Osservatorio Astronomico di Cagliari

Abstract The merger shocks generated by the collision of galaxy clusters elevate the pressure within the intracluster medium, significantly influencing the evolution of embedded cluster galaxies. We detect a merger shock (Mach number ∼1.3) on the northwest side of the closest rich galaxy cluster Norma (A3627), using XMM-Newton and Chandra data. The textbook ram pressure stripping (RPS) galaxy ESO 137-001 appears to be located in the postshock region. The shock boosts RPS and may induce the formation of the brightest known X-ray tail behind a cluster late-type galaxy. Another prominent head–tail radio galaxy, ESO 137-007, with one of the longest radio continuum tails (>500 kpc), is also likely in the postshock region. The shock may have reversed the upstream jet to a one-sided radio head–tail morphology. Moreover, the shock can strip and roll the jet cocoon into a vortex ring structure like a “smoke ring” behind the end of the jet as observed by the ASKAP data. Therefore, the cluster merger shock can remarkably change cluster galaxies. Furthermore, Norma is the second brightest non-cool-core cluster following the Coma cluster, with a cool core remnant on its southeast side. Its original cool core may be disrupted by cluster mergers and/or active galactic nuclei.

Context. Winds are an important ingredient in the evolution of X-ray binary (XRB) systems, particularly those at high accretion rates such as ultra-luminous X-ray sources (ULXs), because they may regulate the accretion of matter onto the compact object. Aims. Our aim is to understand the properties of ULX winds and their link with the source spectral and temporal behaviour. Methods. We performed high-resolution X-ray spectroscopy of the variable source NGC 55 ULX-1 to resolve emission and absorption lines as observed with XMM-Newton at different epochs. Optically thin plasma models were used to characterise the wind. Results. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios of the fluxes of the emission lines favours photoionisation balance and low to moderate densities, which confirm that the lines originate from classical XRB disc winds. An in-depth parameter space exploration shows line emission from a slowly moving, cool, and variable plasma perhaps associated with a thermal wind. Mildly relativistic Doppler shifts (about −0.15 c ) associated with the absorption lines confirm, at higher confidence, the presence of powerful radiatively driven winds. Conclusions. The comparison between the results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum. These variations may be used to understand the actual accretion regime and the nature of the source.

Context. Galaxies fly inside galaxy clusters and ram pressure by the intracluster medium (ICM) can remove a large amount of the interstellar medium (ISM) from the galaxy, and deposit the gas in the ICM. The ISM decoupled from the host galaxy leaves a long trail following the moving galaxy. Such long trails track galaxy motions and they can be detected with sensitive data in H α . Aims. We study the H α tail trailing NGC 4569 in the Virgo cluster. Methods. The initial discovery was made with the deep H α imaging data with CFHT from the VESTIGE project. The follow-up spectroscopic observations were carried out with APO / DIS , MMT / Binospec , and CFHT / SITELLE . Results. Aside from the known 80 kpc H α tail downstream of NGC 4569, the deep H α imaging data allow the H α tail detected to at least 230 kpc from the galaxy. More importantly, we confirmed the H α clumps implied from the imaging data with spectroscopic data. The H α clumps show a smooth radial velocity gradient across ∼1300 km/s, eventually reaching the velocity of the cluster. Conclusions. For the first time, this discovery offers a view of the full deceleration process of the stripped ISM. This discovery also showcases the potential with wide-field H α survey on galaxy clusters to discover intracluster optical emission-line clouds originated from cluster galaxies. These clouds provide kinematic tracers to the infall history of cluster galaxies and the turbulence in the ICM. They also serve as excellent multi-phase objects for studying the classical cloud-crushing problem and other important physical processes.

Here we report on a new survey for pulsars and transients in the 10,pc region around using the Effelsberg radio telescope at frequencies between 4 to 8 GHz. Our calibrated full-Stokes data were searched for pulsars and transients using , , and . Polarisation information is used in the scoring of the candidates. Our periodicity acceleration and jerk searches allowed us to maintain good sensitivity towards binary pulsars in ≳ 10-hr orbits. In addition we performed a dedicated search in linear polarisation for slow transients. While our searches yielded no new discovery beyond the redetection of the magnetar SGR J1745-2900, we report on a faint single pulse candidate in addition to several weak periodicity search candidates. After thoroughly assessing our survey's sensitivity, we determined that it is still not sensitive to a population of millisecond pulsars. Next generation radio interferometers can overcome the limitations of traditional single-dish pulsar searches of the Galactic Centre. PulsarX TransientX PRESTO

The magnetar was discovered in outburst on 2021 June 3 by the Burst Alert Telescope on board the satellite. Early X‐ray follow‐up revealed a spin period P a period derivative dot P ,s,s-1, dozens of short bursts, and an unusual flux decline. We report here on the X‐ray monitoring of -11 over the first ≃29 months of its outburst with ∋cer, νstar, and as well as radio observations with soon after the outburst onset. The observed 0.3–10 keV flux remained at levels erg cm^-2 s^-1 for nearly 500 days before dropping by a factor of ≃ 10$ from its 2021 June peak towards the end of the monitoring campaign. During this time span, the spectrum was dominated by a single blackbody, whose temperature attained an approximately constant value (∼1.2,keV), while the inferred radius shrank from approx1.7,km to approx0.3,km (assuming a source distance of 10,kpc). The long‐term spin‐down rate (dot P ,s,s^-1) is only $ -11 higher than that measured in the first 30 days. No periodic or burst‐like radio emission was detected, in line with what had been previously reported using different radio facilities. The persistently high temperature, shrinking hotspot, and prolonged bright flux plateau followed by a fast dimming observed during the outburst evolution pose a challenge for the outburst mechanisms proposed so far.

Constraining the origin of the nanohertz gravitational-wave background necessitates precise noise modelling to avoid parameter estimation biases. In this work, we find the inferred properties of the putative gravitational wave background in the second data release of the European Pulsar Timing Array to be in better agreement with theoretical expectations under the improved noise model. In particular, our improved noise models show consistency of the background’s strain spectral index with the value of −2/3, favoring the population of supermassive black hole binaries as the origin of the background. Our results further suggest that the observed gravitational wave emission is the dominant source of the binary energy loss, with no evidence of environmental effects or eccentric orbits. At the reference gravitational wave frequency of yr−1, we also find a lower power-law strain amplitude of the background than in previous data analyses. This mitigates some of the tensions of the strain amplitude with the expected number density and mass scale of binaries discussed in the literature. Our analysis demonstrates the importance of accurate modelling of radio pulsar pulse profile variations, hierarchical properties of noise across pulsars, as well as noise model averaging, when inferring properties of the gravitational wave background.

Aims. By exploiting photometry from the Wide-field Infrared Survey Explorer (WISE), we investigate the influence of environment and stellar mass on the prevalence of different excitation types in the interstellar gas of the central regions of galaxies in the Herschel Reference Survey (HRS, z ≈ 0.0047). We extend this analysis to a complementary sample of relatively nearby galaxies that are undergoing ram-pressure stripping (RPS, z ≈ 0.0195). Our goal is to assess whether a connection exists between active galactic nucleus (AGN) activity and either the cluster environment or the ram pressure stripping process. Methods. We computed WISE mid-infrared colour indices from fluxes extracted from central apertures and applied two established mid-infrared diagnostic diagrams to distinguish AGN activity from non-AGN, and star-forming galaxy (SFG) excitation from that associated with ‘retired’ galaxies. The resulting types of excitation were then used in conjunction with stellar mass and environment classifications to construct a stellar mass-excitation fraction relation. Results. The stellar mass-excitation fraction relation reveals that global stellar mass is the primary driver of excitation diversity in galaxy centres. In increasing order of prevalence, excitation types follow a sequence from SFGs to retired galaxies with increasing mass. The number of low-mass galaxies is too small to drive statistical tendencies. In contrast, SFG excitation becomes dominant at intermediate masses. At the highest mass end, the retired-galaxy type clearly prevails. Across the full mass range and in nearly all environments, SFG excitation is the most common, except for H I -gas deficient HRS galaxies, which are mostly retired. The excitation properties of galaxies undergoing RPS resemble those of HRS cluster members, field galaxies, and normal H I -content galaxies, minimising the environmental role. Contrary to previous results, we do not see any increase in AGN activity in H I -deficient cluster galaxies or in those undergoing RPS, since its fractions (∼10% for Seyfert 2 and ∼20% for low-ionisation nuclear emission-line region, LINER) remain largely unaffected along all environments. Conclusions. These findings indicate that while rich environments are associated with certain excitation types, stellar mass remains the primary driver of excitation diversity in galaxy centres.

Recent detections of optical pulsations from both a transitional and an accreting millisecond pulsar have revealed unexpectedly bright signals, suggesting that the presence of an accretion disk enhances the efficiency of optical emission, possibly via synchrotron radiation from accelerated particles. In this work we present optical observations of the redback millisecond pulsar PSR J2339–0533, obtained with the SiFAP2 photometer mounted on the Telescopio Nazionale Galileo . Data accumulated during the campaign with the longest exposure time ($12$ hr) suggest that its ∼18 mag optical counterpart pulsates at the neutron star’s spin frequency. This candidate signal was identified by folding the optical time series using the pulsar ephemeris derived from nearly simultaneous observations with the 64 m Murriyang (Parkes) radio telescope. The detection significance of the candidate optical signal identified in those data lies between 2.9 and 3.5 σ, depending on the statistical test employed. The pulse duty cycle is łesssim 1/32, and the de-reddened pulsed magnitude in the V band is $(26.4 ± 0.6)$ mag. At a distance of 1.7 kpc, this corresponds to a conversion efficiency of ∼ 3 of the pulsar’s spin-down power into pulsed optical luminosity -- which is comparable to the values observed in young, isolated pulsars like the Crab but $50–100$ times lower than in disk-accreting millisecond pulsars. If confirmed, these findings suggest that optical pulsations of MSPs arise independently of an accretion disk and support the notion that such disks boost the optical emission efficiency.

Abstract We present the X-ray polarization observation of G21.5−0.9, a young Galactic supernova remnant (SNR), conducted with the Imaging X-ray Polarimetry Explorer (IXPE) in 2023 October, with a total livetime of approximately 837 ks. Using different analysis methods, such as a space-integrated study of the entire region of the pulsar wind nebula (PWN) and a space-resolved polarization map, we detect significant polarization from the PWN at the center of the SNR, with an average polarization degree of ∼10% oriented at ∼33° (north through east). No significant energy-dependent variation in polarization is observed across the IXPE band (2–8 keV). The polarization map, corrected for the effect of polarization leakage, reveals a consistent pattern in both degree and angle, with little change across the nebula. Our findings indicate the presence of a highly polarized central torus, suggesting low levels of turbulence at particle acceleration sites. Unlike Vela, but similar to the Crab Nebula, we observe substantial differences between radio and X-ray polarization maps. This suggests a clear separation in energy of the emitting particle populations and hints at an important, yet poorly understood, role of instabilities in the turbulence dynamics of PWNe.

ABSTRACT Giant pulses (GPs) occur in high magnetic field millisecond pulsars and young Crab-like pulsars. Motivated by the fast radio bursts (FRBs) discovered in a globular cluster (GC) in the M81, we undertook baseband observations of PSR J1823–3021A, the most active GP emitter in a GC with the Meer Karoo Array Telescope (MeerKAT) ultra high frequency (UHF) band receiver (544–1088 MHz). The steep spectral index of the pulsar yields a GP rate of over 37 000 GPs h−1 with $\mathrm{ S/N}>7$, significantly higher than the 3000 GPs h−1 rate detected by previous work with the L-band (856–1712 MHz) receiver. Similarly to the previous work, we find that the GPs are (1) strongly clustered in two particular phases of its rotation, (2) well described by a power-law in terms of energies, (3) typically broad-band, and have steep spectral indices of ${\approx} {-}3$. Although the integrated pulse profile is not significantly polarized (${<} 1~{{\ \rm per\ cent}}$ linear and ${<} 3~{{\ \rm per\ cent}}$ circular), one of the brightest GPs displays notable polarization of 7 per cent (linear) and 8 per cent (circular). The high-time resolution data reveals the GPs have a range of single-peak and multipeak morphologies, including GPs with three distinct peaks. For the first time, we measured the temporal scattering of the pulsar using 49 bright, narrow, single-pulse GPs, obtaining a mean value of $5.5\pm 0.6$ $\mu$s at 1 GHz. The distinct periodicity and low polarization of GPs differentiate them from typical FRBs, although potential quasi-periodicity substructures in some GPs may suggest a connection to magnetars/FRBs.