Simultaneous Multi-wavelength Data Research Articles

The innermost jet in the hidden ultra-luminous X-ray source Cygnus X-3

ABSTRACT Cygnus X-3 is a high-mass X-ray binary with a compact object accreting matter from a Wolf–Rayet donor star. Recently, it has been revealed by the Imaging X-ray Polarimetry Explorer (IXPE) as a hidden Galactic ultra-luminous X-ray (ULX) source with a luminosity above the Eddington limit, along the direction of a narrow (opening angle $\lesssim 32^{\circ }$) funnel. In between the IXPE observations, we observed Cyg X-3 with the European very long baseline interferometry (VLBI) Network at 22 GHz and the NICER X-ray instrument. To probe possible relations between the X-ray funnel and the potential radio jet from the ULX, we analysed the simultaneous multiwavelength data. Our high-resolution VLBI image reveals an elongated structure with a position angle of −3.2 ± 0.4 °, accurately perpendicular to the direction of the linear X-ray polarization. Because Cyg X-3 was in the radio quiescent state on 2022 November 10, we identify the mas-scale structure as the innermost radio jet. The finding indicates that the radio jet propagates along and within the funnel. Moreover, the jet is marginally resolved in the transverse direction. This possibly results from the strong stellar winds and the rapid orbital motion of the binary system.

Long-term Swift and Metsähovi monitoring of SDSS J164100.10+345452.7 reveals multi-wavelength correlated variability

We report on the first multi-wavelength Swift monitoring campaign performed on SDSS J164100.10+345452.7, a nearby narrow-line Seyfert 1 galaxy that had formerly been considered to be radio-quiet. It has, however, more recently been detected both in the radio (at 37 GHz) and in the γ-ray, a behaviour that hints at the presence of a relativistic jet. During our 20-month Swift campaign, while pursuing the primary goal of assessing the baseline optical/UV and X-ray properties of SDSS J164100.10+345452.7, we observed two radio flaring episodes, namely, one each year. Our strictly simultaneous multi-wavelength data closely match the radio flare and allow us to unambiguously link the jetted radio emission of SDSS J164100.10+345452.7. Indeed, for the X-ray spectra preceding and following the radio flare, a simple absorbed power-law model does not offer an adequate description and, thus, an extra absorption component is required. The average spectrum of SDSS J164100.10+345452.7 can best be described by an absorbed power-law model with a photon index Γ = 1.93 ± 0.12, modified by a partially covering neutral absorber with a covering fraction of f = 0.91−0.03+0.02. On the contrary, the X-ray spectrum closest to the radio flare does not require any such extra absorber and it is much harder (Γflare ∼ 0.7 ± 0.4), thus implying the emergence of an additional, harder spectral component. We interpret this as the jet emission emerging from a gap in the absorber. The fractional variability we derived in the optical/UV and X-ray bands is found to be lower than the typical values reported in the literature because our observations of SDSS J164100.10+345452.7 are dominated by the source being in a low state, as opposed to the literature, where the observations were generally taken as a follow-up of bright flares in other energy bands. Based on the assumption that the origin of the 37 GHz radio flare is the emergence of a jet from an obscuring screen also observed in the X-ray, the derived total jet power is Pjettot = 3.5 × 1042 erg s−1. This result is close to the lowest values measured in the literature.

Multiwavelength study of the quiescent states of six brightest flat-spectrum radio quasars detected by Fermi-LAT

ABSTRACT The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area Telescope) since past 12 yr indicated six sources who exhibited extreme γ-ray outbursts crossing daily flux of 10−5 photons cm−2 s−1. We obtained nearly simultaneous multiwavelength data of these sources in radio to γ-ray waveband from OVRO (Owens Valley Radio Observatory), Steward Observatory, SMARTS (Small and Moderate Aperture Research Telescope System), Swift-UVOT (UV/Optical Telescope), Swift-XRT (X-ray Telescope), and Fermi-LAT. The time-averaged broad-band spectral energy distributions (SEDs) of these sources in quiescent states were studied to get an idea about the underlying baseline radiation processes. We modelled the SEDs using one-zone leptonic synchrotron and inverse Compton emission scenario from broken power-law electron energy distribution inside a spherical plasma blob, relativistically moving down a conical jet. The model takes into account inverse Compton scattering of externally and locally originated seed photons in the jet. The big blue bumps visible in quiescent state SEDs helped to estimate the accretion disc luminosities and central black hole masses. We found a correlation between the magnetic field inside the emission region and the ratio of emission region distance to disc luminosity, which implies that the magnetic field decreases with an increase in emission region distance and decrease in disc luminosity, suggesting a disc–jet connection. The high-energy index of the electron distribution was also found to be correlated with observed γ-ray luminosity as γ-rays are produced by high-energy particles. In most cases, kinetic power carried by electrons can account for jet radiation power as jets become radiatively inefficient during quiescent states.

Differential rotation of the solar corona: A new data-adaptive multiwavelength approach

Context. The characterization of the differential rotation of the extended corona is still lacking conclusive results about the actual rotation rate profiles, and it is also expected to vary along the solar cycle. While some studies supported the quasi-rigidity of coronal rotation, others have found evidence of differential rotation to occur. Aims. For the purpose of investigating the differential rotation of the solar corona, we analyzed ultraviolet (UV) spectral line observations acquired on both the east and west limbs at 1.7 R⊙ by the Ultraviolet Coronagraph Spectrometer (UVCS) on-board the Solar and Heliospheric Observatory during the solar minimum preceding solar cycle 23. To obtain a reliable and statistically robust picture of the rotational profile, we used a set of simultaneous 400 day long spectral line intensities of five different spectral lines: O VI 1032 Å, O VI 1037 Å, Si XII 499 Å, Si XII 521 Å, and H I Lyα 1216 Å, which are routinely observed by UVCS. Methods. The data were analyzed by means of two different techniques: the generalized Lomb-Scargle periodogram and a multivariate data-adaptive technique called multichannel singular spectrum analysis. Among many other positive outcomes, this latter method is unique in its ability to recognize common oscillatory modes between the five time series observed at both limbs. Results. The latitudinal rotation profile obtained in this work emphasizes that the low-latitude region of the UV corona (about ±20° from the solar equator) exhibits differential rotation, while the higher-latitude structures do rotate quasi-rigidly. Moreover, in contrast to previous results obtained using only O VI 1032 Å data over a 365 day time interval during solar minimum activity, the alleged north-south rotational asymmetry of the UV corona, if existent, is much less pronounced. Conclusions. The differential rotation rate of the solar corona as evinced at low-latitudes is consistent with the rotational profile of the near-surface convective zone of the Sun, suggesting that the rotation of the corona at 1.7 R⊙ is linked to intermediate-scale magnetic bipole structures anchored near 0.99 R⊙. The quasi-rigid rotation rate found at mid and high latitudes is instead attributed to the influence of large-scale coronal structures linked to the rigidly rotating coronal holes. We further suggest that the methodology presented in this paper could represent a milestone for future investigations on differential rotation rates when dealing with simultaneous multiwavelength data.

A new lepto-hadronic model applied to the first simultaneous multiwavelength data set for Cygnus X–1

ABSTRACTCygnus X–1 is the first Galactic source confirmed to host an accreting black hole. It has been detected across the entire electromagnetic spectrum from radio to GeV gamma-rays. The source’s radio through mid-infrared radiation is thought to originate from the relativistic jets. The observed high degree of linear polarization in the MeV X-rays suggests that the relativistic jets dominate in this regime as well, whereas a hot accretion flow dominates the soft X-ray band. The origin of the GeV non-thermal emission is still debated, with both leptonic and hadronic scenarios deemed to be viable. In this work, we present results from a new semi-analytical, multizone jet model applied to the broad-band spectral energy distribution of Cygnus X–1 for both leptonic and hadronic scenarios. We try to break this degeneracy by fitting the first-ever high-quality, simultaneous multiwavelength data set obtained from the CHOCBOX campaign (Cygnus X–1 Hard state Observations of a Complete Binary Orbit in X-rays). Our model parametrizes dynamical properties, such as the jet velocity profile, the magnetic field, and the energy density. Moreover, the model combines these dynamical properties with a self-consistent radiative transfer calculation including secondary cascades, both of leptonic and hadronic origin. We conclude that sensitive TeV gamma-ray telescopes like Cherenkov Telescope Array (CTA) will definitively answer the question of whether hadronic processes occur inside the relativistic jets of Cygnus X–1.

Gamma-Ray Flares in the Long-term Light Curve of 3C 454.3

Abstract 3C 454.3 is frequently observed in the flaring state. The long-term light curve of this source has been analyzed with 9 yr (2008 August–2017 July) of data from the Fermi-LAT detector. We have identified five flares and one quiescent state. The flares have substructures with many peaks during the flaring phase. We have estimated the rise and decay time of the flares and compared with flares of other similar sources. The modeling of gamma-ray spectral energy distributions shows in most cases that a log-parabola function gives the best fit to the data. We have done time-dependent leptonic modeling of two of the flares, for which simultaneous multiwavelength data are available. These two long-lasting flares, Flare-2A and Flare-2D, continued for 95 and 133 days, respectively. We have used the average values of Doppler factor, injected luminosity in electrons, size of the emission region, and the magnetic field in the emission region in modeling these flares. The emission region is assumed to be in the broad-line region in our single-zone model. The energy losses (synchrotron, synchrotron self-Compton, external Compton) and escape of electrons from the emission region have been included while doing the modeling. Although the total jet powers required to model these flares with the leptonic model are higher compared to other sources, they are always found to be lower than the Eddington luminosity of 3C 454.3. We also select some flaring peaks and show that the time variation of the Doppler factor or the injected luminosity in electrons over short timescales can explain their light curves.

Unravelling the unusually curved X-ray spectrum of RGB J0710 + 591 using AstroSat observations

ABSTRACT We report the analysis of simultaneous multiwavelength data of the high-energy-peaked blazar RGB J0710 + 591 from the Large Area X-ray Proportional Counters, Soft X-ray focusing Telescope, and Ultraviolet Imaging Telescope (UVIT) instruments onboard AstroSat. The wide band X-ray spectrum (0.35–30 keV) is modelled as synchrotron emission from a non-thermal distribution of high-energy electrons. The spectrum is unusually curved, with a curvature parameter βp ∼ 6.4 for a log parabola particle distribution, or a high-energy spectral index p2 > 4.5 for a broken power-law distribution. The spectrum shows more curvature than an earlier quasi-simultaneous analysis of Swift–XRT/NuSTAR data where the parameters were βp ∼ 2.2 or p2 ∼ 4. It has long been known that a power-law electron distribution can be produced from a region where particles are accelerated under Fermi process and the radiative losses in acceleration site decide the maximum attainable Lorentz factor, γmax. Consequently, this quantity decides the energy at which the spectrum curves steeply. We show that such a distribution provides a more natural explanation for the AstroSat data as well as the earlier XRT/NuSTAR observation, making this as the first well-constrained determination of the photon energy corresponding to γmax. This in turn provides an estimate of the acceleration time-scale as a function of magnetic field and Doppler factor. The UVIT observations are consistent with earlier optical/UV measurements and reconfirm that they plausibly correspond to a different radiative component than the one responsible for the X-ray emission.

A Search of TESS Full-frame Images for a Simultaneous Optical Counterpart to FRB 181228

Abstract FRB 181228 was detected by the Molonglo Synthesis Radio Telescope (MOST) at a position and time coincident with Transiting Exoplanet Survey Satellite (TESS) observations, representing the first simultaneous multiwavelength data collection for a fast radio burst (FRB). The large imaged field of view of TESS allows a search over the uncertainty region produced by MOST. However, the TESS pixel scale of 21″ and the full-frame image (FFI) cadence of 30 minutes is not optimal for the detection of a fast optical burst (FOB) with a possible millisecond duration. We search the TESS FFIs and find no events with a limiting TESS magnitude of 16, assuming a 30 minute event duration, corresponding to an optical flux density upper limit of approximately 2000 Jy for a ∼1 ms signal duration, assuming no signal loss. In addition, the cosmic-ray mitigation method for TESS significantly reduces its sensitivity to short-timescale transients, which we quantify. We compare our results to the predictions of Yang et al. and find that the upper limit is a factor of 2000 higher than the predicted maximum optical flux density. However, we find that if FRB 181228 had occurred in the galaxy thought to host the nearest FRB detection to date (37 Mpc), an FOB may have been detectable by TESS. In the near future, when the Canadian Hydrogen Intensity Mapping Experiment and Australian SKA Pathfinder will detect hundreds to thousands of FRBs, TESS may be able to detect FOBs from those rare bright and nearby FRBs within this large population (if more sophisticated cosmic-ray excision can be implemented).

TeV γ-ray emission from Mrk 421 observed with TACTIC during December 2014–February 2015

We observed the TeV blazar Mrk421 in the very high energy (VHE) region with TACTIC from 21 December, 2014 to 21 February, 2015. The VHE γ-ray signal is detected from the source at a statistical significance of ∼ 9σ with 648 ± 74 γ-ray like events using a total observation time of 65.6 h of clean data. To denote the active period of the source, the entire observation has been divided into two spells: Spell I (21–30 December, 2014; MJD: 57012–57021) and Spell II (15 January-21 February, 2015; MJD: 57037–57074). The Spell I corresponds to relatively higher activity state wherein 337 ± 47 γ-rays are detected at a statistical significance of 7.39σ in 21.7 h. During Spell II, 311 ± 57 γ-ray events have been detected in ∼ 44c at a statistical significance of 5.6σ. The analysis of high energy (HE) γ-ray data from Fermi-LAT has been carried out for the period of TACTIC observations. In addition, near simultaneous multi-wavelength data from Swift-BAT and MAXI in the X-ray region, SPOL in optical V-band and OVRO at 15 GHz radio band have also been considered. The HE γ-ray and optical observations indicate variability during both the spells whereas hard X-ray observations are consistent with the average emission through out the TACTIC observations. The broad-band spectral energy distributions of the source are reproduced by single zone homogeneous synchrotron self Compton process for the two spells. The model parameters estimated for the two spells are similar except for the particle energy density which is obtained to be slightly higher during Spell I.

Spectra and fast multi-wavelength variability of compact jets powered by internal shocks

AbstractThe emission of steady compact jets observed in the hard spectral state of X-ray binaries is likely to be powered by internal shocks caused by fluctuations of the outflow velocity. The dynamics of the internal shocks and the resulting spectral energy distribution (SED) of the jet is very sensitive to the shape of the Power Spectral Density (PSD) of the fluctuations of the jet Lorentz factor. It turns out that Lorentz factor fluctuations injected at the base of the jet with a flicker noise power spectrum (i.e. P(f) ∝1/f) naturally produce the canonical flat SED observed from radio to IR band in X-ray binary systems in the hard state. This model also predicts a strong, wavelength dependent, variability that resembles the observed one. In particular, strong sub-second variability is predicted in the infrared and optical bands. The assumed fluctuations of the jet Lorentz factor are likely to be triggered by the variability of the accretion flow which is best traced by the X-ray emission. In the case of GX339-4 for which high quality and simultaneous multi-wavelength data are available, we performed simulations assuming that the fluctuation of the jet Lorentz factor have the same PSD as the observed X-ray PSD. The synthetic SED calculated under this assumption provides a remarkable match to the observed radio to IR SED. In this case the model also produces strong mid-infrared spectral variability that is comparable to that reported in this source.

Multi-wavelength study of Mrk 421 TeV flare observed with TACTIC telescope in February 2010

We present results from multi-wavelength study of intense flaring activity from a high frequency peaked BL Lac object Mrk 421. The source was observed in its flaring state on February 16, 2010 with the TACTIC at energies above 1.5TeV. Near simultaneous multi-wavelength data were obtained from high energy (MeV–GeV) γ-ray observations with Fermi–LAT, X-ray observations by the Swift and MAXI satellites, optical V-band observation by SPOL at Steward Observatory and radio 15GHz observation at OVRO 40 meter-telescope. We have performed a detailed spectral and temporal analysis of TACTIC, Fermi–LAT and Swift–XRT observations of Mrk 421 during February 10–23, 2010 (MJD 55237–55250). The flaring activity of the source is studied by investigating the properties of daily light curves from radio to TeV energy range and we present the correlation and variability analysis in each energy band. The TeV flare detected by TACTIC on February 16, 2010 is well correlated with the activity in lower energy bands. The differential energy spectrum of the source, in the energy range 1.5–11TeV, as measured by TACTIC on this night is described by a power law (dN/dE∝E-Γ) with spectral index Γ=2.6±0.3. After accounting for absorption of TeV photons by low energy extragalactic background light photons via pair production, the intrinsic TeV spectrum reveals a power law index of 2.3±0.3. Finally the broad band spectral energy distribution of the source in flaring state is reproduced using a simple emission model involving synchrotron and synchrotron self Compton processes. The obtained parameters are then used to understand the energetics of the source during the flaring episode.

Intrinsic VHE gamma-ray spectra of blazars as a probe for extragalactic background light

Very high energy (VHE) γ-rays above 10’s of GeV energy, emitted from distant blazars, are attenuated by photons from the extragalactic background light (EBL). Unfortunately, neither the EBL nor the intrinsic blazar spectrum is accurately known to derive one quantity from the other. In this work we use a homogeneous one zone model involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) emission mechanisms to estimate the intrinsic VHE spectra of blazars. The model is applied on three VHE blazars, namely PKS2155-304, RGB J0710+591 and 3C 279, for which simultaneous multi-wavelength data are available from various observations. The predicted values of the intrinsic VHE fluxes are then compared with the observations by imaging atmospheric Cherenkov telescopes to determine the optical depth of VHE γ-rays. On comparing these optical depth values with those predicted by four different EBL models, we observe a somewhat pronounced systematic deviation for PKS2155-304 and 3C 279 at higher energies, especially for the EBL model proposed by Finke et al. (2010). We attribute this deviation to be an outcome of either the failure of the extrapolation of blazar SED to VHE energies and/or due to various assumptions buried in the EBL models.

Discovery of VHEγ-rays from the blazar 1ES 1215+303 with the MAGIC telescopes and simultaneous multi-wavelength observations

Context. We present the discovery of very high energy (VHE, E > 100GeV) gamma-ray emission from the BL Lac object 1ES 1215+303 by the MAGIC telescopes and simultaneous multi-wavelength data in a broad energy range from radio to gamma-rays. Aims. We study the VHE gamma-ray emission from 1ES 1215+303 and its relation to the emissions in other wavelengths. Methods. Triggered by an optical outburst, MAGIC observed the source in January-February 2011 for 20.3 hrs. The target was monitored in the optical R-band by the KVA telescope that also performed optical polarization measurements. We triggered target of opportunity observations with the Swift satellite and obtained simultaneous and quasi-simultaneous data from the Fermi Large Area Telescope and from the Mets\"ahovi radio telescope. We also present the analysis of older MAGIC data taken in 2010. Results. The MAGIC observations of 1ES 1215+303 carried out in January-February 2011 resulted in the first detection of the source at VHE with a statistical significance of 9.4 sigma. Simultaneously, the source was observed in a high optical and X-ray state. In 2010 the source was observed in a lower state in optical, X-ray, and VHE, while the GeV gamma-ray flux and the radio flux were comparable in 2010 and 2011. The spectral energy distribution obtained with the 2011 data can be modeled with a simple one zone SSC model, but it requires extreme values for the Doppler factor or the electron energy distribution.

Mrk 421 active state in 2008: the MAGIC view, simultaneous multi-wavelength observations and SSC model constrained

Context: The blazar Markarian 421 is one of the brightest TeV gamma-ray sources of the northern sky. From December 2007 until June 2008 it was intensively observed in the very high energy (VHE, E > 100 GeV) band by the single-dish Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC-I). Aims: We aimed to measure the physical parameters of the emitting region of the blazar jet during active states. Methods: We performed a dense monitoring of the source in VHE with MAGIC-I, and also collected complementary data in soft X-rays and optical-UV bands; then, we modeled the spectral energy distributions (SED) derived from simultaneous multi-wavelength data within the synchrotron self--compton (SSC) framework. Results: The source showed intense and prolonged gamma-ray activity during the whole period, with integral fluxes (E > 200 GeV) seldom below the level of the Crab Nebula, and up to 3.6 times this value. Eight datasets of simultaneous optical-UV (KVA, Swift/UVOT), soft X-ray (Swift/XRT) and MAGIC-I VHE data were obtained during different outburst phases. The data constrain the physical parameters of the jet, once the spectral energy distributions obtained are interpreted within the framework of a single-zone SSC leptonic model. Conclusions: The main outcome of the study is that within the homogeneous model high Doppler factors (40 <= delta <= 80) are needed to reproduce the observed SED; but this model cannot explain the observed short time-scale variability, while it can be argued that inhomogeneous models could allow for less extreme Doppler factors, more intense magnetic fields and shorter electron cooling times compatible with hour or sub-hour scale variability.

Discovery of VHEγ-ray emission from the BL Lacertae object B3 2247+381 with the MAGIC telescopes

We study the non-thermal jet emission of the BL Lac object B3 2247+381 during a high optical state. The MAGIC telescopes observed the source during 13 nights between September 30th and October 30th 2010, collecting a total of 14.2 hours of good quality very high energy (VHE) $\gamma$-ray data. Simultaneous multiwavelength data was obtained with X-ray observations by the Swift satellite and optical R-band observations at the KVA-telescope. We also use high energy $\gamma$-ray (HE, 0.1 GeV-100 GeV) data from the Fermi satellite. The BL Lac object B3 2247+381 (z=0.119) was detected, for the first time, at VHE $\gamma$-rays at a statistical significance of 5.6 $\sigma$. A soft VHE spectrum with a photon index of -3.2 $\pm$ 0.6 was determined. No significant short term flux variations were found. We model the spectral energy distribution using a one-zone SSC-model, which can successfully describe our data.

Evidence for synchrotron bubbles from GRS 1915+105

We present GMRT observations of the Galactic microquasar GRS 1915+105 at 1.28 GHz for 8 days from 2001 June 18 to July 1. We have seen several isolated radio flares of varying magnitudes (20–50 mJy) and durations (6–35 min) and we model them as due to adiabatically expanding synchrotron emitting clouds (synchrotron bubbles) ejected from the accretion disk. By applying this model, we provide a new method to estimate the electron power-law index p, hence the spectral index, from single frequency radio observations. This method does not require correction for the optical time delay effects which may be important in the case of optically thick radio emission. Using our estimated value of p and simultaneous multiwavelength data from literature, we have calculated the time of ejection of the synchrotron plasma and the time delays at different observed frequencies. Our estimates are in good agreement with the observed time delays.