Magnetic White Dwarfs

In this paper we review the current status of research on the observational and theoretical characteristics of isolated and binary magnetic white dwarfs (MWDs). Magnetic fields of isolated MWDs are observed to lie in the range 10^3-10^9G. While the upper limit cutoff appears to be real, the lower limit is more difficult to investigate. The incidence of magnetism below a few 10^3G still needs to be established by sensitive spectropolarimetric surveys conducted on 8m class telescopes. Highly magnetic WDs tend to exhibit a complex and non-dipolar field structure with some objects showing the presence of higher order multipoles. There is no evidence that fields of highly magnetic WDs decay over time, which is consistent with the estimated Ohmic decay times scales of ~10^11 yrs. MWDs, as a class, also appear to be more massive than their weakly or non-magnetic counterparts. MWDs are also found in binary systems where they accrete matter from a low-mass donor star. These binaries, called magnetic Cataclysmic Variables (MCVs) and comprise about 20-25\% of all known CVs. Zeeman and cyclotron spectroscopy of MCVs have revealed the presence of fields in the range $\sim 7-230$\,MG. Complex field geometries have been inferred in the high field MCVs (the polars) whilst magnetic field strength and structure in the lower field group (intermediate polars, IPs) are much harder to establish. The origin of fields in MWDs is still being debated. While the fossil field hypothesis remains an attractive possibility, field generation within the common envelope of a binary system has been gaining momentum, since it would explain the absence of MWDs paired with non-degenerate companions and also the lack of relatively wide pre-MCVs.Comment: 73 pages, 22 figures, 2 large tables. Invited review chapter on Magnetic White Dwarfs to appear in Space Science Reviews, Springe

A universal relation for the propeller mechanisms in magnetic rotating stars at different scales

Accretion of matter onto a magnetic, rotating object can be strongly affected by the interaction with its magnetic field. This occurs in a variety of astrophysical settings involving young stellar objects, white dwarfs, and neutron stars. As matter is endowed with angular momentum, its inflow toward the star is often mediated by an accretion disc. The pressure of matter and that originating from the stellar magnetic field balance at the magnetospheric radius: at smaller distances the motion of matter is dominated by the magnetic field, and funnelling towards the magnetic poles ensues. However, if the star, and thus its magnetosphere, is fast spinning, most of the inflowing matter will be halted at the magnetospheric radius by centrifugal forces, resulting in a characteristic reduction of the accretion luminosity. The onset of this mechanism, called the propeller, has been widely adopted to interpret a distinctive knee in the decaying phase of the light curve of several transiently accreting X-ray pulsar systems. By comparing the observed luminosity at the knee for different classes of objects with the value predicted by accretion theory on the basis of the independently measured magnetic field, spin-period, mass, and radius of the star, we disclose here a general relation for the onset of the propeller which spans about eight orders of magnitude in spin period and ten in magnetic moment. The parameter-dependence and normalisation constant that we determine are in agreement with basic accretion theory.Comment: 11 pages, 3 figures. Accepted for publication in A&

Unambiguous Detection of Reflection in Magnetic Cataclysmic Variables: Joint NuSTAR-XMM-Newton Observations of Three Intermediate Polars

In magnetic cataclysmic variables (CVs), X-ray emission regions are located close to the white dwarf surface, which is expected to reflect a significant fraction of intrinsic X-rays above 10 keV, producing a Compton reflection hump. However, up to now, a secure detection of this effect in magnetic CVs has largely proved elusive because of the limited sensitivity of non-imaging X-ray detectors. Here we report our analysis of joint NuSTAR/XMM-Newton observations of three magnetic CVs, V709 Cas, NY Lup, and V1223 Sgr. The improved hard X-ray sensitivity of the imaging NuSTAR data has resulted in the first robust detection of Compton hump in all three objects, with amplitudes of ~1 or greater in NY Lup, and likely <1.0 in the other two. We also confirm earlier report of a strong spin modulation above 10 keV in V709 Cas, and report the first detection of small spin amplitudes in the others. We interpret this as due to different height of the X-ray emitting region among these objects. A height of ~0.2 white dwarf radii provides a plausible explanation for the low reflection amplitude of V709 Cas. Since emission regions above both poles are visible at certain spin phases, this can also explain the strong hard X-ray spin modulation. A shock height of ~0.05 white dwarf radii can explain our results on V1223 Sgr, while the shock height in NY Lup appears negligible.Comment: 16 pages including 3 figures and 2 tables; accepted for publication in Astrophysical Journal Letter

The first orbital period of a very bright and fast Nova in M31: M31N 2013-01b

We present the first X-ray and UV/optical observations of a very bright and fast nova in the disc of M31, M31N 2013-01b. The nova reached a peak magnitude $R\sim$15 mag and decayed by 2 magnitudes in only 3 days, making it one of the brightest and fastest novae ever detected in Andromeda. From archival multi-band data we have been able to trace its fast evolution down to $U>21$ mag in less than two weeks and to uncover for the first time the Super-Soft X-ray phase, whose onset occurred 10-30 days from the optical maximum. The X-ray spectrum is consistent with a blackbody with a temperature of $\sim$50 eV and emitting radius of $\sim$4$\times 10^{9}$ cm, larger than a white dwarf radius, indicating an expanded region. Its peak X-ray luminosity, 3.5$\times 10^{37}$ erg s$^{-1}$, locates M31N 2013-01b among the most luminous novae in M31. We also unambiguously detect a short 1.28$\pm$0.02 h X-ray periodicity that we ascribe to the binary orbital period, possibly due to partial eclipses. This makes M31N 2013-01b the first nova in M31 with an orbital period determined. The short period also makes this nova one of the few known below the 2-3 h orbital period gap. All the observed characteristics strongly indicate that M31N 2013-01b harbours a massive white dwarf and a very low-mass companion, consistent with being a nova belonging to the disc population of the Andromeda Galaxy.Comment: 9 pages, 3 figures, 2 tables; accepted by the Astrophysical Journa

The magnetic CV Swift J0614.0+1709 is not the optical counterpart of Fermi J0614+1713

The Fermi-LAT Gamma-ray Transient Fermi J0614+1713 was detected over 3.5days starting from Jan 28, 2022 with an improved localization of 0.17deg centred at RA=93.48 and DEC=17.18 (ATels #15196, #15199) encompassing the magnetic CV SwiftJ0614.0+1709. We analysed the optical light curve of SwiftJ0614.0+1709 in the ASAS-SN photometry database (Shappee et al. 2014, ApJ 788, 48) with an almost daily coverage up to Feb 6, 2022. In particular it has been observed on Jan 28 from 04:24 to 05:16UT, on Jan 29 from 05:43 to 05:47UT, on Jan 30 from 07:44, on Feb. 03, 2022 from 00:56 to 08:37UT, on Feb. 04 from 02:04 to 09:42UT and on Feb. 6 from 04:08 to 04:56UT. Aperture photometry reveals SwiftJ0614.0+1709 at a stable level with a mean g-band magnitude of 17.13 (rms=0.19), consistent with the Gaia eDR3 measurement (G=17.10) and previous optical measures (Halpern & Thorstensen, 2015, AJ 150, 170). Therefore this mCV is excluded as the counterpart of the Fermi transient. The analysis of about two hundreds optical sources observed by ASAS-SN during the same period located within the 0.17deg localization uncertainty of the gamma-ray source is ongoing

Transitional Millisecond Pulsars

Millisecond pulsars in tight binaries have recently challenged our understanding of physical processes governing the evolution of binaries and the interaction between astrophysical plasma and electromagnetic fields. Transitional systems that showed changes from rotation-powered to accretion-powered states and vice versa have bridged the populations of radio and accreting millisecond pulsars, eventually demonstrating the tight evolutionary link envisaged by the recycling scenario. A decade of discoveries and theoretical efforts have just grasped the complex phenomenology of transitional millisecond pulsars from the radio to the gamma-ray bands. This chapter summarizes the main properties of the three transitional millisecond pulsars discovered so far, as well as of candidates and related systems, discussing the various models proposed to cope with their multifaceted behaviour

Transitional Millisecond Pulsar Binaries

The extremely fast rotation of millisecond pulsars is the outcome of a Gyr-long accretion phase onto a neutron star of material transferred through an accretion disc from a low mass late-type companion star. After this phase during which the binary shines as a bright low-mass X-ray binary (LMXB), the mass transfer rate declines allowing the activation of a radio/gamma-ray pulsar (MSP) powered by the rapid rotation of its magnetic field. The tight link between LMXBs and MSPs was first testified in 2009 by PSRJ1023+0038 that years before was in an accretion state. The recent suprising discovery of three binary systems, dubbed transitional MSPs, switching from accretion to rotation-powered emission and viceversa has shown the existence of a peculiar intermediate evolutionary phase during which LMXB and MSP states interchange on timescales compatible with those of the variations of the mass-inflow. Transitions were observed during outburst but also in an extremely peculiar sub-luminous disc state during which both accretion and ejection may take place. The main observational properties of known and candidate systems in both disc and disc-free states and ongoing efforts to understand the coupling of accretion and ejection and the role of magnetic fields in driving outflows will be presented

New X-ray detections of magnetic period-bounce cataclysmic variables from XMM-Newton and SRG/eROSITA

A great portion of the cataclysmic variable population, between 40% and 70%, is predicted to be made up of period-bouncers, systems with degenerate donors that have evolved past the period minimum. However, due to their intrinsic faintness, only a few of these systems have been observed and confidently identified so far. We have searched for X-ray emission as a proof of accretion in order to confirm period-bounce cataclysmic variables. In a dedicated XMM-Newton observation of the period-bounce candidate SDSS J151415.65+074446.5 we discovered X-ray modulation at the binary orbital period confirming it as an accreting system. The X-ray light curve and the X-ray spectrum display characteristics of magnetic Polar-type systems allowing for the first time the determination of the X-ray luminosity and mass accretion rate for this system. Catalog data from eROSITA on the SRG satellite for V379 Vir and SDSS J125044.42+154957.4 allowed a first look into the X-ray behavior of period-bounce candidates with this new all-sky instrument. From the eROSITA measurements the X-ray luminosity and mass accretion rate were determined for the first time for SDSS J125044.42+154957.4, and the earlier result for V379 Vir from XMM-Newton was confirmed. All three cataclysmic variables with a magnetic white dwarf and very low-mass donor studied in this work present evidence for X-ray emission at a similar level of $L_{\rm x}\,{\rm [erg/s]} \approx 10^{29}$, which, together with the detection of X-ray orbital modulation in two of them, V379 Vir and SDSS J151415.65+074446.5, unambiguously proves the presence of accretion in these systems. The detection of these period-bouncers at faint X-ray luminosity levels with the all-sky X-ray survey eROSITA offers new prospects for the identification of additional period-bouncers, providing impetus for theoretical studies of binary evolution.Comment: 8 pages, 4 figures. Accepted for publication in A&

Multiwavelength study of RX J2015.6+3711: a magnetic cataclysmic variable with a 2-hr spin period

The X-ray source RX J2015.6+3711 was discovered by ROSAT in 1996 and recently proposed to be a cataclysmic variable (CV). Here we report on an XMM-Newton observation of RX J2015.6+3711 performed in 2014, where we detected a coherent X-ray modulation at a period of 7196+/-11 s, and discovered other significant (>6sigma) small-amplitude periodicities which we interpret as the CV spin period and the sidebands of a possible ~12 hr periodicity, respectively. The 0.3-10 keV spectrum can be described by a power law (Gamma = 1.15+/-0.04) with a complex absorption pattern, a broad emission feature at 6.60+/-0.01 keV, and an unabsorbed flux of (3.16+/-0.05)x10^{-12} erg/s/cm^2. We observed a significant spectral variability along the spin phase, which can be ascribed mainly to changes in the density of a partial absorber and the power law normalization. Archival X-ray observations carried out by the Chandra satellite, and two simultaneous X-ray and UV/optical pointings with Swift, revealed a gradual fading of the source in the soft X-rays over the last 13 years, and a rather stable X-ray-to-optical flux ratio (F_X/F_V ~1.4-1.7). Based on all these properties, we identify this source with a magnetic CV, most probably of the intermediate polar type. The 2 hr spin period makes RX J2015.6+3711 the second slowest rotator of the class, after RX J0524+4244 ("Paloma", P_spin~2.3 hr). Although we cannot unambiguously establish the true orbital period with these observations, RX J2015.6+3711 appears to be a key system in the evolution of magnetic CVs.Comment: 11 pages, 8 figures, accepted for publication on MNRA