latter represented a fraction of its radiation reprocessed by the thermal nebula. During the transition phase, both the components were decreasing and during quiescence the SED satisfied radiation produced by a large, optically thick disk (R(disk) > 10 R(Sun)). The mass of the emitting material was (1.6 +/- 0.5) x 1E-4(d/1.6 kpc)**(5/2) M(Sun). The helium ash, deposited on the WD surface during the whole burning period, was around of 8 x 1E-6(d/1.6kpc)**2 M(Sun), which yields an average growing rate of the WD mass, dM(WD)/dt ~ 4 x 1E-7(d/1.6 kpc)**2 M(Sun)/yr. The mass accreted by the WD between outbursts, m(acc) ~ 1.26 x 1E-5 M(Sun), constrains the average accretion rate, dM(acc)/dt ~ 6.3 x 1E-7 M(Sun)/yr. If the wind from the giant is not sufficient to feed the WD at the required rate, the accretion can be realized from the disk-like reservoir of material around the WD. In this case the time between outbursts will extend, with the next explosion beyond 2027. In the opposite case, the wind from the giant has to be focused to the orbital plane to sustain the high accretion rate at a few times 1E-7 M(Sun)/yr. Then the next explosion can occur even prior to 2027.Comment: 14 pages, 6 figures, 2 tables, accepted by New Astron. on May 20, 2014; follow-up of arXiv:1402.612

Observational evidence for gravitationally trapped massive axion(-like) particles

Unexpected astrophysical observations can be explained by gravitationally captured massive particles, which are produced inside the Sun or other Stars and are accumulated over cosmic times. Their radiative decay in solar outer space would give rise to a `self-irradiation' of the whole star, providing the time-independent component of the corona heating source. In analogy with the Sun-irradiated Earth atmosphere, the temperature and density gradient in the corona - chromosphere transition region is suggestive for an omnipresent irradiation of the Sun. The same scenario fits other astrophysical X-ray observations. The radiative decay of a population of such elusive particles mimics a hot gas. X-ray observatories, with an unrivalled sensitivity below ~10 keV, can search for such particles. The elongation angle relative to the Sun is the relevant new parameter.Comment: 35 pages, LaTeX, 9 figures. Accepted by Astroparticle Physic

Solar X-rays from Axions: Rest-Mass Dependent Signatures

The spectral shape of solar X-rays is a power law. The more active the Sun is, the less steep the distribution. This behaviour can be explained by axion regeneration to X-rays occurring ~400km deep into the photosphere. Their down-comptonization reproduces the measured spectral shape, pointing at axions with rest mass m_a~17 meV/c2, without contradicting astrophysical-laboratory limits. Directly measured soft X-ray spectra from the extremely quiet Sun during 2009 (SphinX mission), though hitherto overlooked, fitt the axion scenario.Comment: To appear in Proceedings of the 5th Patras Axion Workshop, Durham 200