Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in recent years. They are understood as neutron stars (NSs) with super-strong magnetic fields, namely B ≳ 10¹⁴ G. However, in the last two years two SGRs with low magnetic fields B ∼ (10¹²–10¹³) G have been detected. Moreover, three fast and very magnetic white dwarfs (WDs) have also been observed in recent years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole moment, characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with the three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. We show that the values for m obtained for several SGRs and AXPs are in agreement with the observed range 10³⁴ emu ≤ m ≤ 10³⁶ emu of isolated and polar magnetic white dwarfs. This result, together with the fact that for magnetic white dwarfs B ∼ (10⁶–10⁸) G their magnetic dipole moments are almost independent of the star rotation period (10⁴ s ≲ P ≲ 10⁶ s)—a phenomenology not shared by neutron star pulsars—suggests a possible magnetic white dwarf nature for some SGRs/AXPs that have much smaller periods (P ∼ 10 s). Moreover, since for pulsars the dipole radiation power is proportional only to m and to the rotational star frequency, we can explain in the WD model—considering only the different scales of the magnetic dipole moment for WDs and NSs—why the steady luminosity L_X for several SGRs/AXPs (and in particular the low-B SGRs) compared to those of X-ray dim isolated neutron stars (XDINs) and high-B pulsars obeys the ratio L_X^SGRs/AXPs/L_X^XDINs ∼ m_WD/m_NS ∼ 10³: all these X-ray sources have essentially the same rotational periods (P ∼ 10 s) and the X-ray luminosity is correlated to the spin-down luminosity, which is equal to the dipole radiation power in the dipole model.

Neutron stars (NSs) are excellent natural laboratories to constrain gravity on strong field regime and nuclear matter in extreme conditions. Motivated by the recent discovery of a compact object with $$2.59^{+0.08}_{-0.09} M_\odot $$2 . 59 - 0.09 + 0.08 M ⊙ in the binary merger GW190814, if this object was a NS, it serves as a strong constraint on the NS equation of state (EoS), ruling out several soft EoSs favored by GW170817 event. In this work, we revisit the question of the maximum mass of NSs considering a chameleon screening (thin-shell effect) on the NS mass-radius relation, where the microscopic physics inside the NS is given by realistic soft EoSs. We find that from appropriate and reasonable combination of modified gravity, rotation effects and realistic soft EoSs, that it is possible to achieve high masses and explain GW190814 secondary component, and in return also NSs like PSR J0740 + 6620 (the most NS massive confirmed to date). It is shown that gravity can play an important role in estimating maximum mass of NSs, and even with soft EoSs, it is possible to generate very high masses. Therefore, in this competition of hydrostatic equilibrium between gravity and pressure (from EoS choice), some soft EoSs, in principle, cannot be completely ruled out without first taking into account gravitational effects.

SGR J1745-2900 was detected from its outburst activity in 2013 April and it was the first soft gamma repeater (SGR) detected near the center of the Galaxy (Sagittarius A*). We use 3.5 yr Chandra X-ray light-curve data to constrain some neutron star (NS) geometric parameters. We assume that the flux modulation comes from hot spots on the stellar surface. Our model includes the NS mass, radius, a maximum of three spots of any size, temperature and positions, and general relativistic effects. We find that the light curve of SGR J1745-2900 could be described by either two or three hot spots. The ambiguity is due to the small amount of data, but our analysis suggests that one should not disregard the possibility of multi-spots (due to a multipolar magnetic field) in highly magnetized stars. For the case of three hot spots, we find that they should be large and have angular semiapertures ranging from 16° to 67°. The large size found for the spots points to a magnetic field with a nontrivial poloidal and toroidal structure (in accordance with magnetohydrodynamics investigations and Neutron Star Interior Composition Explorer’s (NICER) recent findings for PSR J0030+0451) and is consistent with the small characteristic age of the star. Finally, we also discuss possible constraints on the mass and radius of SGR J1745-2900 and briefly envisage possible scenarios accounting for the 3.5 yr evolution of SGR J1745-290 hot spots.

We study the role of time dependent magnetic ellipticities ( ) on the calculation of the braking index of pulsars. Moreover, we study the consequences of such a on the amplitude of gravitational waves (GWs) generated by pulsars with measured braking indices. We show that, since the ellipticity generated by the magnetic dipole is extremely small, the corresponding amplitude of GWs is much smaller than the amplitude obtained via the spindown limit.