The Fermi Large Area Telescope (LAT) provides advantages for radio pulsar searches by enabling efficient target selection. We can confidently point radio telescopes to the positions of Fermi unidentified gamma-ray sources that have a high probability of hosting a pulsar. As part of Transients and Pulsars with MeerKAT (TRAPUM), we conducted a survey of Fermi-LAT sources using the Ultra High Frequency (UHF; 544-1088 MHz) receiver of the MeerKAT radio telescope. We observed 79 sources that were identified as pulsar-like candidates using a random forest technique from the Fermi-LAT Fourth Source Catalogue. We observed each target for 10 min at two separate epochs. As a result, we discovered nine new millisecond pulsars (MSPs) and six slow pulsars. Based on the radio discoveries, we also searched for gamma-ray pulsations, confirming that seven of the newly discovered MSPs are associated with Fermi-LAT sources, and performed joint radio and gamma-ray pulsar timing. Companion mass estimates and evidence of radio eclipses indicate that among the nine MSPs there are three black widows and three redbacks. Lastly, we compared the discovered pulsars in the MeerKAT UHF survey against the previous Fermi sources TRAPUM survey at L band, concluding the superiority of UHF observations in sensitivity to fainter pulsars and in detection rate than L band for finding new gamma-ray MSPs.

We present the discovery and timing results for 15 pulsars discovered in a high-Galactic-latitude survey conducted with the Five-hundred-meter Aperture Spherical Telescope. The survey targeted a region as close as possible to the Galactic center, encompassing an area near the Galactic bulge. The newly discovered pulsars consist of 11 normal pulsars and four millisecond pulsars (MSPs). Among the MSPs, three are identified in binary systems with orbital periods of ∼3.1, 4.6, and 12.5 days, respectively. We have successfully obtained coherent timing solutions for three of the normal pulsars (PSRs J1745−0059, J1746−0156, and J1800−0059). Furthermore, within our data set, we found that four pulsars (three new and one known) show mode-changing and/or subpulse-drifting phenomena. Comparing our discoveries with simulations of the Galactic disk and bulge MSP populations indicates that these new pulsars are most likely located in the disk. Nonetheless, our discoveries demonstrate that deep surveys at high Galactic latitudes have significant potential to enhance our understanding of the MSP population in the direction of the bulge.

Context. The existence of a population of millisecond pulsars in the Galactic bulge is supported, along with other evidence, by the Fermi GeV excess, an anomalous gamma-ray emission detected almost 15 years ago in the direction of the Galactic center. However, radio surveys searching for pulsations have not yet revealed bulge millisecond pulsars. Aims. Identifying promising bulge millisecond pulsar candidates is key to motivating pointed radio pulsation searches. Candidates are often selected among steep-spectrum or polarized radio sources, but multiwavelength information can also be exploited: The aim of this work is to pinpoint strong candidates among the yet unidentified X-ray sources. Methods. We investigated the multiwavelength counterparts of sources detected by the Chandra X-ray observatory that have spectral properties expected for millisecond pulsars in the Galactic bulge. We considered that ultraviolet, optical, and strong infrared counterparts indicate that an X-ray source is not a bulge pulsar, while a radio or a faint infrared counterpart makes it a promising candidate. Results. We identify a large population of more than a thousand X-ray sources without optical, ultraviolet, or strong infrared counterparts. Among them, five are seen for the first time in unpublished radio imaging data from the Very Large Array. We provide the list of promising candidates, for most of which follow-up pulsation searches are ongoing.

Millisecond pulsars (MSPs) are abundant in globular clusters (GCs), which offer favorable environments for their creation. While the advent of recent, powerful facilities led to a rapid increase in MSP discoveries in GCs through pulsation searches, detection biases persist. In this work, we investigate the ability of current and future detections in GCs to constrain the parameters of the MSP population in GCs through a careful study of their luminosity function. Parameters of interest are the number of MSPs hosted by a GC, as well as the mean and the width of their luminosity function, which are typically affected by large uncertainties. While, as we show, likelihood-based studies can lead to ill-behaved posteriors on the size of the MSP population, we introduce a novel, likelihood-free analysis, based on marginal neural ratio estimation, which consistently produces well-behaved posteriors. We focus on the GC Terzan 5 (or Ter 5), which currently counts 48 detected MSPs. We find that 158 ${}_{-104}^{+294}$ MSPs should be hosted in this GC, but the uncertainty on this number remains large. We explore the performance of our new method on simulated Terzan 5-like data sets mimicking possible future observational outcomes. We find that significant improvement on the posteriors can be obtained by adding a reliable measurement of the diffuse radio emission of the GC to the analysis or by improving the detection threshold of current radio pulsation surveys by at least a factor of 2.

We report on the results of an image-based search for pulsar candidates toward the Galactic bulge. We used mosaic images from the MeerKAT radio telescope that were taken as part of a 173 deg2 survey of the bulge and Galactic center of our Galaxy at L band (856-1712 MHz) in all four Stokes I, Q, U, and V. The image rms noise levels of 12-17 mu Jy ba-1 represent a significant increase in sensitivity over past image-based pulsar searches. Our primary search criterion was circular polarization, but we used other criteria, including linear polarization, in-band spectral index, compactness, variability, and multiwavelength counterparts to select pulsar candidates. We first demonstrate the efficacy of this technique by searching for polarized emission from known pulsars and comparing our results with measurements from the literature. Our search resulted in a sample of 75 polarized sources. Bright stars or young stellar objects were associated with 28 of these sources, including a small sample of highly polarized dwarf stars with pulsar-like steep spectra. Comparing the properties of this sample with the known pulsars, we identified 30 compelling candidates for pulsation follow-up, including two sources with both strong circular and linear polarization. The remaining 17 sources are either pulsars or stars, but we cannot rule out an extragalactic origin or image artifacts among the brighter, flat-spectrum objects.

More than 10 years ago, an excess of γ-ray photons coming from the Galactic center was discovered in the Fermi-LAT data. First attributed to dark matter, it has since been shown that it should have at least a partial stellar origin. One hypothesis is the presence of a population of millisecond pulsars (MSPs) confined in the Galactic bulge. We here present our recent progress in the selection of MSP candidates.

Monte Carlo simulation of the millisecond pulsar population in the Galactic field. The simulation includes four spatial components: the disk; the boxy bulge; the nuclear stellar cluster; the nuclear stellar disk. The last 3 components together form the Galactic bulge. There is one file per component, each containing at least 100 Monte Carlo simulations. Each line contains: the longitude L in deg; the latitude B in deg; the line of sight S in kpc; the 0.1-100 GeV gamma-ray flux in erg/cm^2/s; the X-ray spectral index; the gamma-to-X flux ratio, where the gamma-ray flux is the same as in the fourth column and the X-ray flux is the 2-10 keV unabsorbed one of a simulated MSP. More information about the simulation can be found in the related paper.

For more than 20 years, the Compton telescope had provided the best measurements of the Galactic diffuse MeV spectrum. Recently, our analysis of 16 years of data from the SPectrometer on INTEGRAL (SPI) measured this emission with a higher signal-to-noise ratio. At MeV energies, the dominant contribution to the diffuse emission comes from inverse Compton scattering. Nonetheless, sub-dominant emission from Primordial Black Hole (PBH) Dark Matter (DM) can be searched for in these data. Hypothetically formed from the collapse of over-densities before Big Bang nucleosynthesis, PBHs are interesting candidates for DM in the ΛCDM model of cosmology. PBHs of masses between 1016 and 1018 g, in the so-called asteroid mass range, are currently unconstrained and can saturate the DM cosmological abundance. MeV emission from PBH in this mass range is expected to come from PBH evaporation. We searched for the PBH signal with 16 years of SPI data, and demonstrated that PBHs cannot account for all the DM if their mass is smaller than 4 × 1017 g.

ABSTRACT We present the timing of the first five millisecond pulsars discovered in the globular cluster Omega Centauri and the discovery of a pulsar with a spin period of 3.68 ms. With a timing baseline of ∼3.5 yr we are able to measure the derivative of the spin frequency ($\dot{\nu }$) for the first five pulsars. Upper limits on the pulsar line-of-sight acceleration are estimated and compared with predictions based on analytical models of the cluster. We find that PSRs J1326−4728B and D show large negative accelerations, which are in tension with the minimum acceleration predicted by analytical models. We searched for pulsed γ-ray signals using 14.3 yr of data from the Fermi Large Area Telescope. Although we found no evidence for γ-ray pulsations, PSRs J1326−4728A, B, C, and E are associated with X-ray sources. This suggests that the observed γ-ray emission from Omega Centauri is likely caused by the emission of the ensemble of MSPs. Finally, the linearly polarized emission from PSR J1326−4728A yields a rotation measure of −18 ± 8 rad m−2.

More than a decade after its discovery, the Fermi GeV excess is still an exciting subject of research. Thus far, an unresolved population of millisecond pulsars (MSPs) in the Galactic bulge shining in gamma rays is the favorite explanation to the excess, but other explanations exist. Data from the Fermi-LAT have been thoroughly studied and, in order to discriminate between the different hypotheses, a multi-wavelength approach is now needed. In a recent study [1], we demonstrated that if the GeV excess is caused by an MSP population, about a hundred of them could be detectable in X-rays in a region of 6 degrees x 6 degrees about the Galactic Center. The comparison with X-ray data allowed us to conclude that the MSP hypothesis was not excluded, as we found more than 3000 MSP candidates in a conservative approach. Besides, we selected few hundreds of promising candidates, with good X-ray spectral knowledge and no optical counterpart. In our new study, we additionally exploit ultraviolet and infrared data to exclude candidates. Finally, we compute a relation between the X-ray and radio luminosity of MSPs, aiming at predicting the radio luminosity of our candidates, with the ultimate goal of motivating radio observations needed to detect a pulsation and confirm a pulsar detection.

We present a new analysis of the diffuse soft $\gamma$-ray emission towards the inner Galaxy as measured by the SPectrometer aboard the INTEGRAL satellite (SPI) with 16 years of data taking. The analysis implements a spatial template fit of SPI data and an improved instrumental background model. We characterize the contribution of Primordial Black Holes (PBH) as dark matter (DM) candidates evaporating into $\mathcal{O}$(1) MeV photons by including, for the first time, the spatial distribution of their signal into the fitting procedure. No PBH signal is detected, and we set the strongest limit on PBH DM for masses up to $4 \times 10^{17}$ g, significantly closing in into the so-called asteroid mass range.

The last measurement of the diffuse emission spectrum of the Milky Way in the megaelectronvolt (MeV) photon energy range was performed by CGRO/COMPTEL more than 20 yr ago. We report a new analysis with the spectrometer SPI aboard INTEGRAL in the band 0 . 5–8 . 0MeV, finally superseding the signal-to-noise ratio (S/N) of the historic observations. This is possible thanks to an elaborate instrumental background model and careful considerations of the selected data, which are strongly affected by solar activity. We base our analysis on energy-dependent spatial template fitting in a region of ∆ l × ∆ b = 95 ◦ × 95 ◦ around the Galactic centre. Our flux estimates are consistent with COMPTEL measurements and show no ‘MeV bump’. The spectrum follows a power-law shape with index − 1 . 39 ± 0 . 09 stat ± 0 . 10 syst and an integrated flux of (5 . 7 ± 0 . 8 stat ± 1 . 7 syst ) × 10 − 8 ergcm − 2 s − 1 between 0.5 and 8.0MeV. We find that cosmic-ray electrons and propagation models consistent with the latest Fermi /LAT, Voyager 1, and AMS-02 data are broadly in agreement with the inferred inverse Compton spectral shape. However, a mismatch of a factor of 2–3 in normalisation with respect to baseline expectations may point to enhanced target photon densities and/or electron source spectra in the inner Galaxy, slightly modified diffusion properties, or the presence of an unresolved population of MeV γ -ray sources.

If the mysterious Fermi-LAT GeV gamma-ray excess is due to an unresolved population of millisecond pulsars (MSP) in the Galactic bulge, one expects this very same population to shine in x rays. For the first time, we address the question of what is the sensitivity of current x-ray telescopes to an MSP population in the Galactic bulge. To this end, we create a synthetic population of Galactic MSPs, building on an empirical connection between gamma- and x-ray MSP emission based on observed source properties. We compare our model with compact sources in the latest Chandra source catalog, applying selections based on spectral observables and optical astrometry with Gaia. We find a significant number of Chandra sources in the region of interest to be consistent with being bulge MSPs that are as yet unidentified. This motivates dedicated multiwavelength searches for bulge MSPs: Some promising directions are briefly discussed.

Complex, near-saturated, organic molecules have been detected in a wide range of environments, in harsh photon dominated regions, in protoplanetary disks, in the hot cores of star forming regions, and also in cold prestellar cores. In the latter case, the formation scenarios remain debated. While both gas phase or grain surface mechanisms have been considered, both types of scenarios have difficulties accounting for observed gas-phase abundances but none can be ruled out. The observation of structural isomers can help bring further constraints on molecular formation mechanisms, because their relative ratios should be ruled by kinetics, and not thermodynamics, in these low-density environments. Unfortunately so far no structural isomers of complex organic molecules have been detected. In this paper, we present the detection of ethylene oxide (c-C2H4O) in a prestellar core, using the IRAM 30 m telescope. While this species has previously been detected in hot cores of star forming regions, this represents the first detection in the cold (10 K) interstellar medium. Its more stable structural isomer acetaldehyde (CH3CHO) is also detected but not vinyl alcohol (CH2CHOH). We derive the abundances and abundance ratio of both detected species and compare them with predictions from the literature. We find that neither the previously proposed ion-neutral gas-phase nor the thermal grain surface scenarios can account for the observed abundances, while the radical-radical reaction between C2H5 and O remains a possible formation mechanism for both isomers. In addition, experiments have shown that cosmic ray induced electrons in ices containing CO, CO2, and C2H4 can explain the presence of both acetaldehyde and ethylene oxide in cold regions, but more quantitative data are needed to conclude whether the produced abundances can account for the observations.

A 1-parameter class of quadratic equations of state is confronted with the Hubble diagram of supernovae and Baryonic Acoustic Oscillations. The fit is found to be as good as the one using the Lambda CDM model. The corresponding universe has no initial singularity, only a mild bounce at a temperature well above the Planck temperature.