Astrophysics

The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for Type Ia supernovae. The rate has been previously estimated in the range of ??0??00 yr ?? , either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller ( ????0 yr ?? ) than these estimates, even with the advent of all-sky optical time domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time domain survey. Operating in J -band ( ??.2 μ m) that is relatively immune to dust extinction, the extinction distribution of the PGIR sample is highly skewed to large extinction values ( >50 % of events obscured by A V ?? mag). Using recent estimates for the distribution of mass and dust in the Galaxy, we show that the observed extinction distribution of the PGIR sample is commensurate with that expected from dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null hypothesis probability <0.01 %), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a 3? time domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of ??46.0 +12.5 ??2.4 yr ?? . Our results suggest that all-sky near-infrared time-domain surveys are well poised to uncover the Galactic nova population.

We report detection of a line-like feature in the γ -ray spectrum of the blazar B0516 ??621, for which the data obtained with the Large Area Telescope onboard {\it Fermi Gamma-Ray Space Telescope (Fermi)} areanalyzed. The feature is at ??7\,GeV and cannot be explained with radiation processes generally considered for jet emission of blazars. Instead, it could be a signal due to the oscillations between photons and axion-like particles (ALPs) in the source's jet. We investigate this possibility by fitting the spectrum with the photon-ALP oscillation model, and find thatthe parameter space of ALP mass m a ??10 ?? \,eV and the coupling constant (between photons and ALPs) g aγ =1.17--1.49 ? 10 ??0 \,GeV ?? can provide a fit to the line-like feature, while the magnetic field at the emission site of γ -rays is fixed at 0.7\,G. The ranges for m a and g aγ are in tension with those previously obtained from several experiments or methods, but on the other hand in line with some of the others. This spectral-feature case and its possible indication for ALP existence could be checked from similar studies of other blazar systems and also suggest a direction of effort for building future high-energy facilities that would have high sensitivities and spectral resolutions for searching for similar features.

We present a unified model for X-ray quasi-periodic oscillations (QPOs) seen in Narrow-line Seyfert 1 (NLSy1) galaxies, γ -ray and optical band QPOs that are seen in Blazars. The origin of these QPOs is attributed to the plasma motion in corona or jets of these AGN. In the case of X-ray QPOs, we applied the general relativistic precession model for the two simultaneous QPOs seen in NLSy1 1H 0707-945 and deduce orbital parameters, such the radius of the emission region, and spin parameter a for a circular orbit; we obtained the Carter's constant Q , a , and the radius in the case of a spherical orbit solution. In other cases where only one X-ray QPO is seen, we localized the orbital parameters for NLSy1 galaxies REJ 1034+396, 2XMM J123103.2+110648, MS 2254.9-3712, Mrk 766, and MCG-06-30-15. By applying the lighthouse model, we found that a kinematic origin of the jet based γ -ray and optical QPOs, in a relativistic MHD framework, is possible. Based on the inbuilt Hamiltonian formulation with a power-law distribution in the orbital energy of the plasma consisting of only circular or spherical trajectories, we show that the resulting Fourier power spectral density (PSD) has a break corresponding to the energy at ISCO. Further, we derive connection formulae between the slopes in the PSD and that of the energy distribution. Overall, given the preliminary but promising results of these relativistic orbit models to match the observed QPO frequencies and PSD at diverse scales in the inner corona and the jet, it motivates us to build detailed models, including a transfer function for the energy spectrum in the corona and relativistic MHD jet models for plasma flow and its polarization properties.

We present a search for radio afterglows from long gamma-ray bursts using the Australian Square Kilometre Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, covering the entire celestial sphere south of declination + 41 ??, and three epochs of the Variables and Slow Transients Pilot Survey (Phase 1), covering ??,000 square degrees per epoch. The observations we used from these surveys spanned a nine-month period from 2019 April 21 to 2020 January 11. We crossmatched radio sources found in these surveys with 779 well-localised (to ??15 ?��?) long gamma-ray bursts occurring after 2004 and determined whether the associations were more likely afterglow- or host-related through the analysis of optical images. In our search, we detected one radio afterglow candidate associated with GRB 171205A, a local low-luminosity gamma-ray burst with a supernova counterpart SN 2017iuk, in an ASKAP observation 511 days post-burst. We confirmed this detection with further observations of the radio afterglow using the Australia Telescope Compact Array at 859 days and 884 days post-burst. Combining this data with archival data from early-time radio observations, we showed the evolution of the radio spectral energy distribution alone could reveal clear signatures of a wind-like circumburst medium for the burst. Finally, we derived semi-analytical estimates for the microphysical shock parameters of the burst: electron power-law index p=2.84 , normalised wind-density parameter A ??=3 , fractional energy in electrons ϵ e =0.3 , and fractional energy in magnetic fields ϵ B =0.0002 .

Most neutron stars are expected to be born in supernovae, but only about half of supernova remnants (SNRs) are associated with a compact object. In many cases, a supernova progenitor may have resulted in a black hole. However, there are several possible reasons why true pulsar-SNR associations may have been missed in previous surveys: The pulsar's radio beam may not be oriented towards us; the pulsar may be too faint to be detectable; or there may be an offset in the pulsar position caused by a kick. Our goal is to find new pulsars in SNRs and explore their possible association with the remnant. The search and selection of the remnants presented in this paper was inspired by the non-detection of any X-ray bright compact objects in these remnants when previously studied. Five SNRs were searched for radio pulsars with the Green Bank Telescope at 820 MHz with multiple pointings to cover the full spatial extent of the remnants. A periodicity search plus an acceleration search up to 500 m/s^2 and a single pulse search were performed for each pointing in order to detect potential isolated binary pulsars and single pulses, respectively. No new pulsars were detected in the survey. However, we were able to re-detect a known pulsar, PSR J2047+5029, near SNR G89.0+4.7. We were unable to detect the radio-quiet gamma-ray pulsar PSR J2021+4026, but we do find a flux density limit of 0.08 mJy. Our flux density limits make our survey two to 16 times more sensitive than previous surveys, while also covering the whole spatial extent of the same remnants. We discuss potential explanations for the non-detection of a pulsar in the studied SNRs and conclude that sensitivity is still the most likely factor responsible for the lack of pulsars in some remnants.

CTB 80 (G69.0+2.7) is a relatively old (50--80 kyr) supernova remnant (SNR) with a complex radio morphology showing three extended radio arms and a radio and X-ray nebula near the location of the pulsar PSR B1951+32. We report on a study of the GeV emission in the region of CTB 80 with \emph{Fermi}-LAT data. An extended source with a size of 1.3 ??, matching the size of the infrared shell associated to the SNR, was discovered. The GeV emission, detected up to an energy of ??0 GeV, is more significant at the location of the northern radio arm where previous observations imply that the SNR shock is interacting with ambient material. Both hadronic and leptonic scenarios can reproduce the multiwavelength data reasonably well. The hadronic cosmic ray energy density required is considerably larger than the local Galactic value and the gamma-ray leptonic emission is mainly due to bremsstrahlung interactions. We conclude that GeV particles are still trapped or accelerated by the SNR producing the observed high-energy emission when interacting with ambient material.

MAXI J1820+070 is a newly discovered transient black hole X-ray binary, which showed several spectral and temporal features. In this work, we analyse the broadband X-ray spectra from all three simultaneously observing X-ray instruments onboard AstroSat, as well as contemporaneous X-ray spectra from NuSTAR, observed during the hard state of MAXI J1820+070 in March 2018. Implementing a combination of multi-colour disc model, relativistic blurred reflection model relxilllpCp and a distant reflection in the form of xillverCp, we achieve reasonable and consistent fits for AstroSat and NuSTAR spectra. The best-fit model suggests a low temperature disc ( k T in ∼0.3 keV), iron overabundance ( A Fe ∼4−5 solar), a short lamp-post corona height ( h≲8 R g ), and a high corona temperature ( k T e ∼115−150 keV). Addition of a second Comptonisation component leads to a significantly better fit, with the k T e of the second Comptonisation component being ∼14−18 keV. Our results from independent observations with two different satellites in a similar source state indicate an inhomogeneous corona, with decreasing temperature attributed to increasing height. Besides, utilising the broader energy coverage of AstroSat, we estimate the black hole mass to be 6.7−13.9 M ⊙ , consistent with independent measurements reported in the literature.

We study the structure of the accretion flow in the hard state of the black-hole X-ray binary MAXI~J1820+070 with \nicer data. The power spectra show broadband variability which can be fit with four Lorentzian components peaking at different time scales. Extracting power spectra as a function of energy enables the energy spectra of these different power spectral components to be reconstructed. We found significant spectral differences among Lorentzians, with the one corresponding to the shortest variability time scales displaying the hardest spectrum. Both the variability spectra and the time-averaged spectrum are well-modelled by a disc blackbody and thermal Comptonization, but the presence of (at least) two Comptonization zones with different temperatures and optical depths is required. The disc blackbody component is highly variable, but only in the variability components peaking at the longest time scales ($\gsim1$ s). The seed photons for the spectrally harder zone come predominantly from the softer Comptonization zone. Our results require the accretion flow in this source to be structured, and cannot be described by a single Comptonization region upscattering disc blackbody photons, and reflection from the disc.

GRS 1915+105 is a transient black hole X-ray binary consistently emitting 10-100% of the Eddington luminosity in the X-ray band during the last three decades until mid-2018 when the source luminosity suddenly decreased by an order of magnitude. This phase was followed by a change to a state with even lower average X-ray fluxes never seen before during the outburst but presenting renewed flaring activity at different wavelengths. Nevertheless, the mean fluxes were still in decline. GRS 1915+105 has the longest orbital period known among low-mass X-ray binaries, the largest accretion disk size, and therefore the largest mass supply for accretion. The high inclination of the disk allows the study of geometrical effects of the accretion flow such as changes in the height-to-radius ratio or the effect of accretion disk winds on the intrinsic emission that are expected during the outburst decay. In addition, the transient jet is expected to change to a compact, self-absorbed, steady jet. We conducted two full polarization Atacama Large Millimeter Array observations to study the jet properties during the outburst decay by analyzing the spectral, polarization, and intra-epoch variability for both observation epochs. In addition, we analyzed nearly daily Neutron Star Interior Composition Explorer pointing observations consisting of modeling X-ray power spectral densities, spectral energy distributions, and lightcurves with a physically motivated model to follow the changing accretion disk properties throughout the outburst decay and relating them to the jet emission. We show that the X-ray and mm spectral, timing, and polarization properties are consistent with those of a typical decaying X-ray binary outburst and that GRS 1915+105 has descended into the low-luminosity hard X-ray state.

We present the discovery that ASASSN-14ko is a periodically flaring AGN at the center of the galaxy ESO 253-G003. At the time of its discovery by the All-Sky Automated Survey for Supernovae (ASAS-SN), it was classified as a supernova close to the nucleus. The subsequent six years of V- and g-band ASAS-SN observations reveal that ASASSN-14ko has nuclear flares occurring at regular intervals. The seventeen observed outbursts show evidence of a decreasing period over time, with a mean period of P 0 =114.2±0.4 days and a period derivative of P ˙ =−0.0017±0.0003 . The most recent outburst in May 2020, which took place as predicted, exhibited spectroscopic changes during the rise and a had a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). The X-ray flux decreased by a factor of 4 at the beginning of the outburst and then returned to its quiescent flux after ~8 days. TESS observed an outburst during Sectors 4-6, revealing a rise time of 5.60±0.05 days in the optical and a decline that is best fit with an exponential model. We discuss several possible scenarios to explain ASASSN-14ko's periodic outbursts, but currently favor a repeated partial TDE. The next outbursts should peak in the optical on UT 2020-09-7.4 ± 1.1 and UT 2020-12-26.5 ± 1.4.