Optical Studies of 10 Hard X-Ray-selected Cataclysmic Binaries

We conducted time-resolved optical spectroscopy and/or photometry of 10 cataclysmic binaries that were discovered in hard X-ray surveys, with the goal of measuring their orbital periods and searching for evidence that they are magnetic. Four of the objects in this study are new optical identifications: IGR J18017−3542, PBC J1841.1+0138, IGR J18434−0508, and Swift J1909.3+0124. A 311.8 s, coherent optical pulsation is detected from PBC J1841.1+0138, as well as eclipses with a period of 0.221909 days. A 152.49 s coherent period is detected from IGR J18434−0508. A probable period of 389 s is seen in IGR J18151−1052, in agreement with a known X-ray spin period. We also detect a period of 803.5 s in an archival X-ray observation of Swift J0717.8−2156. The last four objects are thus confirmed magnetic cataclysmic variables of the intermediate polar class. An optical period of 1554 s in AX J1832.3−0840 also confirms the known X-ray spin period, but a stronger signal at 2303 s is present whose interpretation is not obvious. We also studied the candidate intermediate polar Swift J0820.6−2805, which has low and high states differing by ≈4 mag and optical periods or quasi-periodic oscillations not in agreement with proposed X-ray periods. Of note is an unusually long 2.06-day orbital period for Swift J1909.3+0124, manifest in the radial velocity variation of photospheric absorption lines of an early K-type companion star. The star must be somewhat evolved if it is to fill its Roche lobe.

Swift/XRT Deep Galactic Plane Survey Discovery of a New Intermediate Polar Cataclysmic Variable, Swift J183920.1-045350

We report on the Swift/XRT Deep Galactic Plane Survey discovery and multiwavelength follow-up observations of a new intermediate polar (IP) cataclysmic variable, Swift J183920.1-045350. A 449.7 s spin period is found in XMM-Newton and NuSTAR data, accompanied by a 459.9 s optical period that is most likely the synodic, or beat period, produced from a 5.6 hr orbital period. The orbital period is seen with moderate significance in independent long-baseline optical photometry observations taken with the ZTF and SAAO telescopes. We find that the X-ray pulse fraction of the source decreases with increasing energy. The X-ray spectra are consistent with the presence of an Fe emission line complex with both local and interstellar absorption. In the optical spectra, strong Hα, H i, He i, and He ii emission lines are observed, all common features in magnetic CVs. The source properties are thus typical of known IPs, with the exception of its estimated distance of 2.26 − 0.83 + 1.93 kpc, which is larger than typical, extending the reach of the CV population in our Galaxy.

An Isolated White Dwarf with a 70 s Spin Period

We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high-speed photometry of three ultramassive white dwarfs within 100 pc and discovered significant variability in one. SDSS J221141.80+113604.4 is a 1.27 M ⊙ (assuming a CO core) magnetic white dwarf that shows 2.9% brightness variations in the BG40 filter with a 70.32 ± 0.04 s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength of B d = 15 MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long-term monitoring of the spin evolution of J2211+1136 and other fast-spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the monoperiodic variables found within the instability strip may be fast-spinning white dwarfs impersonating ZZ Ceti pulsators.

The Green Bank Northern Celestial Cap Pulsar Survey. VI. Discovery and Timing of PSR J1759+5036: A Double Neutron Star Binary Pulsar

The Green Bank North Celestial Cap survey is a 350 MHz all-sky survey for pulsars and fast radio transients using the Robert C. Byrd Green Bank Telescope. To date, the survey has discovered over 190 pulsars, including 33 millisecond pulsars and 24 rotating radio transients. Several exotic pulsars have been discovered in the survey, including PSR J1759+5036, a binary pulsar with a 176 ms spin period in an orbit with a period of 2.04 days, an eccentricity of 0.3, and a projected semi-major axis of 6.8 light seconds. Using seven years of timing data, we are able to measure one post–Keplerian parameter, advance of periastron, which has allowed us to constrain the total system mass to 2.62 ± 0.03 M ⊙. This constraint, along with the spin period and orbital parameters, suggests that this is a double neutron star system, although we cannot entirely rule out a pulsar-white dwarf binary. This pulsar is only detectable in roughly 45% of observations, most likely due to scintillation. However, additional observations are required to determine whether there may be other contributing effects.

Modelling spin evolution of magnetars

The origin and fate of magnetars (young, extremely magnetized neutron stars, NSs) remains unsolved. Probing their evolution is therefore crucial for investigating possible links to other species of isolated NSs, such as the X-ray dim NSs (XDINSs) and rotating radio transients (RRATs). Here we investigate the spin evolution of magnetars. Two avenues of evolution are considered: one with exponentially decaying B-fields, the other with sub- and super-exponential decay. Using Monte Carlo methods, we synthesize magnetar populations using different input distributions and physical parameters, such as for the initial spin period, its time derivative and the B-field decay timescale. Additionally, we introduce a fade-away procedure that can account for the fading of old magnetars, and we briefly discuss the effect of alignment of the B-field and spin axes. Imposing the Galactic core-collapse supernova rate of ∼20 kyr−1 as a strict upper limit on the magnetar birthrate and comparing the synthetic populations to the observed one using both manual and automatic optimization algorithms for our input parameter study, we find that the B-field must decay exponentially or super-exponentially with a characteristic decay timescale of 0.5 − 10 kyr (with a best value of ∼4 kyr). In addition, the initial spin period must be less than 2 sec. If these constraints are kept, we conclude that there are multiple choices of input physics that can reproduce the observed magnetar population reasonably well. We also conclude that magnetars may well be evolutionary linked to the population of XDINSs, whereas they are in general unlikely to evolve into RRATs.

Statistical tests of young radio pulsars with/without supernova remnants: implying two origins of neutron stars

ABSTRACT The properties of the young pulsars and their relations to the supernova remnants (SNRs) have been the interesting topics. At present, 383 SNRs in the Milky Way Galaxy have been published, which are associated with 64 radio pulsars and 46 pulsars with high-energy emissions. However, we noticed that 630 young radio pulsars with the spin periods of less than half a second have been not yet observed the SNRs surrounding or nearby them, which arises a question of that could the two types of young radio pulsars with/without SNRs hold the distinctive characteristics? Here, we employ the statistical tests on the two groups of young radio pulsars with (52) and without (630) SNRs to reveal if they share the different origins. Kolmogorov–Smirnov (K–S) and Mann–Whitney–Wilcoxon (M–W–W) tests indicate that the two samples have the different distributions with parameters of spin period (P), derivative of spin period ($\dot{P}$), surface magnetic field strength (B), and energy loss rate ($\dot{E}$). Meanwhile, the cumulative number ratio between the pulsars with and without SNRs at the different spin-down ages decreases significantly after $\rm 10\!-\!20\, kyr$. So we propose that the existence of the two types of supernovae (SNe), corresponding to their SNR lifetimes, which can be roughly ascribed to the low- and high-energy SNe. Furthermore, the low-energy SNe may be formed from the $\rm 8\!-\!12\, M_{\odot }$ progenitor, e.g. possibly experiencing the electron capture, while the main-sequence stars of $\rm 12\!-\!25\, M_{\odot }$ may produce the high-energy SNe probably by the iron core collapse.

Timing observations of three Galactic millisecond pulsars

We report observed and derived timing parameters for three millisecond pulsars (MSPs) from observations collected with the Parkes 64-m telescope, Murriyang. The pulsars were found during re-processing of archival survey data by Mickaliger et al. One of the new pulsars (PSR J1546–5925) has a spin period P = 7.8 ms and is isolated. The other two (PSR J0921–5202 with P = 9.7 ms and PSR J1146–6610 with P = 3.7 ms) are in binary systems around low-mass (>0.2 M⊙) companions. Their respective orbital periods are 38.2 d and 62.8 d. While PSR J0921–5202 has a low orbital eccentricity e = 1.3 × 10−5, in keeping with many other Galactic MSPs, PSR J1146–6610 has a significantly larger eccentricity, e = 7.4 × 10−3. This makes it a likely member of a group of eccentric MSP–He white dwarf binary systems in the Galactic disk whose formation is poorly understood. Two of the pulsars are co-located with previously unidentified point sources discovered with the Fermi satellite’s Large Area Telescope, but no γ-ray pulsations have been detected, likely due to their low spin-down powers. We also show that, particularly in terms of orbital diversity, the current sample of MSPs is far from complete and is subject to a number of selection biases.

Quantification of Sub-Solar Star Ages from the Symmetry of Conjugate Histograms of Spin Period and Angular Velocity

Empirical laws proposed for the decline in star spin with time have heretofore been tested using ambiguous fitting models. We develop an analytical inverse model that uses histogram data to unequivocally determine the physical law governing how dwarf star spin depends on time (t) and mass (M). We analyze shapes of paired histograms of axial rotation period (П) and angular velocity (ω = 2π/П) to utilize the fact that a variable and its reciprocal are governed by the same physics. Copious data on open clusters are used to test the formula ∂ω/∂t ∝ − ωn where n is unrestricted, and thus covers diverse possibilities. Histogram conjugates for each of 15 clusters with 120 to 812 measurements provide n = 1.13 ± 0.19. Results are independent of initial spin rate, bin size, cluster parameters, and star mass. Notably, 11 large clusters with mostly M-types yield fits with n = 1.07 ± 0.12. Associations behave similarly. Only exponential decay (n = 1) explains the similar shapes of the conjugate histograms for the spin period and angular velocity, despite the asymmetric (inverse) relationship of these variables. This rate law is consistent with viscous dissipation. Forward modeling confirms that n is near unity and further shows that coeval formation of all stars in a cluster does not occur. We therefore explore a constant rate of star production, which is reasonable for tiny stars. Inverse models show that episodic production increases with mass, but is unimportant below ~0.55 MSun. We infer star and cluster ages, and find that star production becomes less regular with time, as interstellar gas and dust are progressively depleted. Our new analytical approach of extracting a physical law from conjugate histograms is general and widely applicable.