Dr. Zahir Ahmad Shah

Ph.D. University of Kashmir (2018)

Thesis: Understanding the Spectral and Temporal Behaviour of High Energy Blazars

Supervisor: Prof. Naseer Iqbal  |  Co-supervisor: Dr. Sunder Sahayanathan

• M.Phil., University of Kashmir (2014)
• M.Sc. Physics, University of Kashmir (2011)
• B.Sc., University of Kashmir (2009)

CSIR-UGC NET: AIR 15

Positions:

• INSPIRE Faculty, CUK (2021–Present)
• Postdoctoral Fellow, IUCAA (2018–2021)

High-Energy Astrophysics & Blazar Physics

• Relativistic jet physics in blazars (FSRQs and BL Lac objects)
• Broadband spectral energy distribution (SED) modelling from radio to VHE gamma-rays
• Temporal variability analysis: flux distributions, hardness ratios, spectral parameter correlations
• High-energy emission mechanisms: leptonic, hadronic, and lepto-hadronic frameworks
• Identification of hadronic contributions via photo-meson interactions in individual blazars
• Quasi-periodic oscillations (QPOs) as probes of jet geometry and precession

Theoretical & Computational Modelling

• Fokker–Planck kinetic equation-based particle evolution (numerical and analytical)
• Convolved broadband SED model development for XSPEC and PyXSPEC
• Time-dependent spectral modelling: linking spectral parameter lags to acceleration and cooling timescales
• Pivot energy analysis and log-normal variability diagnostics
• Statistical methods: Bayesian Blocks, Power Spectral Density, Lomb–Scargle periodograms

Very High Energy (VHE) & Cosmological Studies

• Extragalactic Background Light (EBL) opacity constraints through VHE spectral index correlations
• Redshift estimation for featureless BL Lac objects via Γ_VHE–z correlation
• Identification of VHE FSRQ candidates for MACE and CTAO observations
• EBL model comparison: Franceschini, Inoue, Gilmore, and Dominguez frameworks

Observational Facilities & Software

• Space Telescopes: Fermi-LAT, Swift (XRT/UVOT/BAT), NuSTAR, AstroSat (SXT/LAXPC/CZTI), NICER, XMM-Newton
• Ground-based: TACTIC, MACE
• Analysis Software: XSPEC, PyXSPEC, Fermi Science Tools, HEASoft
• Programming: Python, FORTRAN, Matplotlib, Astropy

📊 Blazar Broadband SED Convolution Code

Numerical modeling of broadband spectral energy distributions (SEDs) of blazars using user-defined particle distributions and synchrotron, SSC, and EC cooling. Implemented as a compiled Cython/PyXSPEC additive model (ssccon) enabling statistically rigorous chi-squared fitting within XSPEC.

Key features: Broken power-law injection • synchrotron + SSC + EC emission • PyXSPEC model interface • multi-group simultaneous fitting (UVOT + XRT + Fermi-LAT)

🔗 View on GitHub → BLAZAR-BROADBAND-SED-CONVOLUTION-CODE

⏳ Time-Dependent One-Zone Leptonic Blazar SED Model

A time-dependent, Fokker–Planck-based broadband SED fitting code for blazars. The electron distribution N(γ, t) is evolved kinetically via the FP equation and convolved with the PyXSPEC sscconv radiation kernel, enabling time-resolved SED fitting with simultaneous UVOT + XRT + Fermi-LAT data.

Fokker–Planck equation: ∂N/∂t  =  ∂/∂γ[b(γ)N − D(γ)∂N/∂γ]  −  N/T_esc  +  Q(γ, t)

• Numerical scheme: Strang operator splitting (2nd-order) — method of characteristics (cooling) + implicit Crank–Nicolson (diffusion) + exponential integration (injection/escape)
• PyXSPEC model chain: tbabs * sscconv * fp_td_conv * bknpower — full absorption + SSC/EC radiation + FP time evolution
• Fitting: 3-stage XSPEC migrad pipeline — norm only → + injection shape → + full jet parameters (Γ, B, θ, log₁₀R, t_obs)
• Optional physics: stochastic acceleration D(γ) = D₀γ^q, self-consistent IC cooling update

Files: fp_td.pyx — Cython FP solver & PyXSPEC convolution model  •  fit_broadband.py — 3-stage SED fitting script  •  setup_fp_td.py — build script

🔗 View on GitHub → BLAZAR-TIMEDEPENDENT-SED-FITTING   📄 README & Documentation

📈 BLAZAR-TIMEDEPENDENT-SED-LC-FITTING 🚧 Currently Being Developed

An extension of the time-dependent FP model that simultaneously fits a broadband SED and a Fermi-LAT gamma-ray light curve during blazar flares. The electron distribution is evolved over multiple days with a time-varying injection profile, enabling joint modelling of both spectral shape and flux variability.

• Flare profile: Asymmetric exponential rise & decay — Q₀(t) = Q_base + Q_peak · exp(∓Δt / t_rise,decay)
• Fitting: 4-stage lmfit Nelder–Mead pipeline — SED-only → LC-only → joint fast → joint full resolution
• Data: Simultaneous UVOT + XRT + Fermi-LAT SED & Fermi-LAT daily light curve
• Resolution modes: Fast (N_g=100, γ_max=10⁶, ~0.3 s/eval) and Full (N_g=200, γ_max=10⁷, ~5 s/eval)

Files: fokker_planck_td.py — FP flare solver  •  flare_sed_models.py — PyXSPEC model wrapper & FP cache  •  fit_flare.py — 4-stage joint fitting pipeline

🔗 View on GitHub → BLAZAR-TIMEDEPENDENT-SED-FITTING

• DST INSPIRE Faculty Fellowship: ₹77.40 Lakh + ₹35 Lakh Research Grant
• Co-I: ISRO AstroSat Data Utilization Project: ₹18.37 Lakh

• PI: Spectral Breaks & Thermal Emission of High-z FSRQs (A03-77)
• PI: Broadband SED of BL Lacs (A04-094)
• PI: FSRQ PMNJ1923-2104 (T03-018)
• Co-PI: Mrk 421, Mrk 180, PKS 0352-686

42 publications including 35+ refereed journal articles in PRD, MNRAS, ApJ, JHEAP, ApJS, RAA, New Astronomy and others.   Google Scholar  |  ORCID

1. Sajad Ahanger, Shah Zahir et al. "Modelling the Time-variable Broadband Emission and Correlation Study of FSRQ S5 1044+71", The Astrophysical Journal, accepted 2026.

2. Nazir Zeeshan, Dar Sikandar, Shah Zahir et al. "Broadband Variability Analysis of FSRQ PKS 0402-362 with Indications of Quasi-Periodic Modulation", The Astrophysical Journal, 2026, Vol. 998, No. 2. [IF: 5.4]

3. Shah Zahir et al. "Comprehensive variability analysis of blazars using Fermi light curves across multiple timescales", Physical Review D, 2025, 111, 123052. DOI [IF: 5.3]

4. Peer Anjum, Dar Athar A, Shah Zahir et al. "Multiwavelength spectral and temporal analysis of VHE Blazar 1ES 1959+650: Tracing emission mechanisms across flux states", MNRAS, 2025. DOI [IF: 4.8]

5. Sikandar A., Shah Zahir et al. "Indication for dual periodic signatures in PKS 0805-07 from multi-technique time series analysis", Physical Review D, 2025. DOI [IF: 5.3]

6. Dar Athar, Shah Zahir et al. "Unraveling the broadband spectrum of B2 1308+326: A clue to its high energy emission mechanism", New Astronomy, 121 (2025) 102463. DOI

7. Shah Zahir et al. "Multi-wavelength variability and broadband SED modeling of BL Lac during a bright flaring period MJD 59000–59943", MNRAS, 2024, 527, 5140–5154. [IF: 4.8]

8. Malik Zahoor, Akbar Sikandar, Shah Zahir et al. "Statistical Insights into Flux and Photon Index Distributions of VHE FSRQs from Fermi-LAT Observations", MNRAS, 2025, 539, 3, 2185–2201. DOI [IF: 4.8]

9. Tantry Javaid, Sharma Ajay, Shah Zahir, Iqbal Naseer, Bose Debanjan. "Study of multiwavelength variability, emission mechanism and quasi-periodic oscillation for transition blazar S5 1803+784", JHEAP, Vol. 47, July 2025, 100372. [IF: 10.2]

10. Ahanger Sajad, Sahayanathan Sunder, Sitha K. Jagan, Shah Zahir, Naseer Iqbal. "Multiwavelength analysis of FSRQ B2 1348+30B: Constraints on the jet power", JHEAP, 2025. [IF: 10.2]

11. Sikandar A., Shah Zahir et al. "Probing spectral evolution and intrinsic variability of Mkn 421: A multi-epoch AstroSat study of X-ray spectra", JHEAP, 2025, 45, 438–455. [IF: 10.2]

12. Shah Zahir et al. "AstroSat observation of the HBL 1ES 1959+650 during its October 2017 flaring", MNRAS, 2022, 504, 5485–5495. [IF: 4.8]

13. Shah Zahir et al. "Unveiling the broadband spectral and temporal properties of PKS 0903-57 during its brightest flare", MNRAS, 2021, 504, 416–427. [IF: 4.8]

14. Tantry J., Shah Zahir et al. "Probing broadband spectral energy distribution and variability of Mrk 501 in the low flux state", JHEAP, 2024, 44, 393–409. [IF: 10.2]

15. Sikandar A., Shah Zahir et al. "Insights into the Long-Term Flaring Events of Blazar PKS 0805-07: A Multi-Wavelength Analysis over the 2009–2023 Period", The Astrophysical Journal, 2024, 977, 111. [IF: 4.8]

16. Shah Zahir et al. "On the determination of log-normal flux distributions for astrophysical systems", MNRAS, 2020, Vol. 496, Issue 3, 3348–3357. [IF: 4.8]

17. Shah Zahir et al. "Study on temporal and spectral behavior of 3C 279 during 2018 January flare", MNRAS, 2019, 484, 3168–3179. [IF: 4.8]

18. Manzoor A., Shah Zahir et al. "Broadband spectral and temporal study of Ton 599 during the brightest January 2023 flare", MNRAS, 2024. [IF: 4.8]

19. Shah Zahir et al. "Log-normal flux distribution of bright Fermi blazars", Research in Astronomy and Astrophysics (RAA), 2018, 18, 11, 141. [IF: 1.8]

20. Shah Zahir et al. "Clues on high-energy emission mechanism from blazar 3C 454.3 during 2015 August flare", MNRAS, 2017, Vol. 470, Issue 3, 3283–3299. [IF: 4.8]

21. Khatoon R., Shah Zahir et al. "Study of long-term flux and photon index distributions of blazars using RXTE observations", MNRAS, 2020, 491, 2. [IF: 4.8]

22. Khatoon R., Shah Zahir et al. "Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift-XRT data", MNRAS, 2022, 515, 3. [IF: 4.8]

23. Rajput B., Shah Zahir et al. "Correlation between optical and γ-ray flux variations in BL Lacs", MNRAS, 2021. [IF: 4.8]

24. Deka K., Shah Zahir et al. "The long-term X-ray flux distribution of Cygnus X-1 using RXTE-ASM and MAXI observations", JHEAP, 31, 2021, 23–30. [IF: 10.2]

25. Hota J., Shah Zahir et al. "Understanding the X-ray spectral curvature of MKN 421 using broadband AstroSat observations", MNRAS, 2021. [IF: 4.8]

26. M. Zahoor, Shah Zahir et al. "Multiwavelength study of blazar 4C+01.02 during its long-term flaring activity in 2014–2017", MNRAS, 2022, 514, 4259–4269. [IF: 4.8]

27. Dar A., Shah Zahir et al. "Can FSRQ 3C 345 be a very high energy blazar candidate?", MNRAS, 2024, 527, 10575–10583. [IF: 4.8]

28. M. Zahoor, Sahayanathan, Shah Zahir et al. "Model-independent Redshift Estimation of BL Lac Objects through VHE Observations", MNRAS, 2021. [IF: 4.8]

29. M. Zahoor, Sahayanathan, Shah Zahir et al. "Very-high-energy flat spectral radio quasar candidates", MNRAS, 2022, 515, 3, 4505–4513. [IF: 4.8]

30. Xiongfei G., ..., Shah Zahir et al. "Exploring γ-Ray Flares in the Long-term Light Curves of CTA 102 at GeV Energies", The Astrophysical Journal Supplement Series, 2022, 260, 48. [IF: 8.8]

31. Manzoor A., Shah Zahir et al. "Understanding the very high energy γ-ray excess in nearby blazars using leptonic model", MNRAS, 2023, 525, 3, 3533–3540. [IF: 4.8]

32. Thekkoth A., Sahayanathan, Shah Zahir et al. "Understanding the broad-band emission process of 3C 279 through long term spectral analysis", MNRAS, 2023, 526, 4, 6364–6380. [IF: 4.8]

33. M. Showkat, I. Naseer, Shah Zahir. "Search of Astrophysical results with LIGO from the Science runs S1 to S5", Journal of Advances in Physics, 6(3).

34. Khatoon R., Prince R., Shah Zahir et al. "Temporal and spectral study of PKS 0208-512 during the 2019–2020 flare", MNRAS, 2022, 513, 611–623. [IF: 4.8]

35. Khan S., H. Abdullah, Shah Zahir et al. "The Fluctuation Theory, Critical Phenomena and Gravitational Clustering of Galaxies", Journal of Astrophysics and Astronomy, 2025, accepted.

36. Khatoon R., Shah Zahir et al. "Flux Distribution Study of Mkn 421 with SPOL, RXTE and Fermi-LAT Telescopes", Springer Proceedings in Physics. DOI

37. Deka K., Shah Zahir et al. "A study of the effects of histogram binning on the accuracy of finding flux distribution of X-ray binaries", Materials Today: Proceedings, Vol. 65, Part 5, 2022, 2862–2864.

38. Deka K., Shah Zahir et al. "Distribution of X-ray flux: RXTE-PCA Observation of Cygnus X-1", Materials Today: Proceedings, Vol. 65, Part 5, 2022.

39. Deka K., Shah Zahir et al. "Distribution of X-ray flux: RXTE-PCA observation of Cygnus X-1", Springer International Conference on Trends in Modern Physics, 26–27 February 2021.

40. Shah Zahir et al. "Understanding the evolution of X-ray spectral properties of blazars using broad-band AstroSat observations", 44th COSPAR Scientific Assembly, Vol. 44, Jul. 2022, p. 2243.

41. J. Hota, Shah Zahir et al. "X-ray Spectral study of MKN 421 using AstroSat observation", 44th COSPAR Scientific Assembly, Vol. 44, Jul. 2022, p. 2245.

42. R. Khatoon, R. Misra, Zahir Shah, J. Hota, R. Gogoi, A. Charan Pradhan. "Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift-XRT data", 44th COSPAR Scientific Assembly, Vol. 44, Jul. 2022, p. 2075.

• Quantum Mechanics
• Relativistic Quantum Mechanics
• Mathematical Physics

17 oral presentations including invited talks, seminars, and resource person roles  |  2 poster presentations  |  10 participations in national and international workshops and schools.

CONTRIBUTED TALK  |  January 2026

International Conference on A Decade of AstroSat Observations: Science Outcomes and Future Prospects
30 January – 1 February 2026
Talk: "Time Resolved X-ray Spectral Study of Blazars with AstroSat: Probing Underlying Particle Distribution"

INVITED SEMINAR  |  January 2026

IIA Astrophysics Seminar, Indian Institute of Astrophysics
27 January 2026
Talk: "Broadband Views of Blazars: Spectral and Temporal Probes of Relativistic Jets"

RESOURCE PERSON  |  February–March 2024

AstroSat & XPoSat Workshop 2024
28 February – 03 March 2024, Providence Women's College, Kozhikode
Participated as resource person.

RESOURCE PERSON  |  January 2023

Advanced AstroSat Data Analysis Workshop
9–15 January 2023
Participated as resource person.

INVITED TALK & SESSION CHAIR  |  September 2023

Himalayan Meet of Astronomers 2023
25–26 September 2023, IUST, Awantipora
Talk: "Understanding Broadband Emission from Blazars"
Also chaired one scientific session on blazars.

INVITED TALK & SESSION CHAIR

4th National Conference on High Energy Emission from Active Galactic Nuclei
Talk: "Temporal Behavior of Blazar Light Curves"
Also chaired one scientific session on blazars.

INVITED TALK  |  December 2021

Introductory Workshop in Astronomy and Astrophysics
18–19 December 2021
Talk: "Active Galaxies — Brightest Objects in the Universe"

INVITED TALK  |  December 2021

2nd China-India Workshop on High Energy Astrophysics
6–10 December 2021
Talk: "Understanding the Evolution of X-ray Spectral Properties of Blazars using Broadband AstroSat Observations"

PRESENTATION  |  2021

IUCAA Annual Seminars by Post-Doctoral Fellows
2021
Talk: "Broadband Spectral and Temporal Study of Blazars"

INVITED TALK  |  July 2021

One-Day Online Meeting on Theoretical Aspects of High Energy Emission from AGN
06 July 2021
Talk: "Time Resolved Spectral Study of 1ES 1959+650 using AstroSat Observations"

TALK  |  April 2021

Astrophysical Jets and Observational Facilities: National Perspective
05–09 April 2021
Talk: "AstroSat Observation of the HBL 1ES 1959+650 during its October 2017 Flaring"

INVITED SEMINAR  |  October 2019

Astronomy Group Seminar Programme, University of Southampton
14 October 2019
Talk: "Flux Distribution Study of Fermi Bright Blazars and Simulated Light Curves"

TALK  |  January 2019

Last Friday Talk, IUCAA
25 January 2019
Talk: "Multiwavelength Study of 3C 279 during 2018 January Flare"

TALK  |  April 2019

4th National Conference on Recent Trends in the Study of Compact Objects (RETCO-IV)
17–20 April 2019, IUCAA, Pune
Talk: "Study on Temporal and Spectral Behaviour of 3C 279 during 2018 January Flare"

TALK  |  November 2017

High Energy Emission from Active Galactic Nuclei — 3rd National Conference
28–30 November 2017, University of Calicut, Kerala

TALK  |  October 2017

Growth Points in Physics (GPP-2017) — UGC Sponsored National Seminar
21–23 October 2017, University of Kashmir, Srinagar

TALK  |  May 2016

Astronomical Society of India (ASI) — 34th Annual Meeting
10–13 May 2016, University of Kashmir, Srinagar

POSTER  |  September 2017

Young Astronomers Meet (YAM) 2017
11–15 September 2017, IUCAA, Pune

POSTER  |  February 2019

IUCAA Science Day Foundation
28 February 2019, IUCAA, Pune

• AstroSat Calibration Meeting, IUCAA, Pune — August 2022
• Online International Workshop on VHE Data Analysis using Open Source Packages, University of Kashmir — February 2022
• AstroSat Data Analysis Workshop, IUCAA — November 2017
• AstroSat AO Proposal Submission Workshop, IUCAA — July 2016
• Extragalactic Relativistic Jets: Cause and Effect, ICTS Bangalore — October 2015
• Radio Astronomy School, National Centre for Radio Astronomy (NCRA), Pune — August 2013
• Astronomical Techniques and Science with Virtual Observatories, University of Kashmir & IUCAA — September 2013
• High Energy Emission from Active Galactic Nuclei, University of Kashmir, BRNS BARC & ISRO — October 2013
• Stellar Astrophysics, University of Kashmir & IUCAA — October 2011
• High Energy Astrophysics Winter School, ApSD BARC, Mount Abu, Rajasthan — January–February 2011

Email: zahir.shah@cukashmir.ac.in

Phone: +91-7780864526

Address: Department of Physics, Central University of Kashmir, Ganderbal-192101, J&K, India