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Register a new userDiscovery of an Outbursting 12.8 Minute Ultracompact X-Ray Binary
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We report the discovery of OGLE-UCXB-01, a 12.8 minute variable object located in the central field of Galactic bulge globular cluster Djorg 2. The presence of frequent, short-duration brightenings at such an ultrashort period in long-term OGLE photometry together with the blue color of the object in Hubble Space Telescope images and the detection of moderately hard X-rays by Chandra observatory point to an ultracompact X-ray binary system. The observed fast period decrease makes the system a particularly interesting target for gravitational-wave detectors such as the planned Laser Interferometer Space Antenna.
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We report the detection of orbital modulation, a model solution, and X-ray properties of a newly discovered contact binary, 2MASS J11201034$-$2201340. We serendipitously found this X-ray point source outside the error ellipse when searching for possible X-ray counterparts of $gamma$-ray millisecond pulsars among the unidentified objects detected by the {it Fermi Gamma-ray Space Telescope}. The optical counterpart of the X-ray source (unrelated to the $gamma$-ray source) was then identified using archival databases. The long-term CRTS survey detected a precise signal with a period of $P=0.28876208(56)$ days. A follow-up observation made by the SLT telescope of Lulin Observatory revealed the binary nature of the object. Utilizing archived photometric data of multi-band surveys, we construct the spectral energy distribution, which is well fitted by a K2V spectral template. The fitting result of the orbital profile using the Wilson--Devinney code suggests that 2MASS J11201034-2201340 is a short-period A-type contact binary and the more massive component has a cool spot. The X-ray emission was first noted in observations made by swift, then further confirmed and characterized by an xmm observation. The X-ray spectrum can be described by a power law or thermal Bremsstrahlung. Unfortunately, we could not observe significant X-ray orbital modulation. Finally, according to the spectral energy distribution, this system is estimated to be 690 pc from Earth with a calculated X-ray intensity of $(0.7-1.5)times 10^{30}$ erg s$^{-1}$, which is in the expected range of an X-ray emitting contact binary.
We report multi-wavelength observations of the unidentified Fermi object 2FGL J1653.6-0159. With the help of high-resolution X-ray observation, we have identified an X-ray and optical counterpart of 2FGL J1653.6-0159. The source exhibits a periodic modulation of 75 min in optical and possibly also in X-ray. We suggest that 2FGL J1653.6-0159 is a compact binary system with an orbital period of 75 min. Combining the gamma-ray and X-ray properties, 2FGL J1653.6-0159 is potentially a black widow/redback type gamma-ray millisecond pulsar (MSP). The optical and X-ray lightcurve profiles show that the companion is mildly heated by the high-energy emission and the X-rays are from intrabinary shock. Although no radio pulsation has been detected yet, we estimated that the spin period of the MSP is ~2ms based on a theoretical model. If pulsation can be confirmed in the future, 2FGL J1653.6-0159 will become the first ultracompact rotation-powered MSP.
We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331-27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331-27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331-27 to be L1. The X-ray flare has an energy of E_X,F ~ 2x10^33 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Msec of XMM data we derive an upper limit of L_X,qui < 10^27 erg/s. The flare peak luminosity L_X,peak = 6.3x10^29 erg/s, flare duration tau_decay ~ 2400 s, and plasma temperature (~16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ~2100 K. However, the absence of any other flares above the detection threshold of E_X,F ~2.5x10^32 erg in a total of ~2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE ~ E^-2, suggesting that magnetic energy release in J0331-27 -- and possibly in all L dwarfs -- takes place predominantly in the form of giant flares.
We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just $8.8$ minutes, the second known eclipsing binary with an orbital period less than ten minutes. The system likely consists of two low-mass white dwarfs, and will merge in approximately 400,000 years to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its $6.91, rm min$ counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational wave sources detectable by the space-based gravitational-wave detector The Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISAs sensitivity. Based on its estimated distance of $d=2120^{+131}_{-115},rm pc$, LISA should detect the source within its first few months of operation, and should achieve a signal-to-noise ratio of $87pm5$ after four years. We find component masses of $M_A= 0.349^{+0.093}_{-0.074},M_odot$ and $M_B=0.384^{+0.114}_{-0.074},M_odot$, radii of $R_A=0.0308^{+0.0026}_{-0.0025},R_odot$ and $R_B = 0.0291^{+0.0032}_{-0.0024},R_odot$, and effective temperatures of $T_A=22200^{+1800}_{-1600},rm K$ and $T_B=16200^{+1200}_{-1000},rm K$. We determined all of these properties, and the distance to this system, using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint ($r>21 , m_{rm AB}$) gravitational-wave sources which the Vera Rubin Observatory (VRO) and LISA should identify.
During normal Type I outbursts, the pulse profiles of Be/X-ray binary pulsars are found to be complex in soft X-ray energy ranges. The profiles in soft X-ray energy ranges are characterized by the presence of narrow absorption dips or dip-like features at several pulse phases. However, in hard X-ray energy ranges, the pulse profiles are rather smooth and single-peaked. Pulse phase-averaged spectroscopy of the these pulsars had been carried out during Type I outbursts. The broad-band spectrum of these pulsars were well described by partial covering high energy cutoff power-law model with interstellar absorption and Iron K_alpha emission line at 6.4 keV. Phase-resolved spectroscopy revealed that the presence of additional matter at certain pulse phases that partially obscured the emitted radiation giving rise to dips in the pulse profiles. The additional absorption is understood to be taking place by matter in the accretion streams that are phase locked with the neutron star. Optical/infrared observations of the companion Be star during these Type I outbursts showed that the increase in the X-ray intensity of the pulsar is coupled with the decrease in the optical/infrared flux of the companion star. There are also several changes in the IR/optical emission line profiles during these X-ray outbursts. The X-ray properties of these pulsars during Type I outbursts and corresponding changes in optical/infrared wavebands are discussed in this paper.
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