ABSTRACT
Neutron star models with maximum mass close to 2 M⊙ reach high central densities, which may activate nucleonic and hyperon direct Urca neutrino emission. To alleviate the tension between ...fast theoretical cooling rates and thermal luminosity observations of moderately magnetized, isolated thermally emitting stars (with Lγ ≳ 1031 erg s−1 at t ≳ 105.3 yr), some internal heating source is required. The power supplied by the internal heater is estimated for both a phenomenological source in the inner crust and Joule heating due to magnetic field decay, assuming different superfluidity models and compositions of the outer stellar envelope. It is found that a thermal power of W(t) ≈ 1034 erg s−1 allows neutron star models to match observations of moderately magnetized, isolated stars with ages t ≳ 105.3 yr. The requisite W(t) can be supplied by Joule heating due to crust-confined initial magnetic configurations with (i) mixed poloidal–toroidal fields, with surface strength Bdip = 1013 G at the pole of the dipolar poloidal component and ∼90 per cent of the magnetic energy stored in the toroidal component; and (ii) poloidal-only configurations with Bdip = 1014 G.
The activity of magnetars is powered by their intense and dynamic magnetic fields and has been proposed as the trigger to extragalactic fast radio bursts. Here we estimate the frequency of crustal ...failures in young magnetars, by computing the magnetic stresses in detailed magnetothermal simulations including Hall drift and ohmic dissipation. The initial internal topology at birth is poorly known but is likely to be much more complex than a dipole. Thus, we explore a wide range of initial configurations, finding that the expected rate of crustal failures varies by orders of magnitude depending on the initial magnetic configuration. Our results show that this rate scales with the crustal magnetic energy, rather than with the often used surface value of the dipolar component related to the spin-down torque. The estimated frequency of crustal failures for a given dipolar component can vary by orders of magnitude for different initial conditions, depending on how much magnetic energy is distributed in the crustal nondipolar components, likely dominant in newborn magnetars. The quantitative reliability of the expected event rate could be improved by a better treatment of the magnetic evolution in the core and the elastic/plastic crustal response, not included here. Regardless of that, our results are useful inputs in modeling the outburst rate of young Galactic magnetars, and their relation with the fast radio bursts in our and other galaxies.
ABSTRACT
The dissipation of intense crustal electric currents produces high Joule heating rates in cooling neutron stars. Here, it is shown that Joule heating can counterbalance fast cooling, making ...it difficult to infer the presence of hyperons (which accelerate cooling) from measurements of the observed thermal luminosity Lγ. Models with and without hyperon cores match Lγ of young magnetars (with poloidal–dipolar field Bdip ≳ 1014 G at the polar surface and Lγ ≳ 1034 erg s−1 at t ≲ 105 yr) as well as mature, moderately magnetized stars (with Bdip ≲ 1014 G and 1031 erg s−1 ≲ Lγ ≲ 1032 erg s−1 at t ≳ 105 yr). In magnetars, the crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether the latter is suppressed or not by hyperon superfluidity. The thermal luminosities of light magnetars without hyperons and heavy magnetars with hyperons have Lγ in the same range and are almost indistinguishable. Likewise, Lγ data of neutron stars with Bdip ≲ 1014 G but with strong internal fields are not suitable to extract information about the equation of state as long as hyperons are superfluid, with maximum amplitude of the energy gaps of the order ≈1 MeV.
A Very Young Radio-loud Magnetar Esposito, P.; Rea, N.; Borghese, A. ...
Astrophysical journal. Letters,
06/2020, Volume:
896, Issue:
2
Journal Article
Peer reviewed
Open access
The magnetar Swift J1818.0-1607 was discovered in 2020 March when Swift detected a 9 ms hard X-ray burst and a long-lived outburst. Prompt X-ray observations revealed a spin period of 1.36 s, soon ...confirmed by the discovery of radio pulsations. We report here on the analysis of the Swift burst and follow-up X-ray and radio observations. The burst average luminosity was Lburst ∼ 2 × 1039 erg s−1 (at 4.8 kpc). Simultaneous observations with XMM-Newton and NuSTAR three days after the burst provided a source spectrum well fit by an absorbed blackbody ( = (1.13 0.03) × 1023 cm−2 and kT = 1.16 0.03 keV) plus a power law (Γ = 0.0 1.3) in the 1-20 keV band, with a luminosity of ∼8 × 1034 erg s−1, dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic field B ∼ 7 × 1014 G, spin-down luminosity erg s−1, and characteristic age of 240 yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity <5.5 × 1033 erg s−1, lower than the value expected from magnetar cooling models at the source characteristic age. A 1 hr radio observation with the Sardinia Radio Telescope taken about 1 week after the X-ray burst detected a number of strong and short radio pulses at 1.5 GHz, in addition to regular pulsed emission; they were emitted at an average rate 0.9 min−1 and accounted for ∼50% of the total pulsed radio fluence. We conclude that Swift J1818.0-1607 is a peculiar magnetar belonging to the small, diverse group of young neutron stars with properties straddling those of rotationally and magnetically powered pulsars. Future observations will make a better estimation of the age possible by measuring the spin-down rate in quiescence.
Abstract
The detection of a short hard X-ray burst and an associated bright soft X-ray source by the Swift satellite in 2020 October heralded a new magnetar in outburst, SGR J1830−0645. Pulsations at ...a period of ∼10.4 s were detected in prompt follow-up X-ray observations. We present here the analysis of the Swift/Burst Alert Telescope burst, of XMM-Newton and the Nuclear Spectroscopic Telescope Array observations performed at the outburst peak, and of a Swift/X-ray Telescope monitoring campaign over the subsequent month. The burst was single-peaked, lasted ∼6 ms, and released a fluence of ≈5 × 10
−9
erg cm
−2
(15–50 keV). The spectrum of the X-ray source at the outburst peak was well described by an absorbed double-blackbody model plus a power-law component detectable up to ∼25 keV. The unabsorbed X-ray flux decreased from ∼5 × 10
−11
to ∼2.5 × 10
−11
erg cm
−2
s
−1
one month later (0.3–10 keV). Based on our timing analysis, we estimate a dipolar magnetic field ≈5.5 × 10
14
G at pole, a spin-down luminosity ≈2.4 × 10
32
erg s
−1
, and a characteristic age ≈24 kyr. The spin modulation pattern appears highly pulsed in the soft X-ray band, and becomes smoother at higher energies. Several short X-ray bursts were detected during our campaign. No evidence for periodic or single-pulse emission was found at radio frequencies in observations performed with the Sardinia Radio Telescope and Parkes. According to magneto-thermal evolutionary models, the real age of SGR J1830−0645 is close to the characteristic age, and the dipolar magnetic field at birth was slightly larger, ∼10
15
G.
Abstract
Swift J1818.0−1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of
B
∼ 3 × 10
14
G, which is very young compared to the Galactic pulsar ...population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in 2020 March until 2021 October, as well as INTEGRAL upper limits on its hard X-ray emission. The 1–10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of
kT
BB
∼ 1.1 keV and apparent reduction in the radius of the emitting region from ∼0.6 to ∼0.2 km. We also confirm the bright diffuse X-ray emission around the source extending between ∼50″ and ∼110″. A timing analysis revealed large torque variability, with an average spin-down rate
ν
̇
∼
−2.3 × 10
−11
Hz
2
that appears to decrease in magnitude over time. We also observed Swift J1818.0−1607 with the Karl G. Jansky Very Large Array on 2021 March 22. We detected the radio counterpart to Swift J1818 measuring a flux density of
S
v
= 4.38 ± 0.05 mJy at 3 GHz and a half-ringlike structure of bright diffuse radio emission located at ∼90″ to the west of the magnetar. We tentatively suggest that the diffuse X-ray emission is due to a dust-scattering halo and that the radio structure may be associated with the supernova remnant of this young pulsar, based on its morphology.
Abstract
We observed the periodic radio transient GLEAM-X J162759.5-523504.3 (GLEAM-X J1627) using the Chandra X-ray Observatory for about 30 ks on 2022 January 22–23, simultaneously with radio ...observations from the Murchison Widefield Array, MeerKAT, and the Australia Telescope Compact Array. Its radio emission and 18 min periodicity led the source to be tentatively interpreted as an extreme magnetar or a peculiar highly magnetic white dwarf. The source was not detected in the 0.3–8 keV energy range with a 3
σ
upper limit on the count rate of 3 × 10
−4
counts s
−1
. No radio emission was detected during our X-ray observations either. Furthermore, we studied the field around GLEAM-X J1627 using archival European Southern Observatory and DECam Plane Survey data, as well as recent Southern African Large Telescope observations. Many sources are present close to the position of GLEAM-X J1627, but only two within the 2″ radio position uncertainty. Depending on the assumed spectral distribution, the upper limits converted to an X-ray luminosity of
L
X
< 6.5 × 10
29
erg s
−1
for a blackbody with temperature
kT
= 0.3 keV, or
L
X
< 9 × 10
29
erg s
−1
for a power law with photon index Γ = 2 (assuming a 1.3 kpc distance). Furthermore, we performed magneto-thermal simulations for neutron stars considering crust- and core-dominated field configurations. Based on our multiband limits, we conclude that (i) in the magnetar scenario, the X-ray upper limits suggest that GLEAM-X J1627 should be older than ∼1 Myr, unless it has a core-dominated magnetic field or has experienced fast cooling; (ii) in the white dwarf scenario, we can rule out most binary systems, a hot sub-dwarf, and a hot magnetic isolated white dwarf (
T
≳ 10.000 K), while a cold isolated white dwarf is still compatible with our limits.
Abstract
Fusion cross sections of the
28
Si +
100
Mo system have been measured near and below the Coulomb barrier by detecting the evaporation residues at forward angles. The excitation function has ...an overall smoother trend than what obtained in a previous experiment, and a large discrepancy is found for the lowest-energy region, where we observe a tendency of the
S
factor to develop a maximum, which would be a clear indication of hindrance. The results have been compared with the theoretical prediction of coupled-channels calculations using a Woods–Saxon nuclear potential, and including the low-energy excitation modes of both nuclei. Good agreement with data is found by including, in the coupling scheme, the three lowest members of the ground state rotational band of the oblate deformed
28
Si, and two-phonons of the strong quadrupole vibration of
100
Mo. The additional coupling, in a schematic way, of the two-neutron pick-up between ground states (
Q
-value = +4.86 MeV) has a minor effect on calculated cross sections, and does not essentially improve the data fit. The excitation function of
28
Si +
100
Mo has been compared with that of (1) the heavier system
60
Ni +
100
Mo having analogous features, and (2) several near-by
28
Si,
32
S + Zr, Mo systems with various nuclear structures and transfer
Q
-values. The role of quadrupole and octupole excitation modes, as well as of transfer channels, in affecting the fusion dynamics, are clarified to some extent. Systematic measurements of fusion barrier distributions and CC calculations properly including transfer couplings, are necessary, in order to shed full light on the influence of the various coupled channels on the fusion cross sections.
The dissipation of intense crustal electric currents produces high Joule heating rates in cooling neutron stars. Here it is shown that Joule heating can counterbalance fast cooling, making it ...difficult to infer the presence of hyperons (which accelerate cooling) from measurements of the observed thermal luminosity \(L_\gamma\). Models with and without hyperon cores match \(L_{\gamma}\) of young magnetars (with poloidal-dipolar field \(B_{\textrm{dip}} \gtrsim 10^{14}\) G at the polar surface and \(L_{\gamma} \gtrsim 10^{34}\) erg s\(^{-1}\) at \(t \lesssim 10^5\) yr) as well as mature, moderately magnetized stars (with \(B_{\textrm{dip}} \lesssim 10^{14}\) G and \(10^{31} \ \textrm{erg s}^{-1} \lesssim L_{\gamma} \lesssim 10^{32}\) erg s\(^{-1}\) at \(t \gtrsim 10^5\) yr). In magnetars, the crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether the latter is suppressed or not by hyperon superfluidity. The thermal luminosities of light magnetars without hyperons and heavy magnetars with hyperons have \(L_{\gamma}\) in the same range and are almost indistinguishable. Likewise, \(L_{\gamma}\) data of neutron stars with \(B_{\textrm{dip}} \lesssim 10^{14}\) G but with strong internal fields are not suitable to extract information about the equation of state as long as hyperons are superfluid, with maximum amplitude of the energy gaps of the order \(\approx 1\) MeV.
Neutron star models with maximum mass close to \(2 \ M_{\odot}\) reach high central densities, which may activate nucleonic and hyperon direct Urca neutrino emission. To alleviate the tension between ...fast theoretical cooling rates and thermal luminosity observations of moderately magnetized, isolated thermally-emitting stars (with \(L_{\gamma} \gtrsim 10^{31}\) erg s\(^{-1}\) at \(t \gtrsim 10^{5.3}\) yr), some internal heating source is required. The power supplied by the internal heater is estimated for both a phenomenological source in the inner crust and Joule heating due to magnetic field decay, assuming different superfluidity models and compositions of the outer stellar envelope. It is found that a thermal power of \(W(t) \approx 10^{34}\) erg s\(^{-1}\) allows neutron star models to match observations of moderately magnetized, isolated stars with ages \(t \gtrsim 10^{5.3}\) yr. The requisite \(W(t)\) can be supplied by Joule heating due to crust-confined initial magnetic configurations with (i) mixed poloidal-toroidal fields, with surface strength \(B_{\textrm{dip}} = 10^{13}\) G at the pole of the dipolar poloidal component and \(\sim 90\) per cent of the magnetic energy stored in the toroidal component; and (ii) poloidal-only configurations with \(B_{\textrm{dip}} = 10^{14}\) G.
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