A tale of two populations: Rotating Radio Transients and X-ray Dim Isolated Neutron Stars

We highlight similarities between recently discovered Rotating Radio Transients and X-ray Dim Isolated Neutron Stars. In particular, it is shown that X-ray Dim Isolated Neutron Stars have a birthrate comparable to that of Rotating Radio Transients. On the contrary, magnetars have too low a formation rate to account for the bulk of the radio transient population. The consequences of the recent detection of a thermal X-ray source associated with one of the Rotating Radio Transients on the proposed scenarios for these sources are also discussed.Comment: 5 pages, accepted to MNRAS Letter

Spin and magnetism in old neutron stars

The thermal, spin and magnetic evolution of neutron stars in the old low mass binaries is first explored. Recycled to very short periods via accretion torques, the neutron stars lose their magnetism progressively. If accretion proceeds undisturbed for 100 Myrs these stars can rotate close to break up with periods far below the minimum observed of 1.558 ms. We investigate their histories using population synthesis models to show that a tail should exist in the period distribution below 1.558 ms. The search of these ultrafastly spinning neutron stars as pulsars can help discriminating among the various equations of state for nuclear matter, and can shed light into the physics of binary evolution. The evolution of isolated neutron stars in the Galaxy is explored beyond the pulsar phase. Moving through the tenuous interstellar medium, these old solitary neutron stars lose their rotational energy. Whether also their magnetism fades is still a mystery. A population synthesis model has revealed that only a tiny fraction of them is able to accrete from the interstellar medium, shining in the X-rays. There is the hope that these solitary stars will eventually appear as faint sources in the Chandra sky survey. This might give insight on the long term evolution of the magnetic field in isolated objects.Comment: 28 pages, 11 PostScript figures. To be published in "Physics of Neutron Star Interiors" (Lecture Notes in Physics), ed. D. Blaschke, N.K. Glendenning and A. Sedrakian (Springer, 2001

A Search for Pulsars in Quiescent Soft X-Ray Transients. I

We have carried out a deep search at 1.4 GHz for radio pulsed emission from six soft X-ray transient sources observed during their X-ray quiescent phase. The commonly accepted model for the formation of the millisecond radio pulsars predicts the presence of a rapidly rotating, weakly magnetized neutron star in the core of these systems. The sudden drop in accretion rate associated with the end of an X-ray outburst causes the Alfv\`en surface to move outside the light cylinder, allowing the pulsar emission process to operate. No pulsed signal was detected from the sources in our sample. We discuss several mechanisms that could hamper the detection and suggest that free-free absorption from material ejected from the system by the pulsar radiation pressure could explain our null result.Comment: accepted by Ap

Peculiar Spin Frequency and Radio Profile Evolution of PSR J1119−-6127 Following Magnetar-like X-ray Bursts

We present the spin frequency and profile evolution of the radio pulsar J1119$-$6127 following magnetar-like X-ray bursts from the system in 2016 July. Using data from the Parkes radio telescope, we observe a smooth and fast spin-down process subsequent to the X-ray bursts resulting in a net change in the pulsar rotational frequency of $\Delta\nu\approx-4\times10^{-4}$\,Hz. During the transition, a net spin-down rate increase of $\Delta\dot\nu\approx-1\times10^{-10}$\,Hz\,s$^{-1}$ is observed, followed by a return of $\dot{\nu}$ to its original value. In addition, the radio pulsations disappeared after the X-ray bursts and reappeared about two weeks later with the flux density at 1.4\,GHz increased by a factor of five. The flux density then decreased and undershot the normal flux density followed by a slow recovery back to normal. The pulsar's integrated profile underwent dramatic and short-term changes in total intensity, polarization and position angle. Despite the complex evolution, we observe correlations between the spin-down rate, pulse profile shape and radio flux density. Strong single pulses have been detected after the X-ray bursts with their energy distributions evolving with time. The peculiar but smooth spin frequency evolution of PSR~J1119$-$6127 accompanied by systematic pulse profile and flux density changes are most likely to be a result of either reconfiguration of the surface magnetic fields or particle winds triggered by the X-ray bursts. The recovery of spin-down rate and pulse profile to normal provides us the best case to study the connection between high magnetic-field pulsars and magnetars.Comment: Accepted for publication in MNRAS on 2018 July 2

Re-examining the X-Ray versus Spin-Down Luminosity Correlation of Rotation Powered Pulsars

The empirical relation between the X-ray luminosity (in the 2-10 keV band) and the rate of spin-down energy loss of a sample of 37 pulsars is re-examined considering recent data from ASCA, RXTE, BeppoSAX, Chandra, and XMM-Newton. The data show a significant scatter around an average correlation computed including statistical and systematic errors. All the X-ray luminosities are found to lie below a critical line corresponding to a maximum efficiency of conversion of rotational energy into X-rays varying between 0.1 and 80%. The large dispersion of the X-ray luminosity below the critical line indicates that other physical parameters uncorrelated with the period and period derivative need to be included to account for the observed emission at X-ray energies. We indicate a few possibilities that all conspire to lower the luminosity

The fingerprint of binary intermediate mass black holes in globular clusters: supra-thermal stars and angular momentum alignment

We explore the signatures that a binary intermediate mass black hole (IMBH) imprints on the velocity and on the angular momentum of globular cluster stars. Simulating 3-body encounters between a star and a binary IMBH, we find that the binary IMBH generates a family of few hundreds of stars (~100-300) which remain bound to the globular cluster (GC) and have velocity significantly higher than the dispersion velocity. For this reason we term them ''supra-thermal'' stars. We also notice that, after the interaction, a considerable fraction (55-70%) of stars tend to align their orbital angular momentum with the angular momentum of the binary IMBH, introducing an anisotropy in the angular momentum distribution of cluster stars. We simulate the dynamical evolution of these supra-thermal stars before thermalization, and find that these stars tend to cluster at a distance of few core radii from the GC center. We conclude that the detectability of such signatures appears problematic with present telescopes.Comment: 15 pages, 9 figures, replaced with revised version, accepted for publication in MNRA

The discovery, monitoring and environment of SGR J1935+2154

We report on the discovery of a new member of the magnetar class, SGR J1935+2154, and on its timing and spectral properties measured by an extensive observational campaign carried out between July 2014 and March 2015 with Chandra and XMM-Newton (11 pointings). We discovered the spin period of SGR J1935+2154 through the detection of coherent pulsations at a period of about 3.24s. The magnetar is slowing-down at a rate of 1.43(1)x10^{-11} s/s and with a decreasing trend due to a negative second period derivative of -3.5(7)x10^{-19} s/s^2. This implies a surface dipolar magnetic field strength of about 2.2x10^{14} G, a characteristic age of about 3.6kyr and, a spin-down luminosity L_{sd} of about 1.7x10^{34} erg/s. The source spectrum is well modelled by a blackbody with temperature of about 500eV plus a power-law component with photon index of about 2. The source showed a moderate long-term variability, with a flux decay of about 25\% during the first four months since its discovery, and a re-brightening of the same amount during the second four months. The X-ray data were also used to study the source environment. In particular, we discovered a diffuse emission extending on spatial scales from about 1" up to at least 1' around SGR J1935+2154 both in Chandra and XMM-Newton data. This component is constant in flux (at least within uncertainties) and its spectrum is well modelled by a power-law spectrum steeper than that of the pulsar. Though a scattering halo origin seems to be more probable we cannot exclude that part, or all, of the diffuse emission is due to a pulsar wind nebula.Comment: To appear in MNRAS; 10 pages, 3 color figures, 4 table

Activation of kinase phosphorylation by heat-shift and mild heat-shock

Most cells activate intracellular signalling to recover from heat damage. An increase of temperature, known as HS (heat shock), induces two major signalling events: the transcriptional induction of HSPs (heat-shock proteins) and the activation of the MAPK (mitogen-activated protein kinase) cascade. We performed the present study to examine the effects of HS, induced by different experimental conditions, on various kinases [ERK (extracellular-signal-regulated kinase), JNK (c-Jun N-terminal kinase), p38, Akt, AMPK (AMP-activated protein kinase) and PKC (protein kinase C)]. We investigated by Western blot analysis the phosphorylation of MAPK as a measure of cellular responsiveness to heat shift (37°C) and mild HS (40°C) in different cell lines. The results of the study indicate that every cell line responded to heat shift, and to a greater extent to HS, increasing ERK and JNK phosphorylation, whereas variable effects on activation or inhibition of PKC, AMPK, Akt and p38 were observed. Besides the implications of intracellular signalling activated by heat variations, these data may be of technical relevance, indicating possible sources of error due to different experimental temperature conditions