K.K. Singh

The XMM–Newton view of the radio-loud narrow-line Seyfert 1 galaxy PMN J0948+0022

ABSTRACT We analysed the archival XMM-Newton data of the radio loud narrow lineSeyfert1 galaxy PMN J0948+0022 in the energy range 0.3–10.0 keV. The X-ray data reveal that the spectrum in the 0.3–10.0 keV energy band is not asimple power-law as previously described in the literature. Instead it consistsof a power-law with soft excess below 2.5 keV. The X-ray spectrum was fit-ted with four different models and it was shown the soft excess component ofthe spectrum in the 0.3–2.5 keV energy range could be described reasonablywell within the framework of the thermal Comptonization model as well asrelativistically blurred reflection model. The power-law component requiredto fit the spectrum beyond 2.5 keV was found to be rather hard compared tothe ones observed in other unobscured active galactic nuclei. It is also shownthat the Swift/XRT spectrum from the source could not reveal the soft excesscomponent due to poor statistics. The fractional variability estimated fromXMM-Newton data indicates the presence of independently varying compo-nents in the spectrum above and below 1 keV.Keywords: extragalactic : Seyfert1 – X-ray – individual:PMN J0948+0022

Multiwavelength study of VHE emission from Markarian 501 using TACTIC observations during April–May, 2012

Abstract We have observed Markarian 501 in Very High Energy (VHE) gamma-ray wavelength band for 70.6 h from 15 April to 30 May, 2012 using TACTIC telescope. Detailed analysis of ∼66.3 h of clean data revealed the presence of a TeV γ-ray signal (686± 77 γ-ray events) from the source direction with a statistical significance of 8.89σ above 850xa0 GeV. Further, a total of 375 ± 47 γ-ray like events were detected in 25.2 h of observation from 22 – 27 May, 2012 with a statistical significance of 8.05σ indicating that the source has possibly switched over to a relatively high gamma-ray emission state. We have derived time-averaged differential energy spectrum of the state in the energy range 850xa0 GeV - 17.24xa0TeV which fits well with a power law function of the form d F / d E = f 0 E − Γ with f 0 = ( 2.27 ± 0.38 ) × 10 − 11 photons cm − 2 s − 1 TeV − 1 and Γ = 2.57 ± 0.15 . In order to investigate the source state, we have also used almost simultaneous multiwavelength observations viz: high energy data collected by Fermi -LAT, X-ray data collected by Swift -XRT and MAXI, optical and UV data collected by Swift -UVOT, and radio data collected by OVRO, and reconstructed broad-band Spectral Energy Distribution (SED). The obtained SED supports leptonic model (homogeneous single zone) for VHE gamma-ray emission involving synchrotron and synchrotron self Compton (SSC) processes.

CFD simulations to study the effects of wall protrusions on microfluidic mixing

In this study the effects of different types of wall protrusions on microfluidic mixing are studied using computational fluid dynamics (CFD) simulations. Two new protrusions, single first bracket protrusions and double opposite first bracket protrusions (DOFBPs), are conceptualized, evaluated through CFD simulations and compared to protrusions having standard geometrical shapes, e.g. rectangular protrusions, triangular protrusions and semicircular protrusions. In the range of Reynolds numbers covered in this study, the microchannel having an opposed T-junction and DOFBPs is found to provide good mixing. A hybrid approach relying on the modification of microfluidic junctions as well as wall protrusions for enhancing microfluidic mixing is also evaluated. The microchannel based on the hybrid approach of an OA 10°-20°-165° WY-junction and DOFBPs is also found to provide very good mixing for a wide range of Reynolds numbers.

A time dependent approach to model X-ray and γ–ray light curves of Mrk 421 observed during the flare in February 2010

Abstract Strong X-ray and γ–ray flares have been detected in February 2010 from the high synchrotron peaked blazar Mrk 421 (zxa0=xa00.031). With the motivation of understanding the physics involved in this flaring activity, we study the variability of the source in X-ray and γ–ray energy bands during the period February 10–23, 2010 (MJD 55237–55250). We use near simultaneous X-ray data collected by MAXI, Swift-XRT and γ–ray data collected by Fermi-LAT and TACTIC along with the optical V-band observations by SPOLnat Steward Observatory. We observe that the variation in the one day averaged flux from the source during the flare is characterized by fast rise and slow decay. Besides, the TeV γ–ray flux shows a strong correlation with the X-ray flux, suggesting the former to be an outcome of synchrotron self Compton emission process. To model the observed X-ray and γ–ray light curves, we numerically solve the kinetic equation describing the evolution of particle distribution in the emission region. The injection of particle distribution into the emission region, from the putative acceleration region, is assumed to be a time dependent power law. The synchrotron and synchrotron self Compton emission from the evolving particle distribution in the emission region are used to reproduce the X-ray and γ–ray flares successfully. Our study suggests that the flaring activity of Mrk 421 can be an outcome of an efficient acceleration process associated with the increase in underlying non-thermal particle distribution.

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

Abstract 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 J0710xa0+xa0591 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.

Search for TeV γ-ray emission from blazar 1ES1218+304 with TACTIC telescope during March–April 2013

Abstract In this paper, we present results of TeV γ-ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (zxa0=xa00.182) with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are primarily motivated by the unusually hard GeV–TeV spectrum of the source despite its relatively large redshift. The source is observed in the TeV energy range with the TACTIC from March 1, 2013 to April 15, 2013 (MJD 56352–56397) for a total observation time of 39.62xa0h and no evidence of TeV γ-ray activity is found from the source. The corresponding 99% confidence level upper limit on the integral flux above a threshold energy of 1.1xa0TeV is estimated to be 3.41xa0 × 10 - 12 xa0photonsxa0cm−2xa0s−1 (i.e.

Study of short term enhanced TeV γ- ray emission from Mrk 421 observed with TACTIC on December 28, 2014

Abstract In this work, we report on the detection of enhanced TeV γ- ray emission from the high synchrotron-peaked blazar Mrk 421 with the TACTIC telescope on the night of December 28, 2014 (MJD 57019). We use data from the TACTIC observations of Mrk 421 for one week during December 25–31, 2014 (MJD 57016–57022) in this study. The TACTIC observation on December 28, 2014 (MJD 57019) alone results in the detection of 86u202f±u202f17 γ- ray like events from Mrk 421 with a statistical significance of 5.17σ in a livetime of u202f∼u202f 2.2 hours above an energy threshold of 0.85 TeV. The high statistics (higher than three Crab Units) of TeV photons enables us to study the very high energy (VHE) γ- ray emission from the source at shorter timescales. A minimum variability timescale of u202f∼u202f 0.72 days is obtained for the TeV γ- ray emission from Mrk 421 during the above flaring activity of the source. The intrinsic VHE spectrum is described by a power law with spectral index of 2.99u202f±u202f0.38 in the energy range 0.85–8.5 TeV. The integral VHE γ- ray flux above 0.85 TeV is determined to be ( 3.68 ± 0.64 ) × 10 − 11 phxa0cm − 2 xa0s − 1 from the TACTIC observations of Mrk 421 on the night of December 28, 2014 (MJD 57019). Near simultaneous measurements by the HAWC observatory give an integral flux of ( 2.90 ± 0.40 ) × 10 − 11 phxa0cm − 2 xa0s − 1 above 2 TeV from Mrk 421 observations on December 29, 2014 (MJD 57020.33–57020.58). We have also analysed the contemporaneous data from Fermi-LAT to study the high energy (HE) γ–ray emission during the high activity state of the source. The HE γ–ray emission is observed to be increasing after the TeV flaring activity detected with the TACTIC. We also use other near simultaneous archival data available from the Swift-BAT in hard X-rays and from SPOL at Steward Observatory in optical V and R bands to characterize the multi-wavelength emission of Mrk 421 during the high activity state observed at TeV energies. The TeV γ- ray emission observed on December 28, 2014 (MJD 57019) is found to be more prominent than the emissions in lower energy bands during the same period. The TeV γ- ray observation of Mrk 421 in high activity state with the TACTIC telescope is also used to understand the physical mechanism for blazar emission under the frame work of the leptonic single zone synchrotron self Compton process.

Multi-wavelength study of the short term TeV flaring activity from the blazar Mrk 501 observed in June 2014

Abstract In this work, we study the short term flaring activity from the high synchrotron peaked blazar Mrk 501 detected by the FACT and H.E.S.S. telescopes in the energy range 2–20 TeV during June 23–24, 2014 (MJD 56831.86–56831.94). We revisit this major TeV flare of the source in the context of near simultaneous multi-wavelength observations of γ –rays in MeV-GeV regime with Fermi-LAT, soft X-rays in 0.3–10 keV range with Swift-XRT, hard X-rays in 10–20 keV and 15–50 keV bands with MAXI and Swift-BAT respectively, UV-Optical with Swift-UVOT and 15 GHz radio with OVRO telescope. We have performed a detailed temporal and spectral analysis of the data from Fermi-LAT, Swift-XRT and Swift-UVOT during the period June 15–30, 2014 (MJD 56823–56838). Near simultaneous archival data available from Swift-BAT, MAXI and OVRO telescope along with the V-band optical polarization measurements from SPOL observatory are also used in the study of giant TeV flare of Mrk 501 detected by the FACT and H.E.S.S. telescopes. No significant change in the multi-wavelength emission from radio to high energy γ –rays during the TeV flaring activity of Mrk 501 is observed except variation in soft X-rays. The varying soft X-ray emission is found to be correlated with the γ –ray emission at TeV energies during the flaring activity of the source. The soft X-ray photon spectral index is observed to be anti-correlated with the integral flux showing harder-when-brighter behavior. An average value of 4.5 % for V-band optical polarization is obtained during the above period whereas the corresponding electric vector position angle changes significantly. We have used the minimum variability timescale from the H.E.S.S. observations to estimate the Doppler factor of the emission region which is found to be consistent with the previous studies of the source.