We report the onset of a large amplitude, statistically significant periodicity (∼46 d) in the Rossi X‐ray Timing Explorer/All Sky Monitor (RXTE/ASM) data of the prototype X‐ray burster 4U 1636‐53, the X‐ray flux of which has been gradually declining over the last 4 yr. This behaviour is remarkably similar to that observed in the neutron star low mass X‐ray binary (LMXB), KS 1731‐260, which is a long‐term transient. We also report on an INTEGRAL/IBIS observation of 4U 1636‐53 during its decline phase, and find that the hard X‐ray flux (20–100 keV) indicates an apparent anticorrelation with soft X‐rays (2–12 keV). We argue that 4U 1636‐53 is transiting from activity to quiescence, as occurred in KS 1731‐260. We also suggest that the variability during the X‐ray decline is the result of an accretion rate variability related to the X‐ray irradiation of the disc.

Using the Rossi X‐ray Timing Explorer and the Nordic Optical Telescope, we have obtained the highest ever quality X‐ray/white‐light high‐speed photometry of XB 1916–053. We refine the X‐ray period (PX) to 3000.6±0.2 s via a restricted cycle counting approach. Using our complete optical light curve, we have extended the optical period (Popt) ephemeris by another 4 yr, providing further evidence for its stability, although a slightly longer period of 3027.555±0.002 s now provides a marginally better fit. Moreover, modulations at both PX and Popt are present in the optical data, with the former dominating the nightly light curves (i.e. a few cycles of data). We have also attempted to determine the ‘beat’ period, as seen in the repeating evolution of the X‐ray dip structure, and the variation in primary dip phase. We find that a quasi‐period of 4.74±0.05 d provides the best fit to the data, even then requiring phase shifts between cycles, with the expected 3.90‐d ‘beat’ of PX and Popt appearing to be less likely. Finally, considering the nature of each of these temporal phenomena, we outline possible models, which could explain all of the observed behaviour of this enigmatic source, focusing on which of PX or Popt is the binary period.

We have discovered an accreting black hole (BH) in a spectroscopically confirmed globular cluster (GC) in NGC 1399 through monitoring of its X-ray activity. The source, with a peak luminosity of L X ≃2×10 39 ergs s −1 , reveals an order of magnitude change in the count rate within ≃10 ks in a Chandra observation. The BH resides in a metal-rich [Fe/H]≃0.2 globular cluster. After RZ 2109 in NGC 4472 this is only the second black-hole X-ray source in a GC confirmed via rapid X-ray variability. Unlike RZ 2109, the X-ray spectrum of this BH source did not change during the period of rapid variability. In addition to the short-term variability the source also exhibits long-term variability. After being bright for at least a decade since 1993 within a span of 2 years it became progressively fainter, and eventually undetectable, or marginally detectable, in deep Chandra and XMM observations. The source also became harder as it faded. The characteristics of the long term variability in itself provide sufficient evidence to identify the source as a BH. The long term decline in the luminosity of this object was likely not recognized in previous studies because the rapid variability within the bright epoch suppressed the average luminosity in that integration. The hardening of the spectrum accompanying the fading would also make this black hole source indistinguishable from an accreting neutron star in some epochs. Therefore some low mass X-ray binaries identified as NS accretors in snapshot studies of nearby galaxies may also be BHs. Thus the discovery of the second confirmed BH in an extragalactic GC through rapid variability at the very least suggests that accreting BHs in GCs are not exceedingly rare occurences.

We present a study of the X-ray spectral properties of the highly variable X-ray emitting black hole in a globular cluster in the elliptical galaxy NGC 4472. The XMM-Newton spectrum of the source in its bright epoch is well described by a multiple blackbody model with a characteristic temperature $kT_{in}\approx$ 0.2 keV. The spectrum of an archival Chandra observation of the source obtained 3.5 years before the XMM data gives similar estimates for the blackbody parameters. We confirm that the fainter interval of the XMM-Newton observation has a spectrum that is consistent with the brighter epoch, except for an additional level of foreground absorption. We also consider other possible mechanisms for the variability. Based on the timescale of the X-ray flux decline and the estimated size of the X-ray emission region we argue that an eclipsing companion is highly unlikely. We find the most likely means of producing the absorption changes on the observed timescale is through partial obscuration by a precessing warped accretion disk.Comment: Accepted for publication in MNRA

XIPE, the X-ray Imaging Polarimetry Explorer, is a mission dedicated to X-ray Astronomy. At the time of writing XIPE is in a competitive phase A as fourth medium size mission of ESA (M4). It promises to reopen the polarimetry window in high energy Astrophysics after more than 4 decades thanks to a detector that efficiently exploits the photoelectric effect and to X-ray optics with large effective area. XIPE uniqueness is time-spectrallyspatially-resolved X-ray polarimetry as a breakthrough in high energy astrophysics and fundamental physics. Indeed the payload consists of three Gas Pixel Detectors at the focus of three X-ray optics with a total effective area larger than one XMM mirror but with a low weight. The payload is compatible with the fairing of the Vega launcher. XIPE is designed as an observatory for X-ray astronomers with 75 % of the time dedicated to a Guest Observer competitive program and it is organized as a consortium across Europe with main contributions from