I am currently an Einstein fellow at MIT. At present, I am working on multi-wavelength spectro-timing analysis of powerful astrophysical transients. Thse include stellar tidal disruption flares, rapidly-evolving optical transients, fast radio bursts, gamma ray bursts, and merging compact objects. For a list of recent publications see here

2015 Sept 21: "Evidence for High-Frequency QPOs with a 3:2 Frequency Ratio From a 5000 Solar Masses Black Hole" published online in ApJ Letters
2014 Sep 02: Stanford Tea Talk, Stanford University, USA
Talk Title "A stable, 3:2 frequency ratio quasi-periodic oscillation from ULX M82 X-1"
2014 Aug 29: Space Sciences Laboratory (SSL) colloquium, UC Berkeley, USA
Talk Title "A near-Eddington 400 solar mass black hole in M82"
2014 Aug 28: SRL Sack lunch talk, Caltech, Pasadena, USA
Talk title "A near-Eddington 400 solar mass black hole in M82"
2014 Aug 25-28: iPTF Summer School August 2014, Caltech, Pasadena, USA
2014 Aug 17-21: High-Energy Astrophysics Division (HEAD) Meeting 2014, Chicago, USA
Poster titled Discovery of 3:2 Ratio High-Frequency QPOs from a ULX: Evidence for a 400 solar mass black hole
2014 Aug 17: "A 400 solar mass Black Hole in M82" published online in Nature
2014 June 27: Ph.D. Thesis defense "X-ray Timing and Spectral Variability as Probes of Ultraluminous X-ray Sources"

2) A 400 solar mass black hole in M82 accreting close to its Eddington limit

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A subset of stellar-mass black holes exhibit the so-called high-frequency quasi-periodic oscillations (QPOs). Often in these systems the high-frequency QPOs occur in pairs of two, with their frequencies in a 3:2 ratio. The power spectra of three such systems showing the twin pair QPOs are shown below.

The respective timescales of these oscillations (~0.01 seconds: 100-450 Hz) are comparable to the Keplerain orbital periods near the innermost stable circular orbit (ISCO) of these black holes (see the equation below). For example, for a non-rotating black hole weighing 10 solar masses the Keplerian frequency at ISCO is 220 Hz. In addition, for a given source, these frequencies appear to be stable to within a few percent for changes in the source luminosity. The fact that their frequencies are stable and appear to be originating from close to the ISCO suggests that they are tied to the mass of the black hole. The physical argument for high-frequency QPO -- mass dependence is as follows:

Some recent results on X-ray timing of stellar-mass and supermassive black holes suggest that supermassive black holes behave as scaled-up stellar-mass black hole systems. Under this black hole unification paradigm (e.g., McHardy et al. 2006), if intermediate-mass black holes exist, some of them should exhibit these 3:2 pairs but at frequencies scaled down (longer timescales) according to their black hole masses.

The Rossi X-ray Timing Explorer (RXTE) monitored the field of view containing the brightest X-ray point source in M82, X-1 for roughly five and a half years. By constructing an average power density spectrum (frequency range of 0.1-16 Hz) using all the X-ray (3-13 keV) data, we found evidence (>4.7σ) for 3:2 frequency ratio oscillations (3.32±0.06 and 5.07±0.06 Hz) that were stable on a few years timescale. The five and a half year average power spectrum is shown below (left). The right panel shows the average power spectrum using the first one year data alone.

We argue that these two oscillations are the analogs of the high-frequency X-ray oscillations of stellar-mass black holes. Assuming a inverse mass scaling, we estimate the mass of the black hole in M82 X-1 to be 428±105 heavier than our sun.
As described earlier, a key signature of high-frequency QPOs is their stability. In order to demonstrate this, we extracted a dynamic power spectrum of M82, which tracks the average power spectrum of the source as a function of time. The signal to noise improves as the movie progresses as more data is being added. This is shown below.

For full details of this work, please see the article here.

1) Can the 62 day X-ray Period of M82 X-1 be due to a Precessing Accretion Disk?

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X-ray monitoring of neutron star and black hole binaries has revealed a plethora of periodic phenomena. Most of these modulations can be classified into two categories: orbital and super-orbital. Any phenomenon associated with the orbit of the binary are referred to as an orbital phenomenon while those that are longer than the binary period are called super-orbital periods. The basic idea behind the X-ray orbital modulations is described in the schematic below:

The underlying mechanism is obscuration of the X-rays, originating from close to the compact source, by an intervening material. This can be the companion star itself, in which case it is known as eclipsing. Other possible obscurers include the hotspot formed at the the accretion stream -- accretion disk interation site, a focused wind from a massive companion, etc. On the other hand, the most likely mechanism for super-orbital periods is that of a precessing accretion disk. A schematic showing a unwarped accretion disk precessing due to the tidal forces of a massive companion is shown below (Image credit: Saeqa Dil Vrtilek):
The key difference between the orbital and the super-orbital periods is that the orbital periods are highly coherent, i.e., their periods and phases do not change on timescales of a few cycles. However, the super-orbital periods often exhibit coherence changes. See below for a sample dynamic power spectrum of SMC X-1 showing strong changes in its super-orbital period (Kotze et al. 2011):
Another famous super-orbital period is that of Her X-1 where the period appears to be stable but its phase changes. Shown below is the dynamic power spectrum (left: Showing the period; Kotze et al. 2011) and the dynamic light curve of Her X-1 (right: Showing the phase; Clarkson et al. 2003).

The ULX M82 X-1 was monitored with RXTE for about five and a half years. The complete light curve of the source is shown below:
The X-rays vary regularly on a timescale of 62 days. This period is stable all throughout the monitoring but when we compared the phase of the 62 day period in the first and the second half of the data, we found that they are significantly offset from each other. Shown below is the phase offset between the first and the second half:
We argue based on this sudden phase jump (evolution timescale of 10 years) and the periodic disk flux variation, that this 62-day period may be due to a precessing accretion disk of the black hole.

For full details of this work, please see the article here.

Research Interests

Time Domain Astronomy: Multi-wavelength analysis of astrophysical transients

X-ray Astrophysics: Periodic and quasi-periodic phenomena from compact X-ray sources

X-ray and optical timing of Ultraluminous X-ray Sources (ULXs) and black hole binaries

Education

2010 - 2014: University of Maryland, College Park, MD, USA

Ph.D., Astronomy, August 2014

Thesis title: X-ray time and spectral variability as probes of ultraluminous X-ray sources

2008 - 2010: University of Maryland, College Park, MD, USA

M.S., Astronomy, December 2010

2004 - 2008: Indian Institute of Technology, Bombay, India

B. Tech, Aerospace Engineering, May 2008

Research Experience

2010 - 2014: Graduate research assistant, University of Maryland & NASA Goddard space flight center

Timing and spectral analysis of ultraluminous X-ray sources

Advisors: Dr. Tod E. Strohmayer, Dr. Coleman Miller & Dr. Richard F. Mushotzky

2009 - 2010: Graduate research assistant, University of Maryland

Near-field cosmological N-body simulations

Advisors: Dr. Ed Shaya & Dr. Massimo Ricotti

     2008      : Research assistant, Ecole Polytechnique (EPFL), Laussane, Switzerland

  May - July    Search for quasar lenses in the SDSS sky survey

Advisor: Dr. Frederic Courbin & Dr. George Meylan

     2007      : Undergraduate research assistant, University of Swinburne, Australia

  May - July    Chemical modeling of proto-planetary disks

Advisor: Dr. Sarah Madison

Awards & Prizes

Spring 2014: Ann G. Wyile dissertation fellowship, merit-based fellowship awarded at the University of Maryland

to doctoral students, stipend of $10,000 plus expenses

April 2012   : Won 1st prize of $500, best oral presentation at the University of Maryland Graduate

Research Interaction Day (GRID)

 Oct. 2012   : Won best poster award of $50, CRESST retreat

     2008      : Dean's award of $1000, graduate school at the University of Maryland

Grants & Accepted Proposals

     Time                                                              Proposal-description

   (Grant)     

  150 ks: Chandra cycle 15

Confronting IC 10 X-1: Does the most massive stellar black hole have the most extreme spin?

James Steiner et al. Dheeraj R. Pasham (co-I)

   1.25ks×30: Swift ToO (Target of Opportunity)

Confirming the 625 day X-ray period of the ultraluminous X-ray source Holmberg IX X-1

Dheeraj R. Pasham (PI)

 33ks×2: Joint XMM-Newton & VLT/FORS2, XMM-Newton cycle 12

     ($65,700) A search for X-ray reverberation in the ultraluminous X-ray source NGC 5408 X-1

Tod E. Strohmayer, Dheeraj R. Pasham (co-I), Margaret L. Trippe, Poshak Gandhi

1 Night: Goodman SOAR spectrograph, NOAO 2013A cycle

Optical spectroscopy of a mysterious periodic X-ray source

Margaret L. Trippe, Dheeraj R. Pasham (co-I)

    2ks×15: Swift ToO (Target of Opportunity)

Search for X-ray dips in the ultraluminous X-ray source NGC 5408 X-1

Dheeraj R. Pasham (PI)

   127ks: XMM-Newton cycle 11

     ($65,734) X-ray timing and eclipse mapping of the massive black hole binary IC 10 X-1

Tod E. Strohmayer, Dheeraj R. Pasham (co-I), Richard F. Mushotzky

Talks & Seminars

2014 Sept: Standford University, CA, USA

A stable, 3:2 frequency ratio quasi-periodic oscillation from ULX M82 X-1

Stanford tea Talk

 2014 Aug: University of Berkeley, Berkeley, CA, USA

A near-Eddington 400 solar mass black hole in M82

Space sciences lab colloquium

 2014 Aug: Caltech, Pasadena, CA, USA

A near-Eddington 400 solar mass black hole in M82

Space radiation lab Sack lunch Talk

  2014 Jan: 223rd AAS Meeting, Washington D.C., USA

X-ray Timing of Ultraluminous X-ray Sources

Dissertation Talk

2013 Sept: University of Michigan, Ann Arbor, MI, USA

News from long-term (3-7 years) X-ray monitoring of a sample of ultraluminous X-ray sources: orbital and super-orbital periodicities

Seminar

2013 June: DC/MD/VA astrophysics summer meeting for graduate students, College Park, MD, USA

The precessing accretion disk of the intermediate-mass black hole candidate ULX M82 X-1

Contributed talk

2013 April: 13th Meeting of The High-Energy Astrophysics Division, Monterey, CA, USA

Quasi-periodic X-ray dips from ULX NGC 5408 X-1: Implications for accretion geometry

Contributed talk

 2012 Aug: IUCAA, Pune, India

A study of the long X-ray periods of ultraluminous X-ray sources: Clues on accretion geometry

Invited seminar

 2012 Aug: Tata Institute of Fundamental Research (TIFR), Bombay, India

Nature of the long X-ray periods from ultraluminous X-ray sources: Orbital or super-orbital?

Invited seminar

 2012 July: 39th COSPAR Scientific Assembly, Mysore, India

Probing the accretion geometry of ultraluminous X-ray sources: News from 3.5 yr Swift monitoring of NGC 5408 X-1

Contributed talk

2012 June: Energetic Astronomy: Richard Mushotzky at 65, Annapolis, MD, USA

Evidence for Quasi-periodic X-ray Dips from an ULX: Implications for the Binary Motion and the Orbital period

Contributed talk

2011 June: The X-ray Universe 2011, Berlin, Germany

ULX, NGC 5408 X-1 : A deep search for correlated timing & spectral behavior

Contributed talk

Poster presentations

 2014 Aug: 14th Meeting of The High-Energy Astrophysics Division, Chicago, IL, USA

Discovery of 3:2 Ratio High-Frequency QPOs from a ULX: Evidence for a 400 solar mass black hole

2013 April: 13th Meeting of The High-Energy Astrophysics Division, Monterey, CA, USA

The nature of the mHz QPOs of ULX M82 X-1: Search for timing-spectral correlations

2013 April: 13th Meeting of The High-Energy Astrophysics Division, Monterey, CA, USA

The 62 day X-ray period of the ultraluminous X-ray source M82 X-1 is likely super-orbital

2011 Sept: 12th Meeting of The High-Energy Astrophysics Division, Newport, RI, USA

ULX NGC 5408 X-1: Coupled spectral-timing behavior similar to StMBHs and AGN?

 2013 Aug: The 2nd ASTRO-H summer school, Tokyo, Japan

Ultraluminous X-ray source NGC 5408 X-1: A scaled-up stellar-mass black hole?