Detection of Gamma-rays from Protostellar Jet in the HH 80-81 System

Considering that the existence of relativistic particles in the protostellar jet has been confirmed by the detection of linearly polarized radioemission from the HH 80-81 jet, we search for gamma-rays from the HH 80-81 system using ten-year {\it Fermi}-LAT observations.A significant point-like $\gamma$-ray excess is found in the direction of the HH 80-81 system with Test-Statistic (TS) value $>$100, which is likely produced in the HH 80-81 jet. The $\gamma$-ray spectrum extends only to 1 GeV with a photon index of 3.5.No significant variability is found in the gamma-ray emission.It is discussed that the properties of HH 80-81 jet suffice for producing the observed $\gamma$-rays.

Precise Timing and Phase-resolved Spectroscopy of the Young Pulsar J1617–5055 with NuSTAR

We report on a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the young, energetic pulsar PSR J1617–5055. Parkes Observatory 3 GHz radio observations of the pulsar (taken about 7 yr before the NuSTAR observations) are also reported here. NuSTAR detected pulsations at a frequency of f ≈ 14.4 Hz (P ≈ 69.44 ms) and, in addition, the observation was long enough to measure the source’s frequency derivative, f ̇ ≈ − 2.8 × 10 − 11 Hz s−1. We find that the pulsar shows one peak per period at both hard X-ray and radio wavelengths, but that the hard X-ray pulse is broader (having a duty cycle of ∼0.7), than the radio pulse (having a duty cycle of ∼0.08). Additionally, the radio pulse is strongly linearly polarized. J1617's phase-integrated hard X-ray spectrum is well fit by an absorbed power-law model, with a photon index Γ = 1.59 ± 0.02. The hard X-ray pulsations are well described by three Fourier harmonics, and have a pulsed fraction that increases with energy. We also fit the phase-resolved NuSTAR spectra with an absorbed power-law model in five phase bins and find that the photon index varies with phase from Γ = 1.52 ± 0.03 at phases around the flux maximum to Γ = 1.79 ± 0.06 around the flux minimum. Last, we compare our results with other pulsars whose magnetospheric emission is detected at hard X-ray energies and find that, similar to previous studies, J1617's hard X-ray properties are more similar to the MeV pulsars than the GeV pulsars.

Detection of the Microquasar V404 Cygni at γ-Rays Revisited: Short Flaring Events in Quiescence

The microquasar V404 Cygni (also known as GS 2023+338) was previously reported to have weak GeV γ-ray emission in subday time periods during its 2015 outburst. In order to provide more detailed information at the high energy range for this black hole binary system, we conduct analysis on the data obtained with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi). Both the LAT database and source catalog used are the latest versions. In addition to the previously reported detection at the peak of the 2015 outburst, we find a possible detection (∼4σ) of the source during the 3 day time period of 2015 August 17–19 (at the end of the 2015 outburst) and one convincing detection (≃7σ) in 2016 August 23–25. The latter high-significance detection shows that the γ-ray emission of the source is soft with photon index Γ ∼ 2.9. As γ-ray emission from microquasars is considered to be associated with their jet activity, we discuss the results by comparing with those well-studied cases, namely, Cyg X-3 and Cyg X-1. The detection helps identify V404 Cygni as a microquasar with detectable γ-ray emission in its quiescent state, and adds interesting features to the microquasar group, or in a more general context to X-ray binaries with jets.

X-Ray Emission of the γ-ray-loud Young Radio Galaxy NGC 3894

The radio source 1146+596 is hosted by the elliptical/S0 galaxy NGC 3894, with a low-luminosity active nucleus. The radio structure is compact, suggesting a very young age for the jets in the system. Recently, the source has been confirmed as a high-energy (HE; >0.1 GeV) γ-ray emitter in the most recent accumulation of Fermi Large Area Telescope data. Here we report on the analysis of the archival Chandra X-ray Observatory data for the central part of the galaxy, consisting of a single 40 ks long exposure. We have found that the core spectrum is best fitted by a combination of an ionized thermal plasma with a temperature of ≃0.8 keV, and a moderately absorbed power-law component (photon index Γ = 1.4 ± 0.4, hydrogen column density N H/1022 cm−2 = 2.4 ± 0.7). We have also detected the iron Kα line at 6.5 ± 0.1 keV, with a large equivalent width of 1.0 − 0.5 + 0.9 keV. Based on the simulations of Chandra's point-spread function, we have concluded that while the soft thermal component is extended on the scale of the galaxy host, the hard X-ray emission within the narrow photon energy range 6.0–7.0 keV originates within the unresolved core (effectively the central kiloparsec radius). The line is therefore indicative of the X-ray reflection from a cold neutral gas in the central regions of NGC 3894. We discuss the implications of our findings in the context of the X-ray Baldwin effect. NGC 3894 is the first young radio galaxy detected in HE γ-rays with the iron Kα line.

Revealing the nature of the transient source MAXI J0637-430 through spectro-temporal analysis

We study the spectral and temporal properties of MAXI J0637-430 during its 2019-2020 outburst using NICER, AstroSat and Swift-XRT data. The source was in a disc dominant state within a day of its detection and traces out a ‘c’ shaped profile in the HID, similar to the ‘mini’-outbursts of the recurrent BHB 4U 1630-472. Energy spectrum is obtained in the 0.5 − 10 keV band with NICER and Swift-XRT, and 0.5 − 25 keV with AstroSat. The spectra can be modelled using a multicolour disc emission (diskbb) convolved with a thermal Comptonisation component (thcomp). The disc temperature decreases from 0.6 keV to 0.1 keV during the decay with a corresponding decrease in photon index (Γ) from 4.6 to 1.8. The fraction of Compton scattered photons (fcov) remains < 0.3 during the decay upto mid-January 2020 and gradually increases to 1 as the source reaches hard state. Power Density Spectra (PDS) generated in the 0.01-100 Hz range display no Quasi-periodic Oscillations (QPOs) although band-limited noise (BLN) is seen towards the end of January 2020. During AstroSat observations, Γ lies in the range 2.3 − 2.6 and rms increases from 11 to 20%, suggesting that the source was in an intermediate state till 21 November 2019. Spectral fitting with the relativistic disc model (kerrbb), in conjunction with the soft-hard transition luminosity, favour a black hole with mass 3 − 19 M⊙ with retrograde spin at a distance <15 kpc. Finally, we discuss the possible implications of our findings.

The Low-Energy Spectral Index of Gamma-Ray Burst Prompt Emission from Internal Shocks

The prompt emission of most gamma-ray bursts (GRBs) typically exhibits a non-thermal Band component. The synchrotron radiation in the popular internal shock model is generally put forward to explain such a non-thermal component. However, the low-energy photon index α∼−1.5 predicted by the synchrotron radiation is inconsistent with the observed value α∼−1. Here, we investigate the evolution of a magnetic field during propagation of internal shocks within an ultrarelativistic outflow, and revisit the fast cooling of shock-accelerated electrons via synchrotron radiation for this evolutional magnetic field. We find that the magnetic field is first nearly constant and then decays as B′∝t−1, which leads to a reasonable range of the low-energy photon index, −3/2<α<−2/3. In addition, if a rising electron injection rate during a GRB is introduced, we find that α reaches −2/3 more easily. We thus fit the prompt emission spectra of GRB 080916c and GRB 080825c.

Probing the circumnuclear environment of NGC 1275 with High-Resolution X-ray spectroscopy

NGC 1275 is the Brightest Cluster Galaxy (BCG) in the Perseus cluster and hosts the active galactic nucleus (AGN) that is heating the central 100 kpc of the intracluster medium (ICM) atmosphere via a regulated feedback loop. Here we use a deep (490 ks) Cycle-19 Chandra High-Energy Transmission Grating (HETG) observation of NGC 1275 to study the anatomy of this AGN. The X-ray continuum is adequately described by an unabsorbed power-law with photon index Γ ≈ 1.9, creating strong tension with the detected column of molecular gas seen via HCN and HCO+ line absorption against the parsec-scale core/jet. This tension is resolved if we permit a composite X-ray source; allowing a column of $N_H\sim 8\times 10^{22}\hbox{${\rm \, cm}^{-2}\, $}$ to cover ∼15 per cent of the X-ray emitter does produce a significant improvement in the statistical quality of the spectral fit. We suggest that the dominant unabsorbed component corresponds to the accretion disk corona, and the sub-dominant X-ray component is the jet working surface and/or jet cocoon that is expanding into clumpy molecular gas. We suggest that this may be a common occurence in BCG-AGN. We conduct a search for photoionized absorbers/winds and fail to detect such a component, ruling out columns and ionization parameters often seen in many other Seyfert galaxies. We detect the 6.4 keV iron-Kα fluorescence line seen previously by XMM-Newton and Hitomi. We describe an analysis methodology which combines dispersive HETG spectra, non-dispersive microcalorimeter spectra, and sensitive XMM-Newton/EPIC spectra in order to constrain (sub)arcsec-scale extensions of the iron-Kα emission region.

NuSTAR and Swift Observations of the Extragalactic Black Hole X-ray Binaries

We present the results obtained from detailed spectral and timing studies of extra-galactic black hole X-ray binaries LMC X–1 and LMC X–3, using simultaneous observations with Nuclear Spectroscopic Telescope Array (NuSTAR) and Neil Gehrels Swift observatories. The combined spectra in the 0.5 − 30 keV energy range, obtained between 2014 and 2019, are investigated for both sources. We do not find any noticeable variability in 0.5 − 30 keV light curves, with 0.1 − 10 Hz fractional rms estimated to be &lt;2 per cent. No evidence of quasi-periodic oscillations is found in the power density spectra. The sources are found to be in the high soft state during the observations with disc temperature Tin ∼ 1 keV, photon index, Γ &gt; 2.5 and thermal emission fraction, fdisc &gt; 80 per cent. An Fe Kα emission line is detected in the spectra of LMC X–1, though no such feature is observed in the spectra of LMC X–3. From the spectral modelling, the spins of the black holes in LMC X–1 and LMC X–3 are estimated to be in the range of 0.92 − 0.95 and 0.19 − 0.29, respectively. The accretion efficiency is found to be, η ∼ 0.13 and η ∼ 0.04 for LMC X–1 and LMC X–3, respectively.

Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow

Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from extragalactic sources followed by fading afterglow emission, are associated with stellar core collapse events. We report the detection of very-high-energy (VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours after the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The low luminosity and redshift of GRB 190829A reduce both internal and external absorption, allowing determination of its intrinsic energy spectrum. Between energies of 0.18 and 3.3 tera–electron volts, this spectrum is described by a power law with photon index of 2.07 ± 0.09, similar to the x-ray spectrum. The x-ray and VHE gamma-ray light curves also show similar decay profiles. These similar characteristics in the x-ray and gamma-ray bands challenge GRB afterglow emission scenarios.

Multi-Wavelength Monitoring and Reverberation Mapping of a Changing Look Event in the Seyfert Galaxy NGC 3516

We present the results of photometric and spectroscopic monitoring campaigns of the changing look AGN NGC 3516 carried out in 2018 to 2020 covering the wavelength range from the X-ray to the optical. The facilities included the telescopes of the CMO SAI MSU, the 2.3-m WIRO telescope, and the XRT and UVOT of Swift. We found that NGC 3516 brightened to a high state and could be classified as Sy1.5 during the late spring of 2020. We have measured time delays in the responses of the Balmer and He II λ4686 lines to continuum variations. In the case of the best-characterized broad Hβ line, the delay to continuum variability is about 17 days in the blue wing and is clearly shorter, 9 days, in the red, which is suggestive of inflow. As the broad lines strengthened, the blue side came to dominate the Balmer lines, resulting in very asymmetric profiles with blueshifted peaks during this high state. During the outburst the X-ray flux reached its maximum on 1 April 2020 and it was the highest value ever observed for NGC 3516 by the Swift observatory. The X-ray hard photon index became softer, ∼1.8 in the maximum on 21 Apr 2020 compared to the mean ∼0.7 during earlier epochs before 2020. We have found that the UV and optical variations correlated well (with a small time delay of 1–2 days) with the X-ray until the beginning of April 2020, but later, until the end of Jun. 2020, these variations were not correlated. We suggest that this fact may be a consequence of partial obscuration by Compton-thick clouds crossing the line of sight.