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Register a new userHard X-ray emission lines from the decay of Ti-44 in the remnant of supernova 1987A
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It is assumed that the radioactive decay of Ti-44 powers the infrared, optical and UV emission of supernova remnants after the complete decay of Co-56 and Co-57 (the isotopes that dominated the energy balance during the first three to four years after the explosion) until the beginning of active interaction of the ejecta with the surrounding matter. Simulations show that the initial mass of Ti-44 synthesized in core-collapse supernovae is (0.02-2.5) x 10^{-4} solar masses (M_sun). Hard X-rays and gamma-rays from the decay of this Ti-44 have been unambiguously observed from Cassiopeia A only, leading to the suggestion that the values of the initial mass of Ti-44 near the upper bound of the predictions occur only in exceptional cases. For the remnant of supernova 1987A, an upper limit to the initial mass of Ti-44 of < 10^{-3} M_sun has been obtained from direct X-ray observations, and an estimate of (1-2) x 10^{-4} M_sun has been made from infrared light curves and ultraviolet spectra by complex model-dependent computations. Here we report observations of hard X-rays from the remnant of supernova 1987A in the narrow band containing two direct-escape lines of Ti-44 at 67.9 and 78.4 keV. The measured line fluxes imply that this decay provided sufficient energy to power the remnant at late times. We estimate that the initial mass of Ti-44 was (3.1+/-0.8) x 10^{-4} M_sun, which is near the upper bound of theoretical predictions.
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We report Swift/BAT survey observations of the Tychos supernova remnant, performed over a period of 104 months since the missions launch. The remnant is detected with high significance (>10 sigma) below 50 keV. We detect significant hard X-ray emission in the 60-85 keV band, above the continuum level predicted by a simple synchrotron model. The location of the observed excess is consistent with line emission from radioactive Titanium-44, so far reported only for Type II supernova explosions. We discuss the implications of these results in the context of the galactic supernova rate, and nucleosynthesis in Type Ia supernova.
Supernova 1987A offers a unique opportunity to study an evolving supernova in unprecedented detail over several decades. The X-ray emission is dominated by interactions between the ejecta and the circumstellar medium, primarily the equatorial ring (ER). We analyze 3.3 Ms of NuSTAR data obtained between 2012 and 2020, and two decades of XMM-Newton data. Since ${sim}$2013, the flux below 2 keV has declined, the 3-8 keV flux has increased, but has started to flatten, and the emission above 10 keV has remained nearly constant. The spectra are well described by a model with three thermal shock components. Two components at 0.3 and 0.9 keV are associated with dense clumps in the ER, and a 4 keV component may be a combination of emission from diffuse gas in the ER and the surrounding low-density H II region. We disfavor models that involve non-thermal X-ray emission and place constraints on non-thermal components, but cannot firmly exclude an underlying power law. Radioactive lines show a $^{44}$Ti redshift of $670^{+520}_{-380}$ km s$^{-1}$, $^{44}$Ti mass of $1.73_{-0.29}^{+0.27}times{}10^{-4}$ solar masses, and $^{55}$Fe mass of $<4.2times{}10^{-4}$ solar masses. The 35-65 keV luminosity limit on the compact object is $2times{}10^{34}$ erg s$^{-1}$, and $<15$% of the 10-20 keV flux is pulsed. Considering previous limits, we conclude that there are currently no indications of a compact object, aside from a possible hint of dust heated by a neutron star in recent ALMA images.
Since the day of its explosion, SN 1987A (SN87A) was closely monitored with the aim to study its evolution and to detect its central compact relic. The detection of neutrinos from the supernova strongly supports the formation of a neutron star (NS). However, the constant and fruitless search for this object has led to different hypotheses on its nature. Up to date, the detection in the ALMA data of a feature somehow compatible with the emission arising from a proto Pulsar Wind Nebula (PWN) is the only hint of the existence of such elusive compact object. Here we tackle this 33-years old issue by analyzing archived observations of SN87A performed Chandra and NuSTAR in different years. We firmly detect nonthermal emission in the $10-20$ kev energy band, due to synchrotron radiation. The possible physical mechanism powering such emission is twofold: diffusive shock acceleration (DSA) or emission arising from an absorbed PWN. By relating a state-of-the-art magneto-hydrodynamic simulation of SN87A to the actual data, we reconstruct the absorption pattern of the PWN embedded in the remnant and surrounded by cold ejecta. We found that, even though the DSA scenario cannot be firmly excluded, the most likely scenario that well explains the data is the PWN emission.
NuSTAR observed G1.9+0.3, the youngest known supernova remnant in the Milky Way, for 350 ks and detected emission up to $sim$30 keV. The remnants X-ray morphology does not change significantly across the energy range from 3 to 20 keV. A combined fit between NuSTAR and CHANDRA shows that the spectrum steepens with energy. The spectral shape can be well fitted with synchrotron emission from a power-law electron energy distribution with an exponential cutoff with no additional features. It can also be described by a purely phenomenological model such as a broken power-law or a power-law with an exponential cutoff, though these descriptions lack physical motivation. Using a fixed radio flux at 1 GHz of 1.17 Jy for the synchrotron model, we get a column density of N$_{rm H}$ = $(7.23pm0.07) times 10^{22}$ cm$^{-2}$, a spectral index of $alpha=0.633pm0.003$, and a roll-off frequency of $ u_{rm rolloff}=(3.07pm0.18) times 10^{17}$ Hz. This can be explained by particle acceleration, to a maximum energy set by the finite remnant age, in a magnetic field of about 10 $mu$G, for which our roll-off implies a maximum energy of about 100 TeV for both electrons and ions. Much higher magnetic-field strengths would produce an electron spectrum that was cut off by radiative losses, giving a much higher roll-off frequency that is independent of magnetic-field strength. In this case, ions could be accelerated to much higher energies. A search for $^{44}$Ti emission in the 67.9 keV line results in an upper limit of $1.5 times 10^{-5}$ $,mathrm{ph},mathrm{cm}^{-2},mathrm{s}^{-1}$ assuming a line width of 4.0 keV (1 sigma).
We present the results of a spectral analysis of the central region of the mixed-morphology supernova remnant HB 9. A prior Ginga observation of this source detected a hard X-ray component above 4 keV and the origin of this particular X-ray component is still unknown. Our results demonstrate that the extracted X-ray spectra are best represented by a model consisting of a collisional ionization equilibrium plasma with a temperature of ~0.1-0.2 keV (interstellar matter component) and an ionizing plasma with a temperature of ~0.6-0.7 keV and an ionization timescale of >1 x 10^{11} cm^{-3} s (ejecta component). No significant X-ray emission was found in the central region above 4 keV. The recombining plasma model reported by a previous work does not explain our spectra.
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